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// 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.
import 'package:flutter/animation.dart';
import 'package:flutter/foundation.dart';
import 'scroll_context.dart';
import 'scroll_physics.dart';
import 'scroll_position.dart';
import 'scroll_position_with_single_context.dart';
/// Controls a scrollable widget.
/// Scroll controllers are typically stored as member variables in [State]
/// objects and are reused in each []. A single scroll controller can
/// be used to control multiple scrollable widgets, but some operations, such
/// as reading the scroll [offset], require the controller to be used with a
/// single scrollable widget.
/// A scroll controller creates a [ScrollPosition] to manage the state specific
/// to an individual [Scrollable] widget. To use a custom [ScrollPosition],
/// subclass [ScrollController] and override [createScrollPosition].
/// A [ScrollController] is a [Listenable]. It notifies its listeners whenever
/// any of the attached [ScrollPosition]s notify _their_ listeners (i.e.
/// whenever any of them scroll). It does not notify its listeners when the list
/// of attached [ScrollPosition]s changes.
/// Typically used with [ListView], [GridView], [CustomScrollView].
/// See also:
/// * [ListView], [GridView], [CustomScrollView], which can be controlled by a
/// [ScrollController].
/// * [Scrollable], which is the lower-level widget that creates and associates
/// [ScrollPosition] objects with [ScrollController] objects.
/// * [PageController], which is an analogous object for controlling a
/// [PageView].
/// * [ScrollPosition], which manages the scroll offset for an individual
/// scrolling widget.
/// * [ScrollNotification] and [NotificationListener], which can be used to watch
/// the scroll position without using a [ScrollController].
class ScrollController extends ChangeNotifier {
/// Creates a controller for a scrollable widget.
/// The values of `initialScrollOffset` and `keepScrollOffset` must not be null.
double initialScrollOffset = 0.0,
this.keepScrollOffset = true,
}) : _initialScrollOffset = initialScrollOffset;
/// The initial value to use for [offset].
/// New [ScrollPosition] objects that are created and attached to this
/// controller will have their offset initialized to this value
/// if [keepScrollOffset] is false or a scroll offset hasn't been saved yet.
/// Defaults to 0.0.
double get initialScrollOffset => _initialScrollOffset;
final double _initialScrollOffset;
/// Each time a scroll completes, save the current scroll [offset] with
/// [PageStorage] and restore it if this controller's scrollable is recreated.
/// If this property is set to false, the scroll offset is never saved
/// and [initialScrollOffset] is always used to initialize the scroll
/// offset. If true (the default), the initial scroll offset is used the
/// first time the controller's scrollable is created, since there's no
/// scroll offset to restore yet. Subsequently the saved offset is
/// restored and [initialScrollOffset] is ignored.
/// See also:
/// * [PageStorageKey], which should be used when more than one
/// scrollable appears in the same route, to distinguish the [PageStorage]
/// locations used to save scroll offsets.
final bool keepScrollOffset;
/// A label that is used in the [toString] output. Intended to aid with
/// identifying scroll controller instances in debug output.
final String? debugLabel;
/// The currently attached positions.
/// This should not be mutated directly. [ScrollPosition] objects can be added
/// and removed using [attach] and [detach].
Iterable<ScrollPosition> get positions => _positions;
final List<ScrollPosition> _positions = <ScrollPosition>[];
/// Whether any [ScrollPosition] objects have attached themselves to the
/// [ScrollController] using the [attach] method.
/// If this is false, then members that interact with the [ScrollPosition],
/// such as [position], [offset], [animateTo], and [jumpTo], must not be
/// called.
bool get hasClients => _positions.isNotEmpty;
/// Returns the attached [ScrollPosition], from which the actual scroll offset
/// of the [ScrollView] can be obtained.
/// Calling this is only valid when only a single position is attached.
ScrollPosition get position {
assert(_positions.isNotEmpty, 'ScrollController not attached to any scroll views.');
assert(_positions.length == 1, 'ScrollController attached to multiple scroll views.');
return _positions.single;
/// The current scroll offset of the scrollable widget.
