在flutter的状态管理中,ValueNotifier会被经常使用,一般会和ValueListenableBuilder一起使用.ValueNotifier负责状态,ValueListenableBuilder负责嵌套组件来刷新

ValueNotifier<int> type = ValueNotifier(0);

ValueListenableBuilder(
  valueListenable: type,
  builder: (context, type, child) {
    return Text(type.toString());
  },
);

这篇文章就来深度剖析一下这个组件内部的具体实现原理
首先我们从ValueNotifier入手,它的继承关系

class ValueNotifier<T> extends ChangeNotifier implements ValueListenable<T>

mixin class ChangeNotifier implements Listenable

abstract class ValueListenable<T> extends Listenable

Listenable

最终根源是节点是Listenable,它提供了addListener,removeListener,用来添加监听和移除监听

ChangeNotifier

再来看ChangeNotifier,这个类在我们使用Provider的时候也是会经常用到,并且很多的controller会是它的子类,比如常用的ScrollController,TabController.ChangeNotifier要实现Listenable的方法

void addListener(VoidCallback listener) {
   assert(ChangeNotifier.debugAssertNotDisposed(this));
   if (kFlutterMemoryAllocationsEnabled) {
     maybeDispatchObjectCreation(this);
   }
   if (_count == _listeners.length) {
     if (_count == 0) {
       _listeners = List<VoidCallback?>.filled(1, null);
     } else {
       final List<VoidCallback?> newListeners =
           List<VoidCallback?>.filled(_listeners.length * 2, null);
       for (int i = 0; i < _count; i++) {
         newListeners[i] = _listeners[i];
       }
       _listeners = newListeners;
     }
   }
   _listeners[_count++] = listener;
}

@override
  void removeListener(VoidCallback listener) {
    for (int i = 0; i < _count; i++) {
      final VoidCallback? listenerAtIndex = _listeners[i];
      if (listenerAtIndex == listener) {
        if (_notificationCallStackDepth > 0) {
          _listeners[i] = null;
          _reentrantlyRemovedListeners++;
        } else {
          _removeAt(i);
        }
        break;
      }
    }
  }

ChangeNotifier内部提供_listeners,用来存放监听的回调,来实现上面两个函数,主要作用是来操作_listeners添加删除回调函数
ChangeNotifier特色函数notifyListeners

void notifyListeners() {
    assert(ChangeNotifier.debugAssertNotDisposed(this));
    if (_count == 0) {
      return;
    }
    _notificationCallStackDepth++;

    final int end = _count;
    for (int i = 0; i < end; i++) {
      try {
        _listeners[i]?.call();
      } catch (exception, stack) {
        FlutterError.reportError(FlutterErrorDetails(
          exception: exception,
          stack: stack,
          library: 'foundation library',
          context: ErrorDescription('while dispatching notifications for $runtimeType'),
          informationCollector: () => <DiagnosticsNode>[
            DiagnosticsProperty<ChangeNotifier>(
              'The $runtimeType sending notification was',
              this,
              style: DiagnosticsTreeStyle.errorProperty,
            ),
          ],
        ));
      }
    }

    _notificationCallStackDepth--;

    if (_notificationCallStackDepth == 0 && _reentrantlyRemovedListeners > 0) {
      final int newLength = _count - _reentrantlyRemovedListeners;
      if (newLength * 2 <= _listeners.length) {
        final List<VoidCallback?> newListeners = List<VoidCallback?>.filled(newLength, null);

        int newIndex = 0;
        for (int i = 0; i < _count; i++) {
          final VoidCallback? listener = _listeners[i];
          if (listener != null) {
            newListeners[newIndex++] = listener;
          }
        }

        _listeners = newListeners;
      } else {
        // Otherwise we put all the null references at the end.
        for (int i = 0; i < newLength; i += 1) {
          if (_listeners[i] == null) {
            // We swap this item with the next not null item.
            int swapIndex = i + 1;
            while (_listeners[swapIndex] == null) {
              swapIndex += 1;
            }
            _listeners[i] = _listeners[swapIndex];
            _listeners[swapIndex] = null;
          }
        }
      }

      _reentrantlyRemovedListeners = 0;
      _count = newLength;
    }
  }

大段代码不想看,其实可以只关注

final int end = _count;
for (int i = 0; i < end; i++) {
  _listeners[i]?.call();
}

主要是调用监听的回调函数,剩下的部分是对_listeners的优化,在执行完所有的回调函数后,才对_listeners进行长度变更,对应removeListener,在_notificationCallStackDepth > 0的时候,并没有对数组长度进行优化,而是在回调结束后,才进行的优化
比如只需要监听一次结果,然后在回调中移除了监听,这里如果正好在_listeners长度变化的节点上,需要重新开辟内存,此时可能是出于性能的优化,并不会在回调执行过程中做数组的内存变更,统一在所有回调结束后去操作,还有一种情况是我的回调函数很多,移除的也很多,可能会出现多次开辟内存的情况,而最后统一处理,最多只需要开辟一次内存,这个就属于很细节的操作了
到这里,我们结束了对ChangeNotifier剖析

ValueListenable

abstract class ValueListenable<T> extends Listenable {
  /// Abstract const constructor. This constructor enables subclasses to provide
  /// const constructors so that they can be used in const expressions.
  const ValueListenable();

  /// The current value of the object. When the value changes, the callbacks
  /// registered with [addListener] will be invoked.
  T get value;
}

