tab_controller.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.

import 'dart:math' as math;

import 'package:flutter/widgets.dart';

import 'constants.dart';

// Examples can assume:
// BuildContext context;

/// Coordinates tab selection between a [TabBar] and a [TabBarView].
///
/// The [index] property is the index of the selected tab and the [animation]
/// represents the current scroll positions of the tab bar and the tab bar view.
/// The selected tab's index can be changed with [animateTo].
///
/// A stateful widget that builds a [TabBar] or a [TabBarView] can create
/// a [TabController] and share it directly.
///
/// When the [TabBar] and [TabBarView] don't have a convenient stateful
/// ancestor, a [TabController] can be shared by providing a
/// [DefaultTabController] inherited widget.
///
/// {@animation 700 540 https://flutter.github.io/assets-for-api-docs/assets/material/tabs.mp4}
///
/// {@tool sample}
///
/// This widget introduces a [Scaffold] with an [AppBar] and a [TabBar].
///
/// ```dart
/// class MyTabbedPage extends StatefulWidget {
///   const MyTabbedPage({ Key key }) : super(key: key);
///   @override
///   _MyTabbedPageState createState() => _MyTabbedPageState();
/// }
///
/// class _MyTabbedPageState extends State<MyTabbedPage> with SingleTickerProviderStateMixin {
///   final List<Tab> myTabs = <Tab>[
///     Tab(text: 'LEFT'),
///     Tab(text: 'RIGHT'),
///   ];
///
///   TabController _tabController;
///
///   @override
///   void initState() {
///     super.initState();
///     _tabController = TabController(vsync: this, length: myTabs.length);
///   }
///
///  @override
///  void dispose() {
///    _tabController.dispose();
///    super.dispose();
///  }
///
///   @override
///   Widget build(BuildContext context) {
///     return Scaffold(
///       appBar: AppBar(
///         bottom: TabBar(
///           controller: _tabController,
///           tabs: myTabs,
///         ),
///       ),
///       body: TabBarView(
///         controller: _tabController,
///         children: myTabs.map((Tab tab) {
///           final String label = tab.text.toLowerCase();
///           return Center(
///             child: Text(
///               'This is the $label tab',
///               style: const TextStyle(fontSize: 36),
///             ),
///           );
///         }).toList(),
///       ),
///     );
///   }
/// }
/// ```
/// {@end-tool}
class TabController extends ChangeNotifier {
  /// Creates an object that manages the state required by [TabBar] and a
  /// [TabBarView].
  ///
  /// The [length] must not be null or negative. Typically it's a value greater
  /// than one, i.e. typically there are two or more tabs. The [length] must
  /// match [TabBar.tabs]'s and [TabBarView.children]'s length.
  ///
  /// The `initialIndex` must be valid given [length] and must not be null. If
  /// [length] is zero, then `initialIndex` must be 0 (the default).
  TabController({ int initialIndex = 0, @required this.length, @required TickerProvider vsync })
    : assert(length != null && length >= 0),
      assert(initialIndex != null && initialIndex >= 0 && (length == 0 || initialIndex < length)),
      _index = initialIndex,
      _previousIndex = initialIndex,
      _animationController = AnimationController.unbounded(
        value: initialIndex.toDouble(),
        vsync: vsync,
      );

  // Private constructor used by `_copyWith`. This allows a new TabController to
  // be created without having to create a new animationController.
  TabController._({
    int index,
    int previousIndex,
    AnimationController animationController,
    @required this.length,
  }) : _index = index,
       _previousIndex = previousIndex,
       _animationController = animationController;


  /// Creates a new [TabController] with `index`, `previousIndex`, and `length`
  /// if they are non-null.
  ///
  /// This method is used by [DefaultTabController].
  ///
  /// When [DefaultTabController.length] is updated, this method is called to
  /// create a new [TabController] without creating a new [AnimationController].
  TabController _copyWith({ int index, int length, int previousIndex }) {
    return TabController._(
      index: index ?? _index,
      length: length ?? this.length,
      animationController: _animationController,
      previousIndex: previousIndex ?? _previousIndex,
    );
  }