/// Requires the controller to be controlling exactly one scrollable widget.
double get offset => position.pixels;
/// Animates the position from its current value to the given value.
/// Any active animation is canceled. If the user is currently scrolling, that
/// action is canceled.
/// The returned [Future] will complete when the animation ends, whether it
/// completed successfully or whether it was interrupted prematurely.
/// An animation will be interrupted whenever the user attempts to scroll
/// manually, or whenever another activity is started, or whenever the
/// animation reaches the edge of the viewport and attempts to overscroll. (If
/// the [ScrollPosition] does not overscroll but instead allows scrolling
/// beyond the extents, then going beyond the extents will not interrupt the
/// animation.)
/// The animation is indifferent to changes to the viewport or content
/// dimensions.
/// Once the animation has completed, the scroll position will attempt to
/// begin a ballistic activity in case its value is not stable (for example,
/// if it is scrolled beyond the extents and in that situation the scroll
/// position would normally bounce back).
/// The duration must not be zero. To jump to a particular value without an
/// animation, use [jumpTo].
/// When calling [animateTo] in widget tests, `await`ing the returned
/// [Future] may cause the test to hang and timeout. Instead, use
/// [WidgetTester.pumpAndSettle].
Future<void> animateTo(
double offset, {
required Duration duration,
required Curve curve,
}) async {
assert(_positions.isNotEmpty, 'ScrollController not attached to any scroll views.');
await Future.wait<void>(<Future<void>>[
for (int i = 0; i < _positions.length; i += 1) _positions[i].animateTo(offset, duration: duration, curve: curve),
/// Jumps the scroll position from its current value to the given value,
/// without animation, and without checking if the new value is in range.
/// Any active animation is canceled. If the user is currently scrolling, that
/// action is canceled.
/// If this method changes the scroll position, a sequence of start/update/end
/// scroll notifications will be dispatched. No overscroll notifications can
/// be generated by this method.
/// Immediately after the jump, a ballistic activity is started, in case the
/// value was out of range.
void jumpTo(double value) {
assert(_positions.isNotEmpty, 'ScrollController not attached to any scroll views.');
for (final ScrollPosition position in List<ScrollPosition>.of(_positions)) {
/// Register the given position with this controller.
/// After this function returns, the [animateTo] and [jumpTo] methods on this
/// controller will manipulate the given position.
void attach(ScrollPosition position) {
/// Unregister the given position with this controller.
/// After this function returns, the [animateTo] and [jumpTo] methods on this
/// controller will not manipulate the given position.
void detach(ScrollPosition position) {
void dispose() {
for (final ScrollPosition position in _positions) {
/// Creates a [ScrollPosition] for use by a [Scrollable] widget.
/// Subclasses can override this function to customize the [ScrollPosition]
/// used by the scrollable widgets they control. For example, [PageController]
/// overrides this function to return a page-oriented scroll position
/// subclass that keeps the same page visible when the scrollable widget
/// resizes.
/// By default, returns a [ScrollPositionWithSingleContext].
/// The arguments are generally passed to the [ScrollPosition] being created:
/// * `physics`: An instance of [ScrollPhysics] that determines how the
/// [ScrollPosition] should react to user interactions, how it should
/// simulate scrolling when released or flung, etc. The value will not be
/// null. It typically comes from the [ScrollView] or other widget that
/// creates the [Scrollable], or, if none was provided, from the ambient
/// [ScrollConfiguration].
/// * `context`: A [ScrollContext] used for communicating with the object
/// that is to own the [ScrollPosition] (typically, this is the
/// [Scrollable] itself).
/// * `oldPosition`: If this is not the first time a [ScrollPosition] has
/// been created for this [Scrollable], this will be the previous instance.
/// This is used when the environment has changed and the [Scrollable]
/// needs to recreate the [ScrollPosition] object. It is null the first
/// time the [ScrollPosition] is created.
