flutter - 如何在 InheritedWidget 中调用 setState 或更新值？

Question

可重现代码:

void main() => runApp(MaterialApp(home: CountInheritedWidget(child: HomePage())));

class CountInheritedWidget extends InheritedWidget {
  CountInheritedWidget({Widget child}) : super(child: child);

  final Map<String, int> _map = {"count": 0};

  // getter
  int get value => _map["count"];

  // setter
  set value(int x) => _map["count"] = x; // is there anything like setState here?

  @override
  bool updateShouldNotify(CountInheritedWidget oldCounter) => true;

  static CountInheritedWidget of(BuildContext context) => context.dependOnInheritedWidgetOfExactType<CountInheritedWidget>();
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            TextWidget(),
            ButtonWidget(),
          ],
        ),
      ),
    );
  }
}

class TextWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    int count = CountInheritedWidget.of(context)?.value ?? -1;
    return Text("Count = $count");
  }
}

class ButtonWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return RaisedButton(
      child: Text("Increment"),
      onPressed: () {
        CountInheritedWidget counter = CountInheritedWidget.of(context);
        int count = counter?.value ?? -1;
        counter.value = ++count;
      },
    );
  }
}

我想更新 count 的值来自 ButtonWidget ，我确定它会在 CounterInheritedWidget 中得到更新类，但它没有反射(reflect)在屏幕上。怎么打电话 setState或来自 InheritedWidget 的类似内容?

任何帮助将不胜感激，我是 Flutter 和 Dart 的新手，因此很难解决此类问题。谢谢你，祝你有美好的一天。

注:我不是在寻找像 Provider 这样的插件, ScopedModel , Redux对于这种工作。

Answer 1

InheritedWidgets 不能这样做。它们是完全不可变的，没有触发更新的机制。

如果要发出更新，则必须将 InheritedWidget 与 StatefulWidget 结合起来，通常以这种方式完成:

class MyWidget extends StatefulWidget {
  const MyWidget({Key key, this.child}) : super(key: key);
  final Widget child;

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

class MyState extends State<MyWidget> {
  String name;
  int age;

  @override
  Widget build(BuildContext context) {
    return MyInherited(
      name: name,
      age: age,
      child: widget.child,
    );
  }
}

哪里 MyInheritedWidget是:

class MyInherited extends InheritedWidget {
  MyInherited({
    Key key,
    this.name,
    this.age,
    Widget child,
  }) : super(key: key, child: child);

  final String name;
  final int age;

  @override
  bool updateShouldNotify(MyInherited oldWidget) {
    return name != oldWidget.name && age != oldWidget.age;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(IntProperty('age', age));
    properties.add(StringProperty('name', name));
  }
}

是的。那是冗长的。这就是为什么 provider存在。