ValueListenable就是在Listenable基础上添加了get value也是一个基类

ValueNotifier

在讲解完上面的部分后,再来说ValueNotifier就相对容易些了

class ValueNotifier<T> extends ChangeNotifier implements ValueListenable<T> {
  ValueNotifier(this._value);

  @override
  T get value => _value;

  T _value;

  set value(T newValue) {
    if (_value == newValue) {
      return;
    }
    _value = newValue;
    notifyListeners();
  }

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

接口类ValueListenable中的添加删除监听的实现,被ValueNotifier的父类ChangeNotifier实现了,ValueListenable增加了成员变量_value,并且实现了ValueListenable的get value,重点是在set value中
调用了父类notifyListeners(),用来调用监听回调函数,所以我们在最开始的例子中,直接执行type.value++,就可以直接通知到被监听对象了

ValueListenableBuilder

这个组件是Widget组件,用来包裹需要刷新的组件,是一个StatefulWidget

typedef ValueWidgetBuilder<T> = Widget Function(BuildContext context, T value, Widget? child);
class ValueListenableBuilder<T> extends StatefulWidget {
  final ValueListenable<T> valueListenable;
  final ValueWidgetBuilder<T> builder;
  final Widget? child;
}

valueListenable抽象成了ValueListenable,所以所有实现了ValueListenable方法的类都可以使用这个组件来进行刷新
具体看下它的 State _ValueListenableBuilderState

class _ValueListenableBuilderState<T> extends State<ValueListenableBuilder<T>> {
  late T value;

  @override
  void initState() {
    super.initState();
    value = widget.valueListenable.value;
    widget.valueListenable.addListener(_valueChanged);
  }

  @override
  void didUpdateWidget(ValueListenableBuilder<T> oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.valueListenable != widget.valueListenable) {
      oldWidget.valueListenable.removeListener(_valueChanged);
      value = widget.valueListenable.value;
      widget.valueListenable.addListener(_valueChanged);
    }
  }

  @override
  void dispose() {
    widget.valueListenable.removeListener(_valueChanged);
    super.dispose();
  }

  void _valueChanged() {
    setState(() { value = widget.valueListenable.value; });
  }

  @override
  Widget build(BuildContext context) {
    return widget.builder(context, value, widget.child);
  }
}

• initState中添加监听addListener -> _valueChanged
• dispose中移除监听removeListener -> _valueChanged
• _valueChanged,就是简单的setState刷新组件
• didUpdateWidget是在widget重新构建后调用,如果监听对象变了,需要移除之前的监听,并且添加新的监听
  到这里,关于ValueNotifier的内容就结束了,但是我们试想一下,如果一个widget组件需要监听多个valueListenable,如何做到,一种方式是ValueListenableBuilder嵌套,能解决问题,但是不是很优雅
  完全可以根据上边的原理,我们实现一套多监听的组件

final ValueListenable<T> valueListenable 变为 final List<ValueListenable> valueListenables;

将之前的单个valueListenable添加回调变成数组遍历添加回调,移除亦然,关于didUpdateWidget中对数组中的每个元素作比较这个方法很多,可以选择

void initState() {
    super.initState();
    value = widget.valueListenables;
    _addListener(widget);
}

@override
  void dispose() {
    _removeListener(widget);
    super.dispose();
  }

  _addListener(ValueListenableListBuilder widget) {
    for (var element in widget.valueListenables) {
      element.addListener(_valueChanged);
    }
  }

  _removeListener(ValueListenableListBuilder widget) {
    for (var element in widget.valueListenables) {
      element.removeListener(_valueChanged);
    }
  }

 @override
  void didUpdateWidget(ValueListenableListBuilder<T> oldWidget) {
    if (!const DeepCollectionEquality().equals(oldWidget.valueListenables, widget.valueListenables)) {
      _removeListener(oldWidget);
      value = widget.valueListenables;
      _addListener(widget);
    }
    super.didUpdateWidget(oldWidget);
  }

  void _valueChanged() {
    setState(() {
      value = List.of(widget.valueListenables);
    });
  }

  @override
  Widget build(BuildContext context) {
    final result = value.map((e) => e.value).toList();
    return widget.builder(context, result, widget.child);
  }

再进一步优化,上边返回的result是List,元素没有具体的类型,在使用的时候会有一些不便利,所以我们可以再上层,添加一层嵌套,以两个为例,创建一个类来接收这两个ValueListenable

class Tuple<T1, T2> {
  T1 value1;
  T2 value2;

  Tuple(this.value1, this.value2);

  List<R> toList<R>() => List.from([value1, value2]);

  factory Tuple.fromList(List list) {
    return Tuple(list[0], list[1]);
  }
}

typedef ValueTupleWidgetBuilder<T1, T2> = Widget Function(
  BuildContext context,
  Tuple<T1, T2> value,
  Widget? child,
);

class ValueListenableList2Builder<T1, T2> extends ValueListenableListBuilder {
  ValueListenableList2Builder({
    super.key,
    required Tuple<ValueListenable<T1>, ValueListenable<T2>> valueListenables,
    required ValueTupleWidgetBuilder<T1, T2> builder,
    super.child,
  }) : super(
          valueListenables: valueListenables.toList(),
          builder: (context, value, child) => builder(
            context,
            Tuple.fromList(value),
            child,
          ),
        );
}

这样一来,我们在使用Tuple的时候就不用再考虑类型问题了