  /// An animation whose value represents the current position of the [TabBar]'s
  /// selected tab indicator as well as the scrollOffsets of the [TabBar]
  /// and [TabBarView].
  ///
  /// The animation's value ranges from 0.0 to [length] - 1.0. After the
  /// selected tab is changed, the animation's value equals [index]. The
  /// animation's value can be [offset] by +/- 1.0 to reflect [TabBarView]
  /// drag scrolling.
  ///
  /// If this [TabController] was disposed, then return null.
  Animation<double> get animation => _animationController?.view;
  AnimationController _animationController;

  /// The total number of tabs.
  ///
  /// Typically greater than one. Must match [TabBar.tabs]'s and
  /// [TabBarView.children]'s length.
  final int length;

  void _changeIndex(int value, { Duration duration, Curve curve }) {
    assert(value != null);
    assert(value >= 0 && (value < length || length == 0));
    assert(duration != null || curve == null);
    assert(_indexIsChangingCount >= 0);
    if (value == _index || length < 2)
      return;
    _previousIndex = index;
    _index = value;
    if (duration != null) {
      _indexIsChangingCount += 1;
      notifyListeners(); // Because the value of indexIsChanging may have changed.
      _animationController
        .animateTo(_index.toDouble(), duration: duration, curve: curve)
        .whenCompleteOrCancel(() {
          _indexIsChangingCount -= 1;
          notifyListeners();
        });
    } else {
      _indexIsChangingCount += 1;
      _animationController.value = _index.toDouble();
      _indexIsChangingCount -= 1;
      notifyListeners();
    }
  }

  /// The index of the currently selected tab.
  ///
  /// Changing the index also updates [previousIndex], sets the [animation]'s
  /// value to index, resets [indexIsChanging] to false, and notifies listeners.
  ///
  /// To change the currently selected tab and play the [animation] use [animateTo].
  ///
  /// The value of [index] must be valid given [length]. If [length] is zero,
  /// then [index] will also be zero.
  int get index => _index;
  int _index;
  set index(int value) {
    _changeIndex(value);
  }

  /// The index of the previously selected tab.
  ///
  /// Initially the same as [index].
  int get previousIndex => _previousIndex;
  int _previousIndex;

  /// True while we're animating from [previousIndex] to [index] as a
  /// consequence of calling [animateTo].
  ///
  /// This value is true during the [animateTo] animation that's triggered when
  /// the user taps a [TabBar] tab. It is false when [offset] is changing as a
  /// consequence of the user dragging (and "flinging") the [TabBarView].
  bool get indexIsChanging => _indexIsChangingCount != 0;
  int _indexIsChangingCount = 0;

  /// Immediately sets [index] and [previousIndex] and then plays the
  /// [animation] from its current value to [index].
  ///
  /// While the animation is running [indexIsChanging] is true. When the
  /// animation completes [offset] will be 0.0.
  void animateTo(int value, { Duration duration = kTabScrollDuration, Curve curve = Curves.ease }) {
    _changeIndex(value, duration: duration, curve: curve);
  }

  /// The difference between the [animation]'s value and [index].
  ///
  /// The offset value must be between -1.0 and 1.0.
  ///
  /// This property is typically set by the [TabBarView] when the user
  /// drags left or right. A value between -1.0 and 0.0 implies that the
  /// TabBarView has been dragged to the left. Similarly a value between
  /// 0.0 and 1.0 implies that the TabBarView has been dragged to the right.
  double get offset => _animationController.value - _index.toDouble();
  set offset(double value) {
    assert(value != null);
    assert(value >= -1.0 && value <= 1.0);
    assert(!indexIsChanging);
    if (value == offset)
      return;
    _animationController.value = value + _index.toDouble();
  }

  @override
  void dispose() {
    _animationController?.dispose();
    _animationController = null;
    super.dispose();
  }
}

class _TabControllerScope extends InheritedWidget {
  const _TabControllerScope({
    Key key,
    this.controller,
    this.enabled,
    Widget child,
  }) : super(key: key, child: child);

  final TabController controller;
  final bool enabled;

