Flutter事件分发源码剖析
Posted xiangzhihong
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概述
不管是原生android、ios还是javascript，只要是涉及手势交互都会有事件的分发处理。和原生Android、iOS的事件分发的步骤和原理一样，Flutter的事件分发总体也由手势触发、拦截和响应等几个部分构成。Flutter所有事件源头是 hooks.dart文件的_dispatchPointerDataