Flutter事件之GestureBinding

Flutter在启动时(runApp)会进行一些浇水类的"粘合"，WidgetsFlutterBinding作为主类，需要粘合一系列的Binding，其中GestureBinding就是事件处理类；

GestureBinding是Flutter中管理手势事件的Binding，是Flutter Framework层处理事件的最起点；

GestureBinding实现了HitTestable, HitTestDispatcher, HitTestTarget，分别具有以下功能

• 
  
• hitTest命中测试
  
• dispatchEvent事件分发
  
• handleEvent处理事件（）
  

成员变量：

//触点路由，由手势识别器注册，会把手势识别器的pointer和handleEvent存入

//以便在GestureBinding.handleEvent调用

final PointerRouter pointerRouter = PointerRouter();


//手势竞技场管理者，管理竞技场们的相关操作

final GestureArenaManager gestureArena = GestureArenaManager();


//hitTest列表，里面存储了被命中测试成员

final Map<int, HitTestResult> _hitTests = <int, HitTestResult>{};

GestureBinding在_handlePointerDataPacket方法接收有Engine层传递过来的触点数据，经过数据包装转换为Framework层可处理的数据:PointerAddedEvent、PointerCancelEvent、PointerDownEvent、PointerMoveEvent、PointerUpEvent等等，随后在_handlePointerEventImmediately方法中进行命中测试和事件分发；

手指按下

当手指按下时，接收到的事件类型是PointerDownEvent

首先是命中测试

当事件类型是event is PointerDownEvent || event is PointerSignalEvent || event is PointerHoverEvent会进行新的命中测试，命中测试相关请看这，得到命中测试列表后，开始调用dispatchEvent进行事件分发。

void _handlePointerEventImmediately(PointerEvent event) {

  HitTestResult? hitTestResult;

  if (event is PointerDownEvent || event is PointerSignalEvent || event is PointerHoverEvent) {

    assert(!_hitTests.containsKey(event.pointer));

    hitTestResult = HitTestResult();

    hitTest(hitTestResult, event.position);

    if (event is PointerDownEvent) {

      _hitTests[event.pointer] = hitTestResult;

    }

    assert(() {

      if (debugPrintHitTestResults)

        debugPrint('$event: $hitTestResult');

      return true;

    }());

  } else if (event is PointerUpEvent || event is PointerCancelEvent) {

    hitTestResult = _hitTests.remove(event.pointer);

  } else if (event.down) {

    //当前事件是按下状态，重用hitTest结果

    hitTestResult = _hitTests[event.pointer];

  }

  if (hitTestResult != null ||

      event is PointerAddedEvent ||

      event is PointerRemovedEvent) {

    assert(event.position != null);

    dispatchEvent(event, hitTestResult);

  }

}

事件分发

事件分发的目的是调用命中对象的handleEvent方法以处理相关逻辑，比如我们熟知的Listener组件，它做的事就是回调相关方法，比如按下时Listener会回调onPointerDown

## GestureBinding ##

@override // from HitTestDispatcher

void dispatchEvent(PointerEvent event, HitTestResult? hitTestResult) {

  assert(!locked);

  if (hitTestResult == null) {

    assert(event is PointerAddedEvent || event is PointerRemovedEvent);

    try {

      pointerRouter.route(event);

    } catch (exception, stack) {

      ...

    }

    return;

  }

  for (final HitTestEntry entry in hitTestResult.path) {

    try {

      entry.target.handleEvent(event.transformed(entry.transform), entry);

    } catch (exception, stack) {

      ...

    }

  }

}

## Listener ##

@override

void handleEvent(PointerEvent event, HitTestEntry entry) {

  assert(debugHandleEvent(event, entry));

  if (event is PointerDownEvent)

    return onPointerDown?.call(event);

  if (event is PointerMoveEvent)

    return onPointerMove?.call(event);

  if (event is PointerUpEvent)

    return onPointerUp?.call(event);

  if (event is PointerHoverEvent)

    return onPointerHover?.call(event);

  if (event is PointerCancelEvent)

    return onPointerCancel?.call(event);

  if (event is PointerSignalEvent)

    return onPointerSignal?.call(event);

}

我们知道命中测试最后会把GestrueBinding本身加入到列表中，所以最后也会执行GestrueBinding的handleEvent方法

handleEvent

GestrueBinding.handleEvent是处理手势识别器相关的逻辑，pointerRouter.route(event)调用了识别器的handleEvent方法（需要提前进行触点注册），随后的是竞技场的相关处理；可以看这里了解手势识别器；

## GestrueBinding ##

@override // from HitTestTarget

void handleEvent(PointerEvent event, HitTestEntry entry) {

  pointerRouter.route(event);


  if (event is PointerDownEvent) {

    gestureArena.close(event.pointer);

  } else if (event is PointerUpEvent) {

    gestureArena.sweep(event.pointer);

  } else if (event is PointerSignalEvent) {

    pointerSignalResolver.resolve(event);

  }

}

手指抬起，

手指抬起会重用之前hitTest结果，并不会重新hitTest，如果是Listener组件，则会回调PointerUpEvent