  @override
  bool updateShouldNotify(_TabControllerScope old) {
    return enabled != old.enabled || controller != old.controller;
  }
}

/// The [TabController] for descendant widgets that don't specify one
/// explicitly.
///
/// [DefaultTabController] is an inherited widget that is used to share a
/// [TabController] with a [TabBar] or a [TabBarView]. It's used when sharing an
/// explicitly created [TabController] isn't convenient because the tab bar
/// widgets are created by a stateless parent widget or by different parent
/// widgets.
///
/// {@animation 700 540 https://flutter.github.io/assets-for-api-docs/assets/material/tabs.mp4}
///
/// ```dart
/// class MyDemo extends StatelessWidget {
///   final List<Tab> myTabs = <Tab>[
///     Tab(text: 'LEFT'),
///     Tab(text: 'RIGHT'),
///   ];
///
///   @override
///   Widget build(BuildContext context) {
///     return DefaultTabController(
///       length: myTabs.length,
///       child: Scaffold(
///         appBar: AppBar(
///           bottom: TabBar(
///             tabs: myTabs,
///           ),
///         ),
///         body: TabBarView(
///           children: myTabs.map((Tab tab) {
///             final String label = tab.text.toLowerCase();
///             return Center(
///               child: Text(
///                 'This is the $label tab',
///                 style: const TextStyle(fontSize: 36),
///               ),
///             );
///           }).toList(),
///         ),
///       ),
///     );
///   }
/// }
/// ```
class DefaultTabController extends StatefulWidget {
  /// Creates a default tab controller for the given [child] widget.
  ///
  /// The [length] argument is typically greater than one. The [length] must
  /// match [TabBar.tabs]'s and [TabBarView.children]'s length.
  ///
  /// The [initialIndex] argument must not be null.
  const DefaultTabController({
    Key key,
    @required this.length,
    this.initialIndex = 0,
    @required this.child,
  }) : assert(initialIndex != null),
       assert(length >= 0),
       assert(length == 0 || (initialIndex >= 0 && initialIndex < length)),
       super(key: key);

  /// The total number of tabs.
  ///
  /// Typically greater than one. Must match [TabBar.tabs]'s and
  /// [TabBarView.children]'s length.
  final int length;

  /// The initial index of the selected tab.
  ///
  /// Defaults to zero.
  final int initialIndex;

  /// The widget below this widget in the tree.
  ///
  /// Typically a [Scaffold] whose [AppBar] includes a [TabBar].
  ///
  /// {@macro flutter.widgets.child}
  final Widget child;

  /// The closest instance of this class that encloses the given context.
  ///
  /// {@tool sample}
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// TabController controller = DefaultTabController.of(context);
  /// ```
  /// {@end-tool}
  static TabController of(BuildContext context) {
    final _TabControllerScope scope = context.dependOnInheritedWidgetOfExactType<_TabControllerScope>();
    return scope?.controller;
  }

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

class _DefaultTabControllerState extends State<DefaultTabController> with SingleTickerProviderStateMixin {
  TabController _controller;

  @override
  void initState() {
    super.initState();
    _controller = TabController(
      vsync: this,
      length: widget.length,
      initialIndex: widget.initialIndex,
    );
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return _TabControllerScope(
      controller: _controller,
      enabled: TickerMode.of(context),
      child: widget.child,
    );
  }

  @override
  void didUpdateWidget(DefaultTabController oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.length != widget.length) {
      // If the length is shortened while the last tab is selected, we should
      // automatically update the index of the controller to be the new last tab.
      int newIndex;
      int previousIndex = _controller.previousIndex;
      if (_controller.index >= widget.length) {
        newIndex = math.max(0, widget.length - 1);
        previousIndex = _controller.index;
      }
      _controller = _controller._copyWith(
        length: widget.length,
        index: newIndex,
        previousIndex: previousIndex,
      );
    }
  }
}