数据传递/状态管理 一ValueNotifier

关于 ValueNotifier的注释

/// A [ChangeNotifier] that holds a single value.
///
/// When [value] is replaced with something that is not equal to the old
/// value as evaluated by the equality operator ==, this class notifies its
/// listeners.

翻译过来大致意思是说 ValueNotifier 包含一个 value 值，当值改变的时候，会通知它的监听者，先通过示例来看如何使用。

• 示例很简单，点击按钮修改值，文本显示的值同步更新。

  Jun-16-2019 13-54-17

代码比较简单，创建一个ValueNotifier 对象，在 initState 和 dispose 分别添加和注销数据变化的监听，按钮点击修改ValueNotifier 里面的值会回调监听update ，完成界面的刷新重建。

class ValueNotifierDemo extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return new _ValueNotifierDemoState();
  }
}
class _ValueNotifierDemoState extends State<ValueNotifierDemo> {
  ValueNotifier<int> _valueNotifier = new ValueNotifier(0);

  @override
  void initState() {
    super.initState();
    ///添加数据的变化监听
    _valueNotifier.addListener(update);
  }

  @override
  void dispose() {
    super.dispose();
    ///组件销毁的时候注销监听
    _valueNotifier.removeListener(update);
  }

  void update() {
    print("数据变化监听回调");
    ///刷新重建界面
    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: "ValueNotifierDemo",
      home: new Scaffold(
        body: new Center(
          child: new Text(_valueNotifier.value.toString()),
        ),
        floatingActionButton: new FloatingActionButton(
          onPressed: () {
            ///修改值
            _valueNotifier.value++;
          },
          child: new Icon(Icons.add),
        ),
      ),
    );
  }
}

实现原理通过源码来看，ValueNotifier

/// A [ChangeNotifier] that holds a single value.
///
/// When [value] is replaced with something that is not equal to the old
/// value as evaluated by the equality operator ==, this class notifies its
/// listeners.
class ValueNotifier<T> extends ChangeNotifier implements ValueListenable<T> {
  /// Creates a [ChangeNotifier] that wraps this value.
  ValueNotifier(this._value);

  /// The current value stored in this notifier.
  ///
  /// When the value is replaced with something that is not equal to the old
  /// value as evaluated by the equality operator ==, this class notifies its
  /// listeners.
  @override
  T get value => _value;
  T _value;
  set value(T newValue) {
    if (_value == newValue)
      return;
    _value = newValue;
    notifyListeners();
  }

  @override
  String toString() => '${describeIdentity(this)}($value)';
}

ChangeNotifier

class ChangeNotifier implements Listenable {
  ObserverList<VoidCallback> _listeners = ObserverList<VoidCallback>();
  
    ///...
  
  @override
  void addListener(VoidCallback listener) {
    assert(_debugAssertNotDisposed());
    _listeners.add(listener);
  }

  @override
  void removeListener(VoidCallback listener) {
    assert(_debugAssertNotDisposed());
    _listeners.remove(listener);
  }


  @mustCallSuper
  void dispose() {
    assert(_debugAssertNotDisposed());
    _listeners = null;
  }

  @protected
  @visibleForTesting
  void notifyListeners() {
    assert(_debugAssertNotDisposed());
    if (_listeners != null) {
      final List<VoidCallback> localListeners = List<VoidCallback>.from(_listeners);
      for (VoidCallback listener in localListeners) {
        try {
          if (_listeners.contains(listener))
            listener();
        } catch (exception, stack) {
         ///...
        }
      }
    }
  }
}

和上一篇文章提到的 scoped_model 库的监听原理基本一直，也是使用观察者模式。ValueNotifier 继承与 ChangeNotifier，里面维护了一个_value值，就是我们需要监听的值，在 set value方法里可以看到，当新的值和旧的值不等的时候会调用 notifyListeners。notifyListeners 里面循环遍历所有的监听方法并调用，所以只要修改了值调用了 set_value 就会回调所有的监听。

ChangeNotifier 主要是在组件内部的监听，如果是组件与组件之间可能更多的是使用 scoped_model 、flutter_redux 、provider 等相关的库了。