packages/flutter/lib/src/widgets/navigator.dart

// Copyright 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:async';
import 'dart:developer' as developer;

import 'package:flutter/foundation.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/scheduler.dart';

import 'basic.dart';
import 'binding.dart';
import 'focus_manager.dart';
import 'focus_scope.dart';
import 'framework.dart';
import 'overlay.dart';
import 'ticker_provider.dart';

// Examples can assume:
// class MyPage extends Placeholder { MyPage({String title}); }
// class MyHomePage extends Placeholder { }
// NavigatorState navigator;
// BuildContext context;

/// Creates a route for the given route settings.
///
/// Used by [Navigator.onGenerateRoute] and [Navigator.onUnknownRoute].
typedef RouteFactory = Route<dynamic> Function(RouteSettings settings);

/// Signature for the [Navigator.popUntil] predicate argument.
typedef RoutePredicate = bool Function(Route<dynamic> route);

/// Signature for a callback that verifies that it's OK to call [Navigator.pop].
///
/// Used by [Form.onWillPop], [ModalRoute.addScopedWillPopCallback],
/// [ModalRoute.removeScopedWillPopCallback], and [WillPopScope].
typedef WillPopCallback = Future<bool> Function();

/// Indicates whether the current route should be popped.
///
/// Used as the return value for [Route.willPop].
///
/// See also:
///
///  * [WillPopScope], a widget that hooks into the route's [Route.willPop]
///    mechanism.
enum RoutePopDisposition {
  /// Pop the route.
  ///
  /// If [Route.willPop] returns [pop] then the back button will actually pop
  /// the current route.
  pop,

  /// Do not pop the route.
  ///
  /// If [Route.willPop] returns [doNotPop] then the back button will be ignored.
  doNotPop,

  /// Delegate this to the next level of navigation.
  ///
  /// If [Route.willPop] return [bubble] then the back button will be handled
  /// by the [SystemNavigator], which will usually close the application.
  bubble,
}

/// An abstraction for an entry managed by a [Navigator].
///
/// This class defines an abstract interface between the navigator and the
/// "routes" that are pushed on and popped off the navigator. Most routes have
/// visual affordances, which they place in the navigators [Overlay] using one
/// or more [OverlayEntry] objects.
///
/// See [Navigator] for more explanation of how to use a Route
/// with navigation, including code examples.
///
/// See [MaterialPageRoute] for a route that replaces the
/// entire screen with a platform-adaptive transition.
abstract class Route<T> {
  /// Initialize the [Route].
  ///
  /// If the [settings] are not provided, an empty [RouteSettings] object is
  /// used instead.
  Route({ RouteSettings settings }) : settings = settings ?? const RouteSettings();

  /// The navigator that the route is in, if any.
  NavigatorState get navigator => _navigator;
  NavigatorState _navigator;

  /// The settings for this route.
  ///
  /// See [RouteSettings] for details.
  final RouteSettings settings;

  /// The overlay entries for this route.
  List<OverlayEntry> get overlayEntries => const <OverlayEntry>[];

  /// Called when the route is inserted into the navigator.
  ///
  /// Use this to populate [overlayEntries] and add them to the overlay
  /// (accessible as [Navigator.overlay]). (The reason the [Route] is
  /// responsible for doing this, rather than the [Navigator], is that the
  /// [Route] will be responsible for _removing_ the entries and this way it's
  /// symmetric.)
  ///
  /// The `insertionPoint` argument will be null if this is the first route
  /// inserted. Otherwise, it indicates the overlay entry to place immediately
  /// below the first overlay for this route.
  @protected
  @mustCallSuper
  void install(OverlayEntry insertionPoint) { }

  /// Called after [install] when the route is pushed onto the navigator.
  ///
  /// The returned value resolves when the push transition is complete.
  ///
  /// The [didChangeNext] and [didChangePrevious] methods are typically called
  /// immediately after this method is called.
  @protected
  TickerFuture didPush() => TickerFuture.complete();

  /// Called after [install] when the route replaced another in the navigator.
  ///
  /// The [didChangeNext] and [didChangePrevious] methods are typically called
  /// immediately after this method is called.
  @protected
  @mustCallSuper
  void didReplace(Route<dynamic> oldRoute) { }

  /// Returns false if this route wants to veto a [Navigator.pop]. This method is
  /// called by [Navigator.maybePop].
  ///
  /// By default, routes veto a pop if they're the first route in the history
  /// (i.e., if [isFirst]). This behavior prevents the user from popping the
  /// first route off the history and being stranded at a blank screen.
  ///
  /// See also:
  ///
  ///  * [Form], which provides a [Form.onWillPop] callback that uses this
  ///    mechanism.
  ///  * [WillPopScope], another widget that provides a way to intercept the
  ///    back button.
  Future<RoutePopDisposition> willPop() async {
    return isFirst ? RoutePopDisposition.bubble : RoutePopDisposition.pop;
  }

  /// Whether calling [didPop] would return false.
  bool get willHandlePopInternally => false;

  /// When this route is popped (see [Navigator.pop]) if the result isn't
  /// specified or if it's null, this value will be used instead.
  T get currentResult => null;

  /// A future that completes when this route is popped off the navigator.
  ///
  /// The future completes with the value given to [Navigator.pop], if any.
  Future<T> get popped => _popCompleter.future;
  final Completer<T> _popCompleter = Completer<T>();

  /// A request was made to pop this route. If the route can handle it
  /// internally (e.g. because it has its own stack of internal state) then
  /// return false, otherwise return true (by return the value of calling
  /// `super.didPop`). Returning false will prevent the default behavior of
  /// [NavigatorState.pop].
  ///
  /// When this function returns true, the navigator removes this route from
  /// the history but does not yet call [dispose]. Instead, it is the route's
  /// responsibility to call [NavigatorState.finalizeRoute], which will in turn
  /// call [dispose] on the route. This sequence lets the route perform an
  /// exit animation (or some other visual effect) after being popped but prior
  /// to being disposed.
  @protected
  @mustCallSuper
  bool didPop(T result) {
    didComplete(result);
    return true;
  }

  /// The route was popped or is otherwise being removed somewhat gracefully.
  ///
  /// This is called by [didPop] and in response to [Navigator.pushReplacement].
  ///
  /// The [popped] future is completed by this method.
  @protected
  @mustCallSuper
  void didComplete(T result) {
    _popCompleter.complete(result);
  }

  /// The given route, which was above this one, has been popped off the
  /// navigator.
  @protected
  @mustCallSuper
  void didPopNext(Route<dynamic> nextRoute) { }

  /// This route's next route has changed to the given new route. This is called
  /// on a route whenever the next route changes for any reason, so long as it
  /// is in the history, including when a route is first added to a [Navigator]
  /// (e.g. by [Navigator.push]), except for cases when [didPopNext] would be
  /// called. `nextRoute` will be null if there's no next route.
  @protected
  @mustCallSuper
  void didChangeNext(Route<dynamic> nextRoute) { }

  /// This route's previous route has changed to the given new route. This is
  /// called on a route whenever the previous route changes for any reason, so
  /// long as it is in the history. `previousRoute` will be null if there's no
  /// previous route.
  @protected
  @mustCallSuper
  void didChangePrevious(Route<dynamic> previousRoute) { }

  /// Called whenever the internal state of the route has changed.
  ///
  /// This should be called whenever [willHandlePopInternally], [didPop],
  /// [offstage], or other internal state of the route changes value. It is used
  /// by [ModalRoute], for example, to report the new information via its
  /// inherited widget to any children of the route.
  ///
  /// See also:
  ///
  ///  * [changedExternalState], which is called when the [Navigator] rebuilds.
  @protected
  @mustCallSuper
  void changedInternalState() { }

  /// Called whenever the [Navigator] has its widget rebuilt, to indicate that
  /// the route may wish to rebuild as well.
  ///
  /// This is called by the [Navigator] whenever the [NavigatorState]'s
  /// [widget] changes, for example because the [MaterialApp] has been rebuilt.
  /// This ensures that routes that directly refer to the state of the widget
  /// that built the [MaterialApp] will be notified when that widget rebuilds,
  /// since it would otherwise be difficult to notify the routes that state they
  /// depend on may have changed.
  ///
  /// See also:
  ///
  ///  * [changedInternalState], the equivalent but for changes to the internal
  ///    state of the route.
  @protected
  @mustCallSuper
  void changedExternalState() { }

  /// The route should remove its overlays and free any other resources.
  ///
  /// This route is no longer referenced by the navigator.
  @mustCallSuper
  @protected
  void dispose() {
    _navigator = null;
  }

  /// Whether this route is the top-most route on the navigator.
  ///
  /// If this is true, then [isActive] is also true.
  bool get isCurrent {
    return _navigator != null && _navigator._history.last == this;
  }

  /// Whether this route is the bottom-most route on the navigator.
  ///
  /// If this is true, then [Navigator.canPop] will return false if this route's
  /// [willHandlePopInternally] returns false.
  ///
  /// If [isFirst] and [isCurrent] are both true then this is the only route on
  /// the navigator (and [isActive] will also be true).
  bool get isFirst {
    return _navigator != null && _navigator._history.first == this;
  }

  /// Whether this route is on the navigator.
  ///
  /// If the route is not only active, but also the current route (the top-most
  /// route), then [isCurrent] will also be true. If it is the first route (the
  /// bottom-most route), then [isFirst] will also be true.
  ///
  /// If a higher route is entirely opaque, then the route will be active but not
  /// rendered. It is even possible for the route to be active but for the stateful
  /// widgets within the route to not be instantiated. See [ModalRoute.maintainState].
  bool get isActive {
    return _navigator != null && _navigator._history.contains(this);
  }
}

