GestureBinding of Flutter events

When Flutter starts ( runApp), it will perform some "bonding" of watering classes. WidgetsFlutterBinding is the main class and needs to glue a series of Bindings, among which GestureBindingis the event processing class;

GestureBinding is the Binding that manages gesture events in Flutter, and is the starting point for the Flutter Framework layer to process events;

GestureBinding implements HitTestable, HitTestDispatcher, HitTestTarget, and has the following functions respectively

• hitTesthit test
• dispatchEventevent distribution
• handleEventhandle event()

Member variables:

//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>{};
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GestureBinding _handlePointerDataPacketreceives the contact data passed by the Engine layer in the method, and converts it into data that can be processed by the Framework layer after data packaging: PointerAddedEvent、PointerCancelEvent、PointerDownEvent、PointerMoveEvent、PointerUpEventetc., and then _handlePointerEventImmediatelyperforms hit testing and event distribution in the method;

finger press

When the finger is pressed, the received event type is PointerDownEvent

first hit test

When the event type is event is PointerDownEvent || event is PointerSignalEvent || event is PointerHoverEventa new hit test, see here for hit test related. After getting the hit test list, start calling dispatchEventfor event distribution.

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);
  }
}
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event distribution

The purpose of event distribution is to call the method of the hit object handleEventto process related logic, such as the well-known Listener component, what it does is to call back related methods, such as Listener will call back when pressedonPointerDown

## 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) {
      ...
    }
  }
}
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## 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);
}
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We know that the hit test will add GestrueBinding itself to the list at the end, so the handleEvent method of GestrueBinding will also be executed at the end

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);
  }
}
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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