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

// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// @dart = 2.8

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

import 'framework.dart';
import 'overscroll_indicator.dart';
import 'scroll_physics.dart';

const Color _kDefaultGlowColor = Color(0xFFFFFFFF);

/// Describes how [Scrollable] widgets should behave.
///
/// Used by [ScrollConfiguration] to configure the [Scrollable] widgets in a
/// subtree.
@immutable
class ScrollBehavior {
  /// Creates a description of how [Scrollable] widgets should behave.
  const ScrollBehavior();

  /// The platform whose scroll physics should be implemented.
  ///
  /// Defaults to the current platform.
  TargetPlatform getPlatform(BuildContext context) => defaultTargetPlatform;

  /// Wraps the given widget, which scrolls in the given [AxisDirection].
  ///
  /// For example, on Android, this method wraps the given widget with a
  /// [GlowingOverscrollIndicator] to provide visual feedback when the user
  /// overscrolls.
  Widget buildViewportChrome(BuildContext context, Widget child, AxisDirection axisDirection) {
    // When modifying this function, consider modifying the implementation in
    // _MaterialScrollBehavior as well.
    switch (getPlatform(context)) {
      case TargetPlatform.iOS:
      case TargetPlatform.linux:
      case TargetPlatform.macOS:
      case TargetPlatform.windows:
        return child;
      case TargetPlatform.android:
      case TargetPlatform.fuchsia:
        return GlowingOverscrollIndicator(
          child: child,
          axisDirection: axisDirection,
          color: _kDefaultGlowColor,
        );
    }
    return null;
  }

  /// Specifies the type of velocity tracker to use in the descendant
  /// [Scrollable]s' drag gesture recognizers, for estimating the velocity of a
  /// drag gesture.
  ///
  /// This can be used to, for example, apply different fling velocity
  /// estimation methods on different platforms, in order to match the
  /// platform's native behavior.
  ///
  /// Typically, the provided [GestureVelocityTrackerBuilder] should return a
  /// fresh velocity tracker. If null is returned, [Scrollable] creates a new
  /// [VelocityTracker] to track the newly added pointer that may develop into
  /// a drag gesture.
  ///
  /// The default implementation provides a new
  /// [IOSScrollViewFlingVelocityTracker] on iOS and macOS for each new pointer,
  /// and a new [VelocityTracker] on other platforms for each new pointer.
  GestureVelocityTrackerBuilder velocityTrackerBuilder(BuildContext context) {
    switch (getPlatform(context)) {
      case TargetPlatform.iOS:
      case TargetPlatform.macOS:
        return (PointerEvent event) => IOSScrollViewFlingVelocityTracker(event.kind);
      case TargetPlatform.android:
      case TargetPlatform.fuchsia:
      case TargetPlatform.linux:
      case TargetPlatform.windows:
        return (PointerEvent event) => VelocityTracker.withKind(event.kind);
    }
    assert(false);
    return (PointerEvent event) => VelocityTracker(event.kind);
  }

  static const ScrollPhysics _bouncingPhysics = BouncingScrollPhysics(parent: RangeMaintainingScrollPhysics());
  static const ScrollPhysics _clampingPhysics = ClampingScrollPhysics(parent: RangeMaintainingScrollPhysics());

  /// The scroll physics to use for the platform given by [getPlatform].
  ///
  /// Defaults to [RangeMaintainingScrollPhysics] mixed with
  /// [BouncingScrollPhysics] on iOS and [ClampingScrollPhysics] on
  /// Android.
  ScrollPhysics getScrollPhysics(BuildContext context) {
    switch (getPlatform(context)) {
      case TargetPlatform.iOS:
      case TargetPlatform.macOS:
        return _bouncingPhysics;
      case TargetPlatform.android:
      case TargetPlatform.fuchsia:
      case TargetPlatform.linux:
      case TargetPlatform.windows:
        return _clampingPhysics;
    }
    return null;
  }

  /// Called whenever a [ScrollConfiguration] is rebuilt with a new
  /// [ScrollBehavior] of the same [runtimeType].
  ///
  /// If the new instance represents different information than the old
  /// instance, then the method should return true, otherwise it should return
  /// false.
  ///
  /// If this method returns true, all the widgets that inherit from the
  /// [ScrollConfiguration] will rebuild using the new [ScrollBehavior]. If this
  /// method returns false, the rebuilds might be optimized away.
  bool shouldNotify(covariant ScrollBehavior oldDelegate) => false;

  @override
  String toString() => objectRuntimeType(this, 'ScrollBehavior');
}

/// Controls how [Scrollable] widgets behave in a subtree.
///
/// The scroll configuration determines the [ScrollPhysics] and viewport
/// decorations used by descendants of [child].
class ScrollConfiguration extends InheritedWidget {
  /// Creates a widget that controls how [Scrollable] widgets behave in a subtree.
  ///
  /// The [behavior] and [child] arguments must not be null.
  const ScrollConfiguration({
    Key key,
    @required this.behavior,
    @required Widget child,
  }) : super(key: key, child: child);

  /// How [Scrollable] widgets that are descendants of [child] should behave.
  final ScrollBehavior behavior;

  /// The [ScrollBehavior] for [Scrollable] widgets in the given [BuildContext].
  ///
  /// If no [ScrollConfiguration] widget is in scope of the given `context`,
  /// a default [ScrollBehavior] instance is returned.
  static ScrollBehavior of(BuildContext context) {
    final ScrollConfiguration configuration = context.dependOnInheritedWidgetOfExactType<ScrollConfiguration>();
    return configuration?.behavior ?? const ScrollBehavior();
  }

  @override
  bool updateShouldNotify(ScrollConfiguration oldWidget) {
    assert(behavior != null);
    return behavior.runtimeType != oldWidget.behavior.runtimeType
        || (behavior != oldWidget.behavior && behavior.shouldNotify(oldWidget.behavior));
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<ScrollBehavior>('behavior', behavior));
  }
}