/// Data that might be useful in constructing a [Route].
@immutable
class RouteSettings {
  /// Creates data used to construct routes.
  const RouteSettings({
    this.name,
    this.isInitialRoute = false,
    this.arguments,
  });

  /// Creates a copy of this route settings object with the given fields
  /// replaced with the new values.
  RouteSettings copyWith({
    String name,
    bool isInitialRoute,
    Object arguments,
  }) {
    return RouteSettings(
      name: name ?? this.name,
      isInitialRoute: isInitialRoute ?? this.isInitialRoute,
      arguments: arguments ?? this.arguments,
    );
  }

  /// The name of the route (e.g., "/settings").
  ///
  /// If null, the route is anonymous.
  final String name;

  /// Whether this route is the very first route being pushed onto this [Navigator].
  ///
  /// The initial route typically skips any entrance transition to speed startup.
  final bool isInitialRoute;

  /// The arguments passed to this route.
  ///
  /// May be used when building the route, e.g. in [Navigator.onGenerateRoute].
  final Object arguments;

  @override
  String toString() => '$runtimeType("$name", $arguments)';
}

/// An interface for observing the behavior of a [Navigator].
class NavigatorObserver {
  /// The navigator that the observer is observing, if any.
  NavigatorState get navigator => _navigator;
  NavigatorState _navigator;

  /// The [Navigator] pushed `route`.
  ///
  /// The route immediately below that one, and thus the previously active
  /// route, is `previousRoute`.
  void didPush(Route<dynamic> route, Route<dynamic> previousRoute) { }

  /// The [Navigator] popped `route`.
  ///
  /// The route immediately below that one, and thus the newly active
  /// route, is `previousRoute`.
  void didPop(Route<dynamic> route, Route<dynamic> previousRoute) { }

  /// The [Navigator] removed `route`.
  ///
  /// If only one route is being removed, then the route immediately below
  /// that one, if any, is `previousRoute`.
  ///
  /// If multiple routes are being removed, then the route below the
  /// bottommost route being removed, if any, is `previousRoute`, and this
  /// method will be called once for each removed route, from the topmost route
  /// to the bottommost route.
  void didRemove(Route<dynamic> route, Route<dynamic> previousRoute) { }

  /// The [Navigator] replaced `oldRoute` with `newRoute`.
  void didReplace({ Route<dynamic> newRoute, Route<dynamic> oldRoute }) { }

  /// The [Navigator]'s route `route` is being moved by a user gesture.
  ///
  /// For example, this is called when an iOS back gesture starts.
  ///
  /// Paired with a call to [didStopUserGesture] when the route is no longer
  /// being manipulated via user gesture.
  ///
  /// If present, the route immediately below `route` is `previousRoute`.
  /// Though the gesture may not necessarily conclude at `previousRoute` if
  /// the gesture is canceled. In that case, [didStopUserGesture] is still
  /// called but a follow-up [didPop] is not.
  void didStartUserGesture(Route<dynamic> route, Route<dynamic> previousRoute) { }

  /// User gesture is no longer controlling the [Navigator].
  ///
  /// Paired with an earlier call to [didStartUserGesture].
  void didStopUserGesture() { }
}