ScrollPosition createScrollPosition(
ScrollPhysics physics,
ScrollContext context,
ScrollPosition? oldPosition,
) {
return ScrollPositionWithSingleContext(
physics: physics,
context: context,
initialPixels: initialScrollOffset,
keepScrollOffset: keepScrollOffset,
oldPosition: oldPosition,
debugLabel: debugLabel,
String toString() {
final List<String> description = <String>[];
return '${describeIdentity(this)}(${description.join(", ")})';
/// Add additional information to the given description for use by [toString].
/// This method makes it easier for subclasses to coordinate to provide a
/// high-quality [toString] implementation. The [toString] implementation on
/// the [ScrollController] base class calls [debugFillDescription] to collect
/// useful information from subclasses to incorporate into its return value.
/// Implementations of this method should start with a call to the inherited
/// method, as in `super.debugFillDescription(description)`.
void debugFillDescription(List<String> description) {
if (debugLabel != null) {
if (initialScrollOffset != 0.0) {
description.add('initialScrollOffset: ${initialScrollOffset.toStringAsFixed(1)}, ');
if (_positions.isEmpty) {
description.add('no clients');
} else if (_positions.length == 1) {
// Don't actually list the client itself, since its toString may refer to us.
description.add('one client, offset ${offset.toStringAsFixed(1)}');
} else {
description.add('${_positions.length} clients');
// Examples can assume:
// TrackingScrollController? _trackingScrollController;
/// A [ScrollController] whose [initialScrollOffset] tracks its most recently
/// updated [ScrollPosition].
/// This class can be used to synchronize the scroll offset of two or more
/// lazily created scroll views that share a single [TrackingScrollController].
/// It tracks the most recently updated scroll position and reports it as its
/// `initialScrollOffset`.
/// {@tool snippet}
/// In this example each [PageView] page contains a [ListView] and all three
/// [ListView]'s share a [TrackingScrollController]. The scroll offsets of all
/// three list views will track each other, to the extent that's possible given
/// the different list lengths.
/// ```dart
/// PageView(
/// children: <Widget>[
/// ListView(
/// controller: _trackingScrollController,
/// children: List<Widget>.generate(100, (int i) => Text('page 0 item $i')).toList(),
/// ),
/// ListView(
/// controller: _trackingScrollController,
/// children: List<Widget>.generate(200, (int i) => Text('page 1 item $i')).toList(),
/// ),
/// ListView(
/// controller: _trackingScrollController,
/// children: List<Widget>.generate(300, (int i) => Text('page 2 item $i')).toList(),
/// ),
/// ],
/// )
/// ```
/// {@end-tool}
/// In this example the `_trackingController` would have been created by the
/// stateful widget that built the widget tree.
class TrackingScrollController extends ScrollController {
/// Creates a scroll controller that continually updates its
/// [initialScrollOffset] to match the last scroll notification it received.
final Map<ScrollPosition, VoidCallback> _positionToListener = <ScrollPosition, VoidCallback>{};
ScrollPosition? _lastUpdated;
double? _lastUpdatedOffset;
/// The last [ScrollPosition] to change. Returns null if there aren't any
/// attached scroll positions, or there hasn't been any scrolling yet, or the
/// last [ScrollPosition] to change has since been removed.
ScrollPosition? get mostRecentlyUpdatedPosition => _lastUpdated;
/// Returns the scroll offset of the [mostRecentlyUpdatedPosition] or, if that
/// is null, the initial scroll offset provided to the constructor.
/// See also:
/// * [ScrollController.initialScrollOffset], which this overrides.
double get initialScrollOffset => _lastUpdatedOffset ?? super.initialScrollOffset;
void attach(ScrollPosition position) {
_positionToListener[position] = () {
_lastUpdated = position;
_lastUpdatedOffset = position.pixels;
void detach(ScrollPosition position) {
if (_lastUpdated == position) {
_lastUpdated = null;
if (_positionToListener.isEmpty) {
_lastUpdatedOffset = null;
void dispose() {
for (final ScrollPosition position in positions) {