Flutter | 数据共享

class ShareDataWidget extends InheritedWidget {
  //需要共享的数据
  final int data;

  ShareDataWidget({@required this.data, Widget child}) : super(child: child);

  //定义一个便捷的方法，方便子树中的 widget获取共享数据
  static ShareDataWidget of(BuildContext context) {
    return context.dependOnInheritedWidgetOfExactType();
  }

  ///该回调决定当 data 发生变化的时候，是否通知子树中依赖 data 的 widget
  @override
  bool updateShouldNotify(covariant ShareDataWidget oldWidget) {
    //返回true：则子树中依赖当前 widget 的 widget 的 didChangeDependencies 会被调用
    return oldWidget.data != data;
  }
}
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class TestShareWidget extends StatefulWidget {
  @override
  _TestShareWidgetState createState() => _TestShareWidgetState();
}

class _TestShareWidgetState extends State<TestShareWidget> {
  @override
  Widget build(BuildContext context) {
    return Text(ShareDataWidget.of(context).data.toString());
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    print('发生改变');
  }
}
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class TestInheritedWidget extends StatefulWidget {
  @override
  _TestInheritedWidgetState createState() => _TestInheritedWidgetState();
}

class _TestInheritedWidgetState extends State<TestInheritedWidget> {
  int count = 0;

  @override
  Widget build(BuildContext context) {
    return Center(
      child: ShareDataWidget(
        data: count,
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Padding(
              padding: const EdgeInsets.only(bottom: 20.0),
              child: TestShareWidget(),
            ),
            RaisedButton(
              child: Text("Increment"),
              //每点击一次，count 自增，然后重新 build，ShareDataWidget 将被更新
              onPressed: () => setState(() => ++count),
            )
          ],
        ),
      ),
    );
  }
}
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class _TestShareWidgetState extends State<TestShareWidget> {
  @override
  Widget build(BuildContext context) {
    // return Text(ShareDataWidget.of(context).data.toString());
    return Text("test");
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    print('发生改变');
  }
}
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//定义一个便捷的方法，方便子树中的 widget获取共享数据
static ShareDataWidget of(BuildContext context) {
  // return context.dependOnInheritedWidgetOfExactType();
    return context.getElementForInheritedWidgetOfExactType<ShareDataWidget>().widget;
}
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@override
InheritedElement? getElementForInheritedWidgetOfExactType<T extends InheritedWidget>() {
  assert(_debugCheckStateIsActiveForAncestorLookup());
  final InheritedElement? ancestor = _inheritedWidgets == null ? null : _inheritedWidgets![T];
  return ancestor;
}
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@override
T? dependOnInheritedWidgetOfExactType<T extends InheritedWidget>({Object? aspect}) {
  assert(_debugCheckStateIsActiveForAncestorLookup());
  final InheritedElement? ancestor = _inheritedWidgets == null ? null : _inheritedWidgets![T];
   //相比与上面的代码，多出的部分
  if (ancestor != null) {
    assert(ancestor is InheritedElement);
    return dependOnInheritedElement(ancestor, aspect: aspect) as T;
  }
  _hadUnsatisfiedDependencies = true;
  return null;
}
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@override
InheritedWidget dependOnInheritedElement(InheritedElement ancestor, { Object? aspect }) {
  assert(ancestor != null);
  _dependencies ??= HashSet<InheritedElement>();
  _dependencies!.add(ancestor);
  ancestor.updateDependencies(this, aspect);
  return ancestor.widget;
}
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///一个通用的 InheritedProvider，保存任何需要跨组件共享的状态
class InheritedProvider<T> extends InheritedWidget {
  ///共享的状态使用泛型
  final T data;

  InheritedProvider({@required this.data, Widget child})
      : super(child: child);

  @override
  bool updateShouldNotify(covariant InheritedWidget oldWidget) {
    ///返回true，表示每次都会调用子孙节点的 didChangeDependencies
    return true;
  }
}
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class ChangeNotifier implements Listenable {
  List listeners=[];
  @override
  void addListener(VoidCallback listener) {
     //添加监听器
     listeners.add(listener);
  }
  @override
  void removeListener(VoidCallback listener) {
    //移除监听器
    listeners.remove(listener);
  }
  
  void notifyListeners() {
    //通知所有监听器，触发监听器回调 
    listeners.forEach((item)=>item());
  }
   