/// A widget that manages a set of child widgets with a stack discipline.
///
/// Many apps have a navigator near the top of their widget hierarchy in order
/// to display their logical history using an [Overlay] with the most recently
/// visited pages visually on top of the older pages. Using this pattern lets
/// the navigator visually transition from one page to another by moving the widgets
/// around in the overlay. Similarly, the navigator can be used to show a dialog
/// by positioning the dialog widget above the current page.
///
/// ## Using the Navigator
///
/// Mobile apps typically reveal their contents via full-screen elements
/// called "screens" or "pages". In Flutter these elements are called
/// routes and they're managed by a [Navigator] widget. The navigator
/// manages a stack of [Route] objects and provides methods for managing
/// the stack, like [Navigator.push] and [Navigator.pop].
///
/// ### Displaying a full-screen route
///
/// Although you can create a navigator directly, it's most common to use
/// the navigator created by a [WidgetsApp] or a [MaterialApp] widget. You
/// can refer to that navigator with [Navigator.of].
///
/// A [MaterialApp] is the simplest way to set things up. The [MaterialApp]'s
/// home becomes the route at the bottom of the [Navigator]'s stack. It is what
/// you see when the app is launched.
///
/// ```dart
/// void main() {
///   runApp(MaterialApp(home: MyAppHome()));
/// }
/// ```
///
/// To push a new route on the stack you can create an instance of
/// [MaterialPageRoute] with a builder function that creates whatever you
/// want to appear on the screen. For example:
///
/// ```dart
/// Navigator.push(context, MaterialPageRoute<void>(
///   builder: (BuildContext context) {
///     return Scaffold(
///       appBar: AppBar(title: Text('My Page')),
///       body: Center(
///         child: FlatButton(
///           child: Text('POP'),
///           onPressed: () {
///             Navigator.pop(context);
///           },
///         ),
///       ),
///     );
///   },
/// ));
/// ```
///
/// The route defines its widget with a builder function instead of a
/// child widget because it will be built and rebuilt in different
/// contexts depending on when it's pushed and popped.
///
/// As you can see, the new route can be popped, revealing the app's home
/// page, with the Navigator's pop method:
///
/// ```dart
/// Navigator.pop(context);
/// ```
///
/// It usually isn't necessary to provide a widget that pops the Navigator
/// in a route with a [Scaffold] because the Scaffold automatically adds a
/// 'back' button to its AppBar. Pressing the back button causes
/// [Navigator.pop] to be called. On Android, pressing the system back
/// button does the same thing.
///
/// ### Using named navigator routes
///
/// Mobile apps often manage a large number of routes and it's often
/// easiest to refer to them by name. Route names, by convention,
/// use a path-like structure (for example, '/a/b/c').
/// The app's home page route is named '/' by default.
///
/// The [MaterialApp] can be created
/// with a [Map<String, WidgetBuilder>] which maps from a route's name to
/// a builder function that will create it. The [MaterialApp] uses this
/// map to create a value for its navigator's [onGenerateRoute] callback.
///
/// ```dart
/// void main() {
///   runApp(MaterialApp(
///     home: MyAppHome(), // becomes the route named '/'
///     routes: <String, WidgetBuilder> {
///       '/a': (BuildContext context) => MyPage(title: 'page A'),
///       '/b': (BuildContext context) => MyPage(title: 'page B'),
///       '/c': (BuildContext context) => MyPage(title: 'page C'),
///     },
///   ));
/// }
/// ```
///
/// To show a route by name:
///
/// ```dart
/// Navigator.pushNamed(context, '/b');
/// ```
///
/// ### Routes can return a value
///
/// When a route is pushed to ask the user for a value, the value can be
/// returned via the [pop] method's result parameter.
///
/// Methods that push a route return a [Future]. The Future resolves when the
/// route is popped and the [Future]'s value is the [pop] method's `result`
/// parameter.
///
/// For example if we wanted to ask the user to press 'OK' to confirm an
/// operation we could `await` the result of [Navigator.push]:
///
/// ```dart
/// bool value = await Navigator.push(context, MaterialPageRoute<bool>(
///   builder: (BuildContext context) {
///     return Center(
///       child: GestureDetector(
///         child: Text('OK'),
///         onTap: () { Navigator.pop(context, true); }
///       ),
///     );
///   }
/// ));
/// ```
///
/// If the user presses 'OK' then value will be true. If the user backs
/// out of the route, for example by pressing the Scaffold's back button,
/// the value will be null.
///
/// When a route is used to return a value, the route's type parameter must
/// match the type of [pop]'s result. That's why we've used
/// `MaterialPageRoute<bool>` instead of `MaterialPageRoute<void>` or just
/// `MaterialPageRoute`. (If you prefer to not specify the types, though, that's
/// fine too.)
///
/// ### Popup routes
///
/// Routes don't have to obscure the entire screen. [PopupRoute]s cover the
/// screen with a [ModalRoute.barrierColor] that can be only partially opaque to
/// allow the current screen to show through. Popup routes are "modal" because
/// they block input to the widgets below.
///
/// There are functions which create and show popup routes. For
/// example: [showDialog], [showMenu], and [showModalBottomSheet]. These
/// functions return their pushed route's Future as described above.
/// Callers can await the returned value to take an action when the
/// route is popped, or to discover the route's value.
///
/// There are also widgets which create popup routes, like [PopupMenuButton] and
/// [DropdownButton]. These widgets create internal subclasses of PopupRoute
/// and use the Navigator's push and pop methods to show and dismiss them.
///
/// ### Custom routes
///
/// You can create your own subclass of one of the widget library route classes
/// like [PopupRoute], [ModalRoute], or [PageRoute], to control the animated
/// transition employed to show the route, the color and behavior of the route's
/// modal barrier, and other aspects of the route.
///
/// The [PageRouteBuilder] class makes it possible to define a custom route
/// in terms of callbacks. Here's an example that rotates and fades its child
/// when the route appears or disappears. This route does not obscure the entire
/// screen because it specifies `opaque: false`, just as a popup route does.
///
/// ```dart
/// Navigator.push(context, PageRouteBuilder(
///   opaque: false,
///   pageBuilder: (BuildContext context, _, __) {
///     return Center(child: Text('My PageRoute'));
///   },
///   transitionsBuilder: (___, Animation<double> animation, ____, Widget child) {
///     return FadeTransition(
///       opacity: animation,
///       child: RotationTransition(
///         turns: Tween<double>(begin: 0.5, end: 1.0).animate(animation),
///         child: child,
///       ),
///     );
///   }
/// ));
/// ```
///
/// The page route is built in two parts, the "page" and the
/// "transitions". The page becomes a descendant of the child passed to
/// the `buildTransitions` method. Typically the page is only built once,
/// because it doesn't depend on its animation parameters (elided with `_`
/// and `__` in this example). The transition is built on every frame
/// for its duration.
///
/// ### Nesting Navigators
///
/// An app can use more than one Navigator. Nesting one Navigator below
/// another Navigator can be used to create an "inner journey" such as tabbed
/// navigation, user registration, store checkout, or other independent journeys
/// that represent a subsection of your overall application.
///
/// #### Real World Example
///
/// It is standard practice for iOS apps to use tabbed navigation where each
/// tab maintains its own navigation history. Therefore, each tab has its own
/// [Navigator], creating a kind of "parallel navigation."
///
/// In addition to the parallel navigation of the tabs, it is still possible to
/// launch full-screen pages that completely cover the tabs. For example: an
/// on-boarding flow, or an alert dialog. Therefore, there must exist a "root"
/// [Navigator] that sits above the tab navigation. As a result, each of the
/// tab's [Navigator]s are actually nested [Navigator]s sitting below a single
/// root [Navigator].
///
/// The nested [Navigator]s for tabbed navigation sit in [WidgetApp] and
/// [CupertinoTabView], so you don't need to worry about nested [Navigator]s
/// in this situation, but it's a real world example where nested [Navigator]s
/// are used.
///
/// {@tool snippet --template=freeform}
/// The following example demonstrates how a nested [Navigator] can be used to
/// present a standalone user registration journey.
///
/// Even though this example uses two [Navigator]s to demonstrate nested
/// [Navigator]s, a similar result is possible using only a single [Navigator].
///
/// Run this example with `flutter run --route=/signup` to start it with
/// the signup flow instead of on the home page.
///
/// ```dart imports
/// import 'package:flutter/material.dart';
/// ```
///
/// ```dart main
/// void main() => runApp(new MyApp());
/// ```
///
/// ```dart
/// class MyApp extends StatelessWidget {
///   @override
///   Widget build(BuildContext context) {
///     return MaterialApp(
///       title: 'Flutter Code Sample for Navigator',
///       // MaterialApp contains our top-level Navigator
///       initialRoute: '/',
///       routes: {
///         '/': (BuildContext context) => HomePage(),
///         '/signup': (BuildContext context) => SignUpPage(),
///       },
///     );
///   }
/// }
///
/// class HomePage extends StatelessWidget {
///   @override
///   Widget build(BuildContext context) {
///     return DefaultTextStyle(
///       style: Theme.of(context).textTheme.display1,
///       child: Container(
///         color: Colors.white,
///         alignment: Alignment.center,
///         child: Text('Home Page'),
///       ),
///     );
///   }
/// }
///
/// class CollectPersonalInfoPage extends StatelessWidget {
///   @override
///   Widget build(BuildContext context) {
///     return DefaultTextStyle(
///       style: Theme.of(context).textTheme.display1,
///       child: GestureDetector(
///         onTap: () {
///           // This moves from the personal info page to the credentials page,
///           // replacing this page with that one.
///           Navigator.of(context)
///             .pushReplacementNamed('signup/choose_credentials');
///         },
///         child: Container(
///           color: Colors.lightBlue,
///           alignment: Alignment.center,
///           child: Text('Collect Personal Info Page'),
///         ),
///       ),
///     );
///   }
/// }
///
/// class ChooseCredentialsPage extends StatelessWidget {
///   const ChooseCredentialsPage({
///     this.onSignupComplete,
///   });
///
///   final VoidCallback onSignupComplete;
///
///   @override
///   Widget build(BuildContext context) {
///     return GestureDetector(
///       onTap: onSignupComplete,
///       child: DefaultTextStyle(
///         style: Theme.of(context).textTheme.display1,
///         child: Container(
///           color: Colors.pinkAccent,
///           alignment: Alignment.center,
///           child: Text('Choose Credentials Page'),
///         ),
///       ),
///     );
///   }
/// }
///
/// class SignUpPage extends StatelessWidget {
///   @override
///   Widget build(BuildContext context) {
///     // SignUpPage builds its own Navigator which ends up being a nested
///     // Navigator in our app.
///     return Navigator(
///       initialRoute: 'signup/personal_info',
///       onGenerateRoute: (RouteSettings settings) {
///         WidgetBuilder builder;
///         switch (settings.name) {
///           case 'signup/personal_info':
///           // Assume CollectPersonalInfoPage collects personal info and then
///           // navigates to 'signup/choose_credentials'.
///             builder = (BuildContext _) => CollectPersonalInfoPage();
///             break;
///           case 'signup/choose_credentials':
///           // Assume ChooseCredentialsPage collects new credentials and then
///           // invokes 'onSignupComplete()'.
///             builder = (BuildContext _) => ChooseCredentialsPage(
///               onSignupComplete: () {
///                 // Referencing Navigator.of(context) from here refers to the
///                 // top level Navigator because SignUpPage is above the
///                 // nested Navigator that it created. Therefore, this pop()
///                 // will pop the entire "sign up" journey and return to the
///                 // "/" route, AKA HomePage.
///                 Navigator.of(context).pop();
///               },
///             );
///             break;
///           default:
///             throw Exception('Invalid route: ${settings.name}');
///         }
///         return MaterialPageRoute(builder: builder, settings: settings);
///       },
///     );
///   }
/// }
/// ```
/// {@end-tool}
///
/// [Navigator.of] operates on the nearest ancestor [Navigator] from the given
/// [BuildContext]. Be sure to provide a [BuildContext] below the intended
/// [Navigator], especially in large [build] methods where nested [Navigator]s
/// are created. The [Builder] widget can be used to access a [BuildContext] at
/// a desired location in the widget subtree.
class Navigator extends StatefulWidget {
  /// Creates a widget that maintains a stack-based history of child widgets.
  ///
  /// The [onGenerateRoute] argument must not be null.
  const Navigator({
    Key key,
    this.initialRoute,
    @required this.onGenerateRoute,
    this.onUnknownRoute,
    this.observers = const <NavigatorObserver>[]
  }) : assert(onGenerateRoute != null),
       super(key: key);

  /// The name of the first route to show.
  ///
  /// By default, this defers to [dart:ui.Window.defaultRouteName].
  ///
  /// If this string contains any `/` characters, then the string is split on
  /// those characters and substrings from the start of the string up to each
  /// such character are, in turn, used as routes to push.
  ///
  /// For example, if the route `/stocks/HOOLI` was used as the [initialRoute],
  /// then the [Navigator] would push the following routes on startup: `/`,
  /// `/stocks`, `/stocks/HOOLI`. This enables deep linking while allowing the
  /// application to maintain a predictable route history.
  final String initialRoute;

  /// Called to generate a route for a given [RouteSettings].
  final RouteFactory onGenerateRoute;

  /// Called when [onGenerateRoute] fails to generate a route.
  ///
  /// This callback is typically used for error handling. For example, this
  /// callback might always generate a "not found" page that describes the route
  /// that wasn't found.
  ///
  /// Unknown routes can arise either from errors in the app or from external
  /// requests to push routes, such as from Android intents.
  final RouteFactory onUnknownRoute;

  /// A list of observers for this navigator.
  final List<NavigatorObserver> observers;

  /// The default name for the [initialRoute].
  ///
  /// See also:
  ///
  ///  * [dart:ui.Window.defaultRouteName], which reflects the route that the
  ///    application was started with.
  static const String defaultRouteName = '/';

  /// Push a named route onto the navigator that most tightly encloses the given
  /// context.
  ///
  /// {@template flutter.widgets.navigator.pushNamed}
  /// The route name will be passed to that navigator's [onGenerateRoute]
  /// callback. The returned route will be pushed into the navigator.
  ///
  /// The new route and the previous route (if any) are notified (see
  /// [Route.didPush] and [Route.didChangeNext]). If the [Navigator] has any
  /// [Navigator.observers], they will be notified as well (see
  /// [NavigatorObserver.didPush]).
  ///
  /// Ongoing gestures within the current route are canceled when a new route is
  /// pushed.
  ///
  /// Returns a [Future] that completes to the `result` value passed to [pop]
  /// when the pushed route is popped off the navigator.
  ///
  /// The `T` type argument is the type of the return value of the route.
  /// {@endtemplate}
  ///
  /// {@template flutter.widgets.navigator.pushNamed.arguments}
  /// The provided `arguments` are passed to the pushed route via
  /// [RouteSettings.arguments]. Any object can be passed as `arguments` (e.g. a
  /// [String], [int], or an instance of a custom `MyRouteArguments` class).
  /// Often, a [Map] is used to pass key-value pairs.
  ///
  /// The `arguments` may be used in [Navigator.onGenerateRoute] or
  /// [Navigator.onUnknownRoute] to construct the route.
  /// {@endtemplate}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _didPushButton() {
  ///   Navigator.pushNamed(context, '/settings');
  /// }
  /// ```
  /// {@end-tool}
  ///
  /// {@tool sample}
  ///
  /// The following example shows how to pass additional `arguments` to the
  /// route:
  ///
  /// ```dart
  /// void _showBerlinWeather() {
  ///   Navigator.pushNamed(
  ///     context,
  ///     '/weather',
  ///     arguments: <String, String>{
  ///       'city': 'Berlin',
  ///       'country': 'Germany',
  ///     },
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  ///
  /// {@tool sample}
  ///
  /// The following example shows how to pass a custom Object to the route:
  ///
  /// ```dart
  /// class WeatherRouteArguments {
  ///   WeatherRouteArguments({ this.city, this.country });
  ///   final String city;
  ///   final String country;
  ///
  ///   bool get isGermanCapital {
  ///     return country == 'Germany' && city == 'Berlin';
  ///   }
  /// }
  ///
  /// void _showWeather() {
  ///   Navigator.pushNamed(
  ///     context,
  ///     '/weather',
  ///     arguments: WeatherRouteArguments(city: 'Berlin', country: 'Germany'),
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  static Future<T> pushNamed<T extends Object>(
    BuildContext context,
    String routeName, {
    Object arguments,
   }) {
    return Navigator.of(context).pushNamed<T>(routeName, arguments: arguments);
  }

