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>{};
复制代码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);
}
}
复制代码事件分发
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) {
...
}
}
}
复制代码## 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);
}
复制代码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);
}
}
复制代码手指抬起，
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