  ... //省略无关代码
}
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class ChangeNotifierProvider<T extends ChangeNotifier> extends StatefulWidget {
  final Widget child;
  final T data;

  ChangeNotifierProvider({Key key, this.child, this.data});

  @override
  _ChangeNotifierProviderState<T> createState() =>
      _ChangeNotifierProviderState<T>();

  ///定义一个便捷的方法，方便子树中的 widget 获取共享数据
  static T of<T>(BuildContext context) {
    final provider =
        context.dependOnInheritedWidgetOfExactType<InheritedProvider<T>>();
    return provider.data;
  }
}

class _ChangeNotifierProviderState<T extends ChangeNotifier>
    extends State<ChangeNotifierProvider<T>> {

  void update() {
    setState(() {});
  }

  @override
  void initState() {
    //给 model 添加监听器
    widget.data.addListener(update);
    super.initState();
  }

  @override
  void dispose() {
    //移除 model 的监听器
    super.dispose();
  }

  @override
  void didUpdateWidget(covariant ChangeNotifierProvider<T> oldWidget) {
    //当 Provider 更新时，如果旧数据不 ==，则解绑旧数据的监听，同时添加新数据的监听
    if (widget.data != oldWidget.data) {
      oldWidget.data.removeListener(update);
      widget.data.addListener(update);
    }
    super.didUpdateWidget(oldWidget);
  }

  @override
  Widget build(BuildContext context) {
    return InheritedProvider<T>(
      data: widget.data,
      child: widget.child,
    );
  }
}
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///Item类，用于表示商品的信息
class Item {
  final price;
  int count;

  Item(this.price, this.count);
}

class CarMode extends ChangeNotifier {
  //用户保存购物车中商品列表
  final List<Item> _items = [];

  //禁止修改购物车里的商品信息
  UnmodifiableListView get items => UnmodifiableListView(_items);

  //购物车商品的总价
  double get totalPrice =>
      _items.fold(0, (value, item) => value + item.count * item.price);

  void add(Item item) {
    _items.add(item);
    //通知监听器(观察者)，重新构建 InheritedProvider，更新状态
    notifyListeners();
  }
}

class ProviderTest extends StatefulWidget {
  @override
  _ProviderTestState createState() => _ProviderTestState();
}

class _ProviderTestState extends State<ProviderTest> {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: ChangeNotifierProvider(
        data: CarMode(),
        child: Builder(
          builder: (context) {
            return Column(
              children: [
                Builder(builder: (context) {
                  var cart = ChangeNotifierProvider.of<CarMode>(context);
                  return Text("总价 ：${cart.totalPrice}");
                }),
                Builder(
                  builder: (context) {
                    return RaisedButton(
                      child: Text("添加商品"),
                      onPressed: () {
                        ChangeNotifierProvider.of<CarMode>(context)
                            .add(Item(20, 1));
                      },
                    );
                  },
                )
              ],
            );
          },
        ),
      ),
    );
  }
}
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Builder(builder: (context) {
  var cart = ChangeNotifierProvider.of<CarMode>(context);
  return Text("总价 ：${cart.totalPrice}");
}),
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class Consumer<T> extends StatelessWidget {
  
  final Widget child;
  final Widget Function(BuildContext context, T value) builder;

  Consumer({Key key, @required this.builder, this.child});

  @override
  Widget build(BuildContext context) {
    return builder(context, ChangeNotifierProvider.of<T>(context)); //自动获取 model
  }
  
}
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Consumer<CarMode>(
  builder: (context, cart) => Text("总价：${cart.totalPrice}"),
),
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Builder(
  builder: (context) {
    return RaisedButton(
      child: Text("添加商品"),
      onPressed: () {
        ChangeNotifierProvider.of<CarMode>(context)
            .add(Item(20, 1));
      },
    );
  },
)
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///listen：是否建立以来关系
static T of<T>(BuildContext context, {bool listen = true}) {
  final provider = listen
      ? context.dependOnInheritedWidgetOfExactType<InheritedProvider<T>>()
      : context
          .getElementForInheritedWidgetOfExactType<InheritedProvider<T>>()
          ?.widget as InheritedProvider<T>;
  return provider.data;
}
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