  /// Replace the current route of the navigator that most tightly encloses the
  /// given context by pushing the route named [routeName] and then disposing
  /// the previous route once the new route has finished animating in.
  ///
  /// {@template flutter.widgets.navigator.pushReplacementNamed}
  /// If non-null, `result` will be used as the result of the route that is
  /// removed; the future that had been returned from pushing that old route
  /// will complete with `result`. Routes such as dialogs or popup menus
  /// typically use this mechanism to return the value selected by the user to
  /// the widget that created their route. The type of `result`, if provided,
  /// must match the type argument of the class of the old route (`TO`).
  ///
  /// The route name will be passed to the navigator's [onGenerateRoute]
  /// callback. The returned route will be pushed into the navigator.
  ///
  /// The new route and the route below the removed route are notified (see
  /// [Route.didPush] and [Route.didChangeNext]). If the [Navigator] has any
  /// [Navigator.observers], they will be notified as well (see
  /// [NavigatorObserver.didReplace]). The removed route is notified once the
  /// new route has finished animating (see [Route.didComplete]). The removed
  /// route's exit animation is not run (see [popAndPushNamed] for a variant
  /// that does animated the removed route).
  ///
  /// Ongoing gestures within the current route are canceled when a new route is
  /// pushed.
  ///
  /// Returns a [Future] that completes to the `result` value passed to [pop]
  /// when the pushed route is popped off the navigator.
  ///
  /// The `T` type argument is the type of the return value of the new route,
  /// and `TO` is the type of the return value of the old route.
  /// {@endtemplate}
  ///
  /// {@macro flutter.widgets.navigator.pushNamed.arguments}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _switchToBrightness() {
  ///   Navigator.pushReplacementNamed(context, '/settings/brightness');
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  static Future<T> pushReplacementNamed<T extends Object, TO extends Object>(
    BuildContext context,
    String routeName, {
    TO result,
    Object arguments,
  }) {
    return Navigator.of(context).pushReplacementNamed<T, TO>(routeName, arguments: arguments, result: result);
  }

  /// Pop the current route off the navigator that most tightly encloses the
  /// given context and push a named route in its place.
  ///
  /// {@template flutter.widgets.navigator.popAndPushNamed}
  /// If non-null, `result` will be used as the result of the route that is
  /// popped; the future that had been returned from pushing the popped route
  /// will complete with `result`. Routes such as dialogs or popup menus
  /// typically use this mechanism to return the value selected by the user to
  /// the widget that created their route. The type of `result`, if provided,
  /// must match the type argument of the class of the popped route (`TO`).
  ///
  /// The route name will be passed to the navigator's [onGenerateRoute]
  /// callback. The returned route will be pushed into the navigator.
  ///
  /// The new route, the old route, and the route below the old route (if any)
  /// are all notified (see [Route.didPop], [Route.didComplete],
  /// [Route.didPopNext], [Route.didPush], and [Route.didChangeNext]). If the
  /// [Navigator] has any [Navigator.observers], they will be notified as well
  /// (see [NavigatorObserver.didPop] and [NavigatorObservers.didPush]). The
  /// animations for the pop and the push are performed simultaneously, so the
  /// route below may be briefly visible even if both the old route and the new
  /// route are opaque (see [TransitionRoute.opaque]).
  ///
  /// Ongoing gestures within the current route are canceled when a new route is
  /// pushed.
  ///
  /// Returns a [Future] that completes to the `result` value passed to [pop]
  /// when the pushed route is popped off the navigator.
  ///
  /// The `T` type argument is the type of the return value of the new route,
  /// and `TO` is the return value type of the old route.
  /// {@endtemplate}
  ///
  /// {@macro flutter.widgets.navigator.pushNamed.arguments}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _selectAccessibility() {
  ///   Navigator.popAndPushNamed(context, '/settings/accessibility');
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  static Future<T> popAndPushNamed<T extends Object, TO extends Object>(
    BuildContext context,
    String routeName, {
    TO result,
    Object arguments,
   }) {
    return Navigator.of(context).popAndPushNamed<T, TO>(routeName, arguments: arguments, result: result);
  }

  /// Push the route with the given name onto the navigator that most tightly
  /// encloses the given context, and then remove all the previous routes until
  /// the `predicate` returns true.
  ///
  /// {@template flutter.widgets.navigator.pushNamedAndRemoveUntil}
  /// The predicate may be applied to the same route more than once if
  /// [Route.willHandlePopInternally] is true.
  ///
  /// To remove routes until a route with a certain name, use the
  /// [RoutePredicate] returned from [ModalRoute.withName].
  ///
  /// To remove all the routes below the pushed route, use a [RoutePredicate]
  /// that always returns false (e.g. `(Route<dynamic> route) => false`).
  ///
  /// The removed routes are removed without being completed, so this method
  /// does not take a return value argument.
  ///
  /// The new route's name (`routeName`) will be passed to the navigator's
  /// [onGenerateRoute] callback. The returned route will be pushed into the
  /// navigator.
  ///
  /// The new route and the route below the bottommost removed route (which
  /// becomes the route below the new route) are notified (see [Route.didPush]
  /// and [Route.didChangeNext]). If the [Navigator] has any
  /// [Navigator.observers], they will be notified as well (see
  /// [NavigatorObservers.didPush] and [NavigatorObservers.didRemove]). The
  /// removed routes are disposed, without being notified, once the new route
  /// has finished animating. The futures that had been returned from pushing
  /// those routes will not complete.
  ///
  /// Ongoing gestures within the current route are canceled when a new route is
  /// pushed.
  ///
  /// Returns a [Future] that completes to the `result` value passed to [pop]
  /// when the pushed route is popped off the navigator.
  ///
  /// The `T` type argument is the type of the return value of the new route.
  /// {@endtemplate}
  ///
  /// {@macro flutter.widgets.navigator.pushNamed.arguments}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _resetToCalendar() {
  ///   Navigator.pushNamedAndRemoveUntil(context, '/calendar', ModalRoute.withName('/'));
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  static Future<T> pushNamedAndRemoveUntil<T extends Object>(
    BuildContext context,
    String newRouteName,
    RoutePredicate predicate, {
    Object arguments,
  }) {
    return Navigator.of(context).pushNamedAndRemoveUntil<T>(newRouteName, predicate, arguments: arguments);
  }

  /// Push the given route onto the navigator that most tightly encloses the
  /// given context.
  ///
  /// {@template flutter.widgets.navigator.push}
  /// The new route and the previous route (if any) are notified (see
  /// [Route.didPush] and [Route.didChangeNext]). If the [Navigator] has any
  /// [Navigator.observers], they will be notified as well (see
  /// [NavigatorObserver.didPush]).
  ///
  /// Ongoing gestures within the current route are canceled when a new route is
  /// pushed.
  ///
  /// Returns a [Future] that completes to the `result` value passed to [pop]
  /// when the pushed route is popped off the navigator.
  ///
  /// The `T` type argument is the type of the return value of the route.
  /// {@endtemplate}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _openMyPage() {
  ///   Navigator.push(context, MaterialPageRoute(builder: (BuildContext context) => MyPage()));
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  static Future<T> push<T extends Object>(BuildContext context, Route<T> route) {
    return Navigator.of(context).push(route);
  }

  /// Replace the current route of the navigator that most tightly encloses the
  /// given context by pushing the given route and then disposing the previous
  /// route once the new route has finished animating in.
  ///
  /// {@template flutter.widgets.navigator.pushReplacement}
  /// If non-null, `result` will be used as the result of the route that is
  /// removed; the future that had been returned from pushing that old route will
  /// complete with `result`. Routes such as dialogs or popup menus typically
  /// use this mechanism to return the value selected by the user to the widget
  /// that created their route. The type of `result`, if provided, must match
  /// the type argument of the class of the old route (`TO`).
  ///
  /// The new route and the route below the removed route are notified (see
  /// [Route.didPush] and [Route.didChangeNext]). If the [Navigator] has any
  /// [Navigator.observers], they will be notified as well (see
  /// [NavigatorObserver.didReplace]). The removed route is notified once the
  /// new route has finished animating (see [Route.didComplete]).
  ///
  /// Ongoing gestures within the current route are canceled when a new route is
  /// pushed.
  ///
  /// Returns a [Future] that completes to the `result` value passed to [pop]
  /// when the pushed route is popped off the navigator.
  ///
  /// The `T` type argument is the type of the return value of the new route,
  /// and `TO` is the type of the return value of the old route.
  /// {@endtemplate}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _completeLogin() {
  ///   Navigator.pushReplacement(
  ///       context, MaterialPageRoute(builder: (BuildContext context) => MyHomePage()));
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  static Future<T> pushReplacement<T extends Object, TO extends Object>(BuildContext context, Route<T> newRoute, { TO result }) {
    return Navigator.of(context).pushReplacement<T, TO>(newRoute, result: result);
  }

  /// Push the given route onto the navigator that most tightly encloses the
  /// given context, and then remove all the previous routes until the
  /// `predicate` returns true.
  ///
  /// {@template flutter.widgets.navigator.pushAndRemoveUntil}
  /// The predicate may be applied to the same route more than once if
  /// [Route.willHandlePopInternally] is true.
  ///
  /// To remove routes until a route with a certain name, use the
  /// [RoutePredicate] returned from [ModalRoute.withName].
  ///
  /// To remove all the routes below the pushed route, use a [RoutePredicate]
  /// that always returns false (e.g. `(Route<dynamic> route) => false`).
  ///
  /// The removed routes are removed without being completed, so this method
  /// does not take a return value argument.
  ///
  /// The new route and the route below the bottommost removed route (which
  /// becomes the route below the new route) are notified (see [Route.didPush]
  /// and [Route.didChangeNext]). If the [Navigator] has any
  /// [Navigator.observers], they will be notified as well (see
  /// [NavigatorObservers.didPush] and [NavigatorObservers.didRemove]). The
  /// removed routes are disposed, without being notified, once the new route
  /// has finished animating. The futures that had been returned from pushing
  /// those routes will not complete.
  ///
  /// Ongoing gestures within the current route are canceled when a new route is
  /// pushed.
  ///
  /// Returns a [Future] that completes to the `result` value passed to [pop]
  /// when the pushed route is popped off the navigator.
  ///
  /// The `T` type argument is the type of the return value of the new route.
  /// {@endtemplate}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _finishAccountCreation() {
  ///   Navigator.pushAndRemoveUntil(
  ///     context,
  ///     MaterialPageRoute(builder: (BuildContext context) => MyHomePage()),
  ///     ModalRoute.withName('/'),
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  static Future<T> pushAndRemoveUntil<T extends Object>(BuildContext context, Route<T> newRoute, RoutePredicate predicate) {
    return Navigator.of(context).pushAndRemoveUntil<T>(newRoute, predicate);
  }

  /// Replaces a route on the navigator that most tightly encloses the given
  /// context with a new route.
  ///
  /// {@template flutter.widgets.navigator.replace}
  /// The old route must not be current visible, as this method skips the
  /// animations and therefore the removal would be jarring if it was visible.
  /// To replace the top-most route, consider [pushReplacement] instead, which
  /// _does_ animate the new route, and delays removing the old route until the
  /// new route has finished animating.
  ///
  /// The removed route is removed without being completed, so this method does
  /// not take a return value argument.
  ///
  /// The new route, the route below the new route (if any), and the route above
  /// the new route, are all notified (see [Route.didReplace],
  /// [Route.didChangeNext], and [Route.didChangePrevious]). If the [Navigator]
  /// has any [Navigator.observers], they will be notified as well (see
  /// [NavigatorObservers.didReplace]). The removed route is disposed without
  /// being notified. The future that had been returned from pushing that routes
  /// will not complete.
  ///
  /// This can be useful in combination with [removeRouteBelow] when building a
  /// non-linear user experience.
  ///
  /// The `T` type argument is the type of the return value of the new route.
  /// {@endtemplate}
  ///
  /// See also:
  ///
  ///  * [replaceRouteBelow], which is the same but identifies the route to be
  ///    removed by reference to the route above it, rather than directly.
  @optionalTypeArgs
  static void replace<T extends Object>(BuildContext context, { @required Route<dynamic> oldRoute, @required Route<T> newRoute }) {
    return Navigator.of(context).replace<T>(oldRoute: oldRoute, newRoute: newRoute);
  }

  /// Replaces a route on the navigator that most tightly encloses the given
  /// context with a new route. The route to be replaced is the one below the
  /// given `anchorRoute`.
  ///
  /// {@template flutter.widgets.navigator.replaceRouteBelow}
  /// The old route must not be current visible, as this method skips the
  /// animations and therefore the removal would be jarring if it was visible.
  /// To replace the top-most route, consider [pushReplacement] instead, which
  /// _does_ animate the new route, and delays removing the old route until the
  /// new route has finished animating.
  ///
  /// The removed route is removed without being completed, so this method does
  /// not take a return value argument.
  ///
  /// The new route, the route below the new route (if any), and the route above
  /// the new route, are all notified (see [Route.didReplace],
  /// [Route.didChangeNext], and [Route.didChangePrevious]). If the [Navigator]
  /// has any [Navigator.observers], they will be notified as well (see
  /// [NavigatorObservers.didReplace]). The removed route is disposed without
  /// being notified. The future that had been returned from pushing that routes
  /// will not complete.
  ///
  /// The `T` type argument is the type of the return value of the new route.
  /// {@endtemplate}
  ///
  /// See also:
  ///
  ///  * [replace], which is the same but identifies the route to be removed
  ///    directly.
  @optionalTypeArgs
  static void replaceRouteBelow<T extends Object>(BuildContext context, { @required Route<dynamic> anchorRoute, Route<T> newRoute }) {
    return Navigator.of(context).replaceRouteBelow<T>(anchorRoute: anchorRoute, newRoute: newRoute);
  }

  /// Whether the navigator that most tightly encloses the given context can be
  /// popped.
  ///
  /// {@template flutter.widgets.navigator.canPop}
  /// The initial route cannot be popped off the navigator, which implies that
  /// this function returns true only if popping the navigator would not remove
  /// the initial route.
  ///
  /// If there is no [Navigator] in scope, returns false.
  /// {@endtemplate}
  ///
  /// See also:
  ///
  ///  * [Route.isFirst], which returns true for routes for which [canPop]
  ///    returns false.
  static bool canPop(BuildContext context) {
    final NavigatorState navigator = Navigator.of(context, nullOk: true);
    return navigator != null && navigator.canPop();
  }

  /// Returns the value of the current route's [Route.willPop] method for the
  /// navigator that most tightly encloses the given context.
  ///
  /// {@template flutter.widgets.navigator.maybePop}
  /// This method is typically called before a user-initiated [pop]. For example
  /// on Android it's called by the binding for the system's back button.
  ///
  /// The `T` type argument is the type of the return value of the current
  /// route.
  /// {@endtemplate}
  ///
  /// See also:
  ///
  ///  * [Form], which provides an `onWillPop` callback that enables the form
  ///    to veto a [pop] initiated by the app's back button.
  ///  * [ModalRoute], which provides a `scopedWillPopCallback` that can be used
  ///    to define the route's `willPop` method.
  @optionalTypeArgs
  static Future<bool> maybePop<T extends Object>(BuildContext context, [ T result ]) {
    return Navigator.of(context).maybePop<T>(result);
  }

  /// Pop the top-most route off the navigator that most tightly encloses the
  /// given context.
  ///
  /// {@template flutter.widgets.navigator.pop}
  /// The current route's [Route.didPop] method is called first. If that method
  /// returns false, then this method returns true but nothing else is changed
  /// (the route is expected to have popped some internal state; see e.g.
  /// [LocalHistoryRoute]). Otherwise, the rest of this description applies.
  ///
  /// If non-null, `result` will be used as the result of the route that is
  /// popped; the future that had been returned from pushing the popped route
  /// will complete with `result`. Routes such as dialogs or popup menus
  /// typically use this mechanism to return the value selected by the user to
  /// the widget that created their route. The type of `result`, if provided,
  /// must match the type argument of the class of the popped route (`T`).
  ///
  /// The popped route and the route below it are notified (see [Route.didPop],
  /// [Route.didComplete], and [Route.didPopNext]). If the [Navigator] has any
  /// [Navigator.observers], they will be notified as well (see
  /// [NavigatorObserver.didPop]).
  ///
  /// The `T` type argument is the type of the return value of the popped route.
  ///
  /// Returns true if a route was popped (including if [Route.didPop] returned
  /// false); returns false if there are no further previous routes.
  /// {@endtemplate}
  ///
  /// {@tool sample}
  ///
  /// Typical usage for closing a route is as follows:
  ///
  /// ```dart
  /// void _close() {
  ///   Navigator.pop(context);
  /// }
  /// ```
  /// {@end-tool}
  ///
  /// A dialog box might be closed with a result:
  ///
  /// ```dart
  /// void _accept() {
  ///   Navigator.pop(context, true); // dialog returns true
  /// }
  /// ```
  @optionalTypeArgs
  static bool pop<T extends Object>(BuildContext context, [ T result ]) {
    return Navigator.of(context).pop<T>(result);
  }

  /// Calls [pop] repeatedly on the navigator that most tightly encloses the
  /// given context until the predicate returns true.
  ///
  /// {@template flutter.widgets.navigator.popUntil}
  /// The predicate may be applied to the same route more than once if
  /// [Route.willHandlePopInternally] is true.
  ///
  /// To pop until a route with a certain name, use the [RoutePredicate]
  /// returned from [ModalRoute.withName].
  ///
  /// The routes are closed with null as their `return` value.
  ///
  /// See [pop] for more details of the semantics of popping a route.
  /// {@endtemplate}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _logout() {
  ///   Navigator.popUntil(context, ModalRoute.withName('/login'));
  /// }
  /// ```
  /// {@end-tool}
  static void popUntil(BuildContext context, RoutePredicate predicate) {
    Navigator.of(context).popUntil(predicate);
  }

  /// Immediately remove `route` from the navigator that most tightly encloses
  /// the given context, and [Route.dispose] it.
  ///
  /// {@template flutter.widgets.navigator.removeRoute}
  /// The removed route is removed without being completed, so this method does
  /// not take a return value argument. No animations are run as a result of
  /// this method call.
  ///
  /// The routes below and above the removed route are notified (see
  /// [Route.didChangeNext] and [Route.didChangePrevious]). If the [Navigator]
  /// has any [Navigator.observers], they will be notified as well (see
  /// [NavigatorObserver.didRemove]). The removed route is disposed without
  /// being notified. The future that had been returned from pushing that routes
  /// will not complete.
  ///
  /// The given `route` must be in the history; this method will throw an
  /// exception if it is not.
  ///
  /// Ongoing gestures within the current route are canceled.
  /// {@endtemplate}
  ///
  /// This method is used, for example, to instantly dismiss dropdown menus that
  /// are up when the screen's orientation changes.
  static void removeRoute(BuildContext context, Route<dynamic> route) {
    return Navigator.of(context).removeRoute(route);
  }

  /// Immediately remove a route from the navigator that most tightly encloses
  /// the given context, and [Route.dispose] it. The route to be replaced is the
  /// one below the given `anchorRoute`.
  ///
  /// {@template flutter.widgets.navigator.removeRouteBelow}
  /// The removed route is removed without being completed, so this method does
  /// not take a return value argument. No animations are run as a result of
  /// this method call.
  ///
  /// The routes below and above the removed route are notified (see
  /// [Route.didChangeNext] and [Route.didChangePrevious]). If the [Navigator]
  /// has any [Navigator.observers], they will be notified as well (see
  /// [NavigatorObserver.didRemove]). The removed route is disposed without
  /// being notified. The future that had been returned from pushing that routes
  /// will not complete.
  ///
  /// The given `anchorRoute` must be in the history and must have a route below
  /// it; this method will throw an exception if it is not or does not.
  ///
  /// Ongoing gestures within the current route are canceled.
  /// {@endtemplate}
  static void removeRouteBelow(BuildContext context, Route<dynamic> anchorRoute) {
    return Navigator.of(context).removeRouteBelow(anchorRoute);
  }

  /// The state from the closest instance of this class that encloses the given context.
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// Navigator.of(context)
  ///   ..pop()
  ///   ..pop()
  ///   ..pushNamed('/settings');
  /// ```
  ///
  /// If `rootNavigator` is set to true, the state from the furthest instance of
  /// this class is given instead. Useful for pushing contents above all subsequent
  /// instances of [Navigator].
  static NavigatorState of(
    BuildContext context, {
    bool rootNavigator = false,
    bool nullOk = false,
  }) {
    final NavigatorState navigator = rootNavigator
        ? context.rootAncestorStateOfType(const TypeMatcher<NavigatorState>())
        : context.ancestorStateOfType(const TypeMatcher<NavigatorState>());
    assert(() {
      if (navigator == null && !nullOk) {
        throw FlutterError(
          'Navigator operation requested with a context that does not include a Navigator.\n'
          'The context used to push or pop routes from the Navigator must be that of a '
          'widget that is a descendant of a Navigator widget.'
        );
      }
      return true;
    }());
    return navigator;
  }

  @override
  NavigatorState createState() => NavigatorState();
}

/// The state for a [Navigator] widget.
class NavigatorState extends State<Navigator> with TickerProviderStateMixin {
  final GlobalKey<OverlayState> _overlayKey = GlobalKey<OverlayState>();
  final List<Route<dynamic>> _history = <Route<dynamic>>[];
  final Set<Route<dynamic>> _poppedRoutes = Set<Route<dynamic>>();

  /// The [FocusScopeNode] for the [FocusScope] that encloses the routes.
  final FocusScopeNode focusScopeNode = FocusScopeNode();

  final List<OverlayEntry> _initialOverlayEntries = <OverlayEntry>[];

  @override
  void initState() {
    super.initState();
    for (NavigatorObserver observer in widget.observers) {
      assert(observer.navigator == null);
      observer._navigator = this;
    }
    String initialRouteName = widget.initialRoute ?? Navigator.defaultRouteName;
    if (initialRouteName.startsWith('/') && initialRouteName.length > 1) {
      initialRouteName = initialRouteName.substring(1); // strip leading '/'
      assert(Navigator.defaultRouteName == '/');
      final List<String> plannedInitialRouteNames = <String>[
        Navigator.defaultRouteName,
      ];
      final List<Route<dynamic>> plannedInitialRoutes = <Route<dynamic>>[
        _routeNamed<dynamic>(Navigator.defaultRouteName, allowNull: true, arguments: null),
      ];
      final List<String> routeParts = initialRouteName.split('/');
      if (initialRouteName.isNotEmpty) {
        String routeName = '';
        for (String part in routeParts) {
          routeName += '/$part';
          plannedInitialRouteNames.add(routeName);
          plannedInitialRoutes.add(_routeNamed<dynamic>(routeName, allowNull: true, arguments: null));
        }
      }
      if (plannedInitialRoutes.contains(null)) {
        assert(() {
          FlutterError.reportError(
            FlutterErrorDetails(
              exception:
                'Could not navigate to initial route.\n'
                'The requested route name was: "/$initialRouteName"\n'
                'The following routes were therefore attempted:\n'
                ' * ${plannedInitialRouteNames.join("\n * ")}\n'
                'This resulted in the following objects:\n'
                ' * ${plannedInitialRoutes.join("\n * ")}\n'
                'One or more of those objects was null, and therefore the initial route specified will be '
                'ignored and "${Navigator.defaultRouteName}" will be used instead.'
            ),
          );
          return true;
        }());
        push(_routeNamed<Object>(Navigator.defaultRouteName, arguments: null));
      } else {
        plannedInitialRoutes.forEach(push);
      }
    } else {
      Route<Object> route;
      if (initialRouteName != Navigator.defaultRouteName)
        route = _routeNamed<Object>(initialRouteName, allowNull: true, arguments: null);
      route ??= _routeNamed<Object>(Navigator.defaultRouteName, arguments: null);
      push(route);
    }
    for (Route<dynamic> route in _history)
      _initialOverlayEntries.addAll(route.overlayEntries);
  }

  @override
  void didUpdateWidget(Navigator oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.observers != widget.observers) {
      for (NavigatorObserver observer in oldWidget.observers)
        observer._navigator = null;
      for (NavigatorObserver observer in widget.observers) {
        assert(observer.navigator == null);
        observer._navigator = this;
      }
    }
    for (Route<dynamic> route in _history)
      route.changedExternalState();
  }

  @override
  void dispose() {
    assert(!_debugLocked);
    assert(() { _debugLocked = true; return true; }());
    for (NavigatorObserver observer in widget.observers)
      observer._navigator = null;
    final List<Route<dynamic>> doomed = _poppedRoutes.toList()..addAll(_history);
    for (Route<dynamic> route in doomed)
      route.dispose();
    _poppedRoutes.clear();
    _history.clear();
    focusScopeNode.detach();
    super.dispose();
    assert(() { _debugLocked = false; return true; }());
  }

  /// The overlay this navigator uses for its visual presentation.
  OverlayState get overlay => _overlayKey.currentState;

  OverlayEntry get _currentOverlayEntry {
    for (Route<dynamic> route in _history.reversed) {
      if (route.overlayEntries.isNotEmpty)
        return route.overlayEntries.last;
    }
    return null;
  }

  bool _debugLocked = false; // used to prevent re-entrant calls to push, pop, and friends

  Route<T> _routeNamed<T>(String name, { @required Object arguments, bool allowNull = false }) {
    assert(!_debugLocked);
    assert(name != null);
    final RouteSettings settings = RouteSettings(
      name: name,
      isInitialRoute: _history.isEmpty,
      arguments: arguments,
    );
    Route<T> route = widget.onGenerateRoute(settings);
    if (route == null && !allowNull) {
      assert(() {
        if (widget.onUnknownRoute == null) {
          throw FlutterError(
            'If a Navigator has no onUnknownRoute, then its onGenerateRoute must never return null.\n'
            'When trying to build the route "$name", onGenerateRoute returned null, but there was no '
            'onUnknownRoute callback specified.\n'
            'The Navigator was:\n'
            '  $this'
          );
        }
        return true;
      }());
      route = widget.onUnknownRoute(settings);
      assert(() {
        if (route == null) {
          throw FlutterError(
            'A Navigator\'s onUnknownRoute returned null.\n'
            'When trying to build the route "$name", both onGenerateRoute and onUnknownRoute returned '
            'null. The onUnknownRoute callback should never return null.\n'
            'The Navigator was:\n'
            '  $this'
          );
        }
        return true;
      }());
    }
    return route;
  }

  /// Push a named route onto the navigator.
  ///
  /// {@macro flutter.widgets.navigator.pushNamed}
  ///
  /// {@macro flutter.widgets.navigator.pushNamed.arguments}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _aaronBurrSir() {
  ///   navigator.pushNamed('/nyc/1776');
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  Future<T> pushNamed<T extends Object>(
    String routeName, {
    Object arguments,
  }) {
    return push<T>(_routeNamed<T>(routeName, arguments: arguments));
  }

  /// Replace the current route of the navigator by pushing the route named
  /// [routeName] and then disposing the previous route once the new route has
  /// finished animating in.
  ///
  /// {@macro flutter.widgets.navigator.pushReplacementNamed}
  ///
  /// {@macro flutter.widgets.navigator.pushNamed.arguments}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _startBike() {
  ///   navigator.pushReplacementNamed('/jouett/1781');
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  Future<T> pushReplacementNamed<T extends Object, TO extends Object>(
    String routeName, {
    TO result,
    Object arguments,
  }) {
    return pushReplacement<T, TO>(_routeNamed<T>(routeName, arguments: arguments), result: result);
  }

  /// Pop the current route off the navigator and push a named route in its
  /// place.
  ///
  /// {@macro flutter.widgets.navigator.popAndPushNamed}
  ///
  /// {@macro flutter.widgets.navigator.pushNamed.arguments}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _begin() {
  ///   navigator.popAndPushNamed('/nyc/1776');
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  Future<T> popAndPushNamed<T extends Object, TO extends Object>(
    String routeName, {
    TO result,
    Object arguments,
  }) {
    pop<TO>(result);
    return pushNamed<T>(routeName, arguments: arguments);
  }

  /// Push the route with the given name onto the navigator, and then remove all
  /// the previous routes until the `predicate` returns true.
  ///
  /// {@macro flutter.widgets.navigator.pushNamedAndRemoveUntil}
  ///
  /// {@macro flutter.widgets.navigator.pushNamed.arguments}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _handleOpenCalendar() {
  ///   navigator.pushNamedAndRemoveUntil('/calendar', ModalRoute.withName('/'));
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  Future<T> pushNamedAndRemoveUntil<T extends Object>(
    String newRouteName,
    RoutePredicate predicate, {
    Object arguments,
  }) {
    return pushAndRemoveUntil<T>(_routeNamed<T>(newRouteName, arguments: arguments), predicate);
  }

  /// Push the given route onto the navigator.
  ///
  /// {@macro flutter.widgets.navigator.push}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _openPage() {
  ///   navigator.push(MaterialPageRoute(builder: (BuildContext context) => MyPage()));
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  Future<T> push<T extends Object>(Route<T> route) {
    assert(!_debugLocked);
    assert(() { _debugLocked = true; return true; }());
    assert(route != null);
    assert(route._navigator == null);
    final Route<dynamic> oldRoute = _history.isNotEmpty ? _history.last : null;
    route._navigator = this;
    route.install(_currentOverlayEntry);
    _history.add(route);
    route.didPush();
    route.didChangeNext(null);
    if (oldRoute != null) {
      oldRoute.didChangeNext(route);
      route.didChangePrevious(oldRoute);
    }
    for (NavigatorObserver observer in widget.observers)
      observer.didPush(route, oldRoute);
    assert(() { _debugLocked = false; return true; }());
    _afterNavigation();
    return route.popped;
  }

  void _afterNavigation() {
    if (!kReleaseMode) {
      // This event is used by performance tools that show stats for the
      // time interval since the last navigation event occurred ensuring that
      // stats only reflect the current page.
      // These tools do not need to know exactly what the new route is so no
      // attempt is made to describe the current route as part of the event.
      developer.postEvent('Flutter.Navigation', <String, dynamic>{});
    }
    _cancelActivePointers();
  }

  /// Replace the current route of the navigator by pushing the given route and
  /// then disposing the previous route once the new route has finished
  /// animating in.
  ///
  /// {@macro flutter.widgets.navigator.pushReplacement}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _doOpenPage() {
  ///   navigator.pushReplacement(
  ///       MaterialPageRoute(builder: (BuildContext context) => MyHomePage()));
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  Future<T> pushReplacement<T extends Object, TO extends Object>(Route<T> newRoute, { TO result }) {
    assert(!_debugLocked);
    assert(() { _debugLocked = true; return true; }());
    final Route<dynamic> oldRoute = _history.last;
    assert(oldRoute != null && oldRoute._navigator == this);
    assert(oldRoute.overlayEntries.isNotEmpty);
    assert(newRoute._navigator == null);
    assert(newRoute.overlayEntries.isEmpty);
    final int index = _history.length - 1;
    assert(index >= 0);
    assert(_history.indexOf(oldRoute) == index);
    newRoute._navigator = this;
    newRoute.install(_currentOverlayEntry);
    _history[index] = newRoute;
    newRoute.didPush().whenCompleteOrCancel(() {
      // The old route's exit is not animated. We're assuming that the
      // new route completely obscures the old one.
      if (mounted) {
        oldRoute
          ..didComplete(result ?? oldRoute.currentResult)
          ..dispose();
      }
    });
    newRoute.didChangeNext(null);
    if (index > 0) {
      _history[index - 1].didChangeNext(newRoute);
      newRoute.didChangePrevious(_history[index - 1]);
    }
    for (NavigatorObserver observer in widget.observers)
      observer.didReplace(newRoute: newRoute, oldRoute: oldRoute);
    assert(() { _debugLocked = false; return true; }());
    _afterNavigation();
    return newRoute.popped;
  }

  /// Push the given route onto the navigator, and then remove all the previous
  /// routes until the `predicate` returns true.
  ///
  /// {@macro flutter.widgets.navigator.pushAndRemoveUntil}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _resetAndOpenPage() {
  ///   navigator.pushAndRemoveUntil(
  ///     MaterialPageRoute(builder: (BuildContext context) => MyHomePage()),
  ///     ModalRoute.withName('/'),
  ///   );
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  Future<T> pushAndRemoveUntil<T extends Object>(Route<T> newRoute, RoutePredicate predicate) {
    assert(!_debugLocked);
    assert(() { _debugLocked = true; return true; }());
    final List<Route<dynamic>> removedRoutes = <Route<dynamic>>[];
    while (_history.isNotEmpty && !predicate(_history.last)) {
      final Route<dynamic> removedRoute = _history.removeLast();
      assert(removedRoute != null && removedRoute._navigator == this);
      assert(removedRoute.overlayEntries.isNotEmpty);
      removedRoutes.add(removedRoute);
    }
    assert(newRoute._navigator == null);
    assert(newRoute.overlayEntries.isEmpty);
    final Route<dynamic> oldRoute = _history.isNotEmpty ? _history.last : null;
    newRoute._navigator = this;
    newRoute.install(_currentOverlayEntry);
    _history.add(newRoute);
    newRoute.didPush().whenCompleteOrCancel(() {
      if (mounted) {
        for (Route<dynamic> route in removedRoutes)
          route.dispose();
      }
    });
    newRoute.didChangeNext(null);
    if (oldRoute != null)
      oldRoute.didChangeNext(newRoute);
    for (NavigatorObserver observer in widget.observers) {
      observer.didPush(newRoute, oldRoute);
      for (Route<dynamic> removedRoute in removedRoutes)
        observer.didRemove(removedRoute, oldRoute);
    }
    assert(() { _debugLocked = false; return true; }());
    _afterNavigation();
    return newRoute.popped;
  }

  /// Replaces a route on the navigator with a new route.
  ///
  /// {@macro flutter.widgets.navigator.replace}
  ///
  /// See also:
  ///
  ///  * [replaceRouteBelow], which is the same but identifies the route to be
  ///    removed by reference to the route above it, rather than directly.
  @optionalTypeArgs
  void replace<T extends Object>({ @required Route<dynamic> oldRoute, @required Route<T> newRoute }) {
    assert(!_debugLocked);
    assert(oldRoute != null);
    assert(newRoute != null);
    if (oldRoute == newRoute)
      return;
    assert(() { _debugLocked = true; return true; }());
    assert(oldRoute._navigator == this);
    assert(newRoute._navigator == null);
    assert(oldRoute.overlayEntries.isNotEmpty);
    assert(newRoute.overlayEntries.isEmpty);
    assert(!overlay.debugIsVisible(oldRoute.overlayEntries.last));
    final int index = _history.indexOf(oldRoute);
    assert(index >= 0);
    newRoute._navigator = this;
    newRoute.install(oldRoute.overlayEntries.last);
    _history[index] = newRoute;
    newRoute.didReplace(oldRoute);
    if (index + 1 < _history.length) {
      newRoute.didChangeNext(_history[index + 1]);
      _history[index + 1].didChangePrevious(newRoute);
    } else {
      newRoute.didChangeNext(null);
    }
    if (index > 0) {
      _history[index - 1].didChangeNext(newRoute);
      newRoute.didChangePrevious(_history[index - 1]);
    }
    for (NavigatorObserver observer in widget.observers)
      observer.didReplace(newRoute: newRoute, oldRoute: oldRoute);
    oldRoute.dispose();
    assert(() { _debugLocked = false; return true; }());
  }

  /// Replaces a route on the navigator with a new route. The route to be
  /// replaced is the one below the given `anchorRoute`.
  ///
  /// {@macro flutter.widgets.navigator.replaceRouteBelow}
  ///
  /// See also:
  ///
  ///  * [replace], which is the same but identifies the route to be removed
  ///    directly.
  @optionalTypeArgs
  void replaceRouteBelow<T extends Object>({ @required Route<dynamic> anchorRoute, Route<T> newRoute }) {
    assert(anchorRoute != null);
    assert(anchorRoute._navigator == this);
    assert(_history.indexOf(anchorRoute) > 0);
    replace<T>(oldRoute: _history[_history.indexOf(anchorRoute) - 1], newRoute: newRoute);
  }

  /// Whether the navigator can be popped.
  ///
  /// {@macro flutter.widgets.navigator.canPop}
  ///
  /// See also:
  ///
  ///  * [Route.isFirst], which returns true for routes for which [canPop]
  ///    returns false.
  bool canPop() {
    assert(_history.isNotEmpty);
    return _history.length > 1 || _history[0].willHandlePopInternally;
  }

  /// Returns the value of the current route's [Route.willPop] method for the
  /// navigator.
  ///
  /// {@macro flutter.widgets.navigator.maybePop}
  ///
  /// See also:
  ///
  ///  * [Form], which provides an `onWillPop` callback that enables the form
  ///    to veto a [pop] initiated by the app's back button.
  ///  * [ModalRoute], which provides a `scopedWillPopCallback` that can be used
  ///    to define the route's `willPop` method.
  @optionalTypeArgs
  Future<bool> maybePop<T extends Object>([ T result ]) async {
    final Route<T> route = _history.last;
    assert(route._navigator == this);
    final RoutePopDisposition disposition = await route.willPop();
    if (disposition != RoutePopDisposition.bubble && mounted) {
      if (disposition == RoutePopDisposition.pop)
        pop(result);
      return true;
    }
    return false;
  }

  /// Pop the top-most route off the navigator.
  ///
  /// {@macro flutter.widgets.navigator.pop}
  ///
  /// {@tool sample}
  ///
  /// Typical usage for closing a route is as follows:
  ///
  /// ```dart
  /// void _handleClose() {
  ///   navigator.pop();
  /// }
  /// ```
  /// {@end-tool}
  /// {@tool sample}
  ///
  /// A dialog box might be closed with a result:
  ///
  /// ```dart
  /// void _handleAccept() {
  ///   navigator.pop(true); // dialog returns true
  /// }
  /// ```
  /// {@end-tool}
  @optionalTypeArgs
  bool pop<T extends Object>([ T result ]) {
    assert(!_debugLocked);
    assert(() { _debugLocked = true; return true; }());
    final Route<dynamic> route = _history.last;
    assert(route._navigator == this);
    bool debugPredictedWouldPop;
    assert(() { debugPredictedWouldPop = !route.willHandlePopInternally; return true; }());
    if (route.didPop(result ?? route.currentResult)) {
      assert(debugPredictedWouldPop);
      if (_history.length > 1) {
        _history.removeLast();
        // If route._navigator is null, the route called finalizeRoute from
        // didPop, which means the route has already been disposed and doesn't
        // need to be added to _poppedRoutes for later disposal.
        if (route._navigator != null)
          _poppedRoutes.add(route);
        _history.last.didPopNext(route);
        for (NavigatorObserver observer in widget.observers)
          observer.didPop(route, _history.last);
      } else {
        assert(() { _debugLocked = false; return true; }());
        return false;
      }
    } else {
      assert(!debugPredictedWouldPop);
    }
    assert(() { _debugLocked = false; return true; }());
    _afterNavigation();
    return true;
  }

  /// Calls [pop] repeatedly until the predicate returns true.
  ///
  /// {@macro flutter.widgets.navigator.popUntil}
  ///
  /// {@tool sample}
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// void _doLogout() {
  ///   navigator.popUntil(ModalRoute.withName('/login'));
  /// }
  /// ```
  /// {@end-tool}
  void popUntil(RoutePredicate predicate) {
    while (!predicate(_history.last))
      pop();
  }

  /// Immediately remove `route` from the navigator, and [Route.dispose] it.
  ///
  /// {@macro flutter.widgets.navigator.removeRoute}
  void removeRoute(Route<dynamic> route) {
    assert(route != null);
    assert(!_debugLocked);
    assert(() { _debugLocked = true; return true; }());
    assert(route._navigator == this);
    final int index = _history.indexOf(route);
    assert(index != -1);
    final Route<dynamic> previousRoute = index > 0 ? _history[index - 1] : null;
    final Route<dynamic> nextRoute = (index + 1 < _history.length) ? _history[index + 1] : null;
    _history.removeAt(index);
    previousRoute?.didChangeNext(nextRoute);
    nextRoute?.didChangePrevious(previousRoute);
    for (NavigatorObserver observer in widget.observers)
      observer.didRemove(route, previousRoute);
    route.dispose();
    assert(() { _debugLocked = false; return true; }());
    _afterNavigation();
  }

  /// Immediately remove a route from the navigator, and [Route.dispose] it. The
  /// route to be replaced is the one below the given `anchorRoute`.
  ///
  /// {@macro flutter.widgets.navigator.removeRouteBelow}
  void removeRouteBelow(Route<dynamic> anchorRoute) {
    assert(!_debugLocked);
    assert(() { _debugLocked = true; return true; }());
    assert(anchorRoute._navigator == this);
    final int index = _history.indexOf(anchorRoute) - 1;
    assert(index >= 0);
    final Route<dynamic> targetRoute = _history[index];
    assert(targetRoute._navigator == this);
    assert(targetRoute.overlayEntries.isEmpty || !overlay.debugIsVisible(targetRoute.overlayEntries.last));
    _history.removeAt(index);
    final Route<dynamic> nextRoute = index < _history.length ? _history[index] : null;
    final Route<dynamic> previousRoute = index > 0 ? _history[index - 1] : null;
    if (previousRoute != null)
      previousRoute.didChangeNext(nextRoute);
    if (nextRoute != null)
      nextRoute.didChangePrevious(previousRoute);
    targetRoute.dispose();
    assert(() { _debugLocked = false; return true; }());
  }

  /// Complete the lifecycle for a route that has been popped off the navigator.
  ///
  /// When the navigator pops a route, the navigator retains a reference to the
  /// route in order to call [Route.dispose] if the navigator itself is removed
  /// from the tree. When the route is finished with any exit animation, the
  /// route should call this function to complete its lifecycle (e.g., to
  /// receive a call to [Route.dispose]).
  ///
  /// The given `route` must have already received a call to [Route.didPop].
  /// This function may be called directly from [Route.didPop] if [Route.didPop]
  /// will return true.
  void finalizeRoute(Route<dynamic> route) {
    _poppedRoutes.remove(route);
    route.dispose();
  }

  /// Whether a route is currently being manipulated by the user, e.g.
  /// as during an iOS back gesture.
  bool get userGestureInProgress => _userGesturesInProgress > 0;
  int _userGesturesInProgress = 0;

  /// The navigator is being controlled by a user gesture.
  ///
  /// For example, called when the user beings an iOS back gesture.
  ///
  /// When the gesture finishes, call [didStopUserGesture].
  void didStartUserGesture() {
    _userGesturesInProgress += 1;
    if (_userGesturesInProgress == 1) {
      final Route<dynamic> route = _history.last;
      final Route<dynamic> previousRoute = !route.willHandlePopInternally && _history.length > 1
          ? _history[_history.length - 2]
          : null;
      // Don't operate the _history list since the gesture may be cancelled.
      // In case of a back swipe, the gesture controller will call .pop() itself.

      for (NavigatorObserver observer in widget.observers)
        observer.didStartUserGesture(route, previousRoute);
    }
  }

  /// A user gesture completed.
  ///
  /// Notifies the navigator that a gesture regarding which the navigator was
  /// previously notified with [didStartUserGesture] has completed.
  void didStopUserGesture() {
    assert(_userGesturesInProgress > 0);
    _userGesturesInProgress -= 1;
    if (_userGesturesInProgress == 0) {
      for (NavigatorObserver observer in widget.observers)
        observer.didStopUserGesture();
    }
  }

  final Set<int> _activePointers = Set<int>();

  void _handlePointerDown(PointerDownEvent event) {
    _activePointers.add(event.pointer);
  }

  void _handlePointerUpOrCancel(PointerEvent event) {
    _activePointers.remove(event.pointer);
  }

  void _cancelActivePointers() {
    // TODO(abarth): This mechanism is far from perfect. See https://github.com/flutter/flutter/issues/4770
    if (SchedulerBinding.instance.schedulerPhase == SchedulerPhase.idle) {
      // If we're between frames (SchedulerPhase.idle) then absorb any
      // subsequent pointers from this frame. The absorbing flag will be
      // reset in the next frame, see build().
      final RenderAbsorbPointer absorber = _overlayKey.currentContext?.ancestorRenderObjectOfType(const TypeMatcher<RenderAbsorbPointer>());
      setState(() {
        absorber?.absorbing = true;
        // We do this in setState so that we'll reset the absorbing value back
        // to false on the next frame.
      });
    }
    _activePointers.toList().forEach(WidgetsBinding.instance.cancelPointer);
  }

  @override
  Widget build(BuildContext context) {
    assert(!_debugLocked);
    assert(_history.isNotEmpty);
    return Listener(
      onPointerDown: _handlePointerDown,
      onPointerUp: _handlePointerUpOrCancel,
      onPointerCancel: _handlePointerUpOrCancel,
      child: AbsorbPointer(
        absorbing: false, // it's mutated directly by _cancelActivePointers above
        child: FocusScope(
          node: focusScopeNode,
          autofocus: true,
          child: Overlay(
            key: _overlayKey,
            initialEntries: _initialOverlayEntries,
          ),
        ),
      ),
    );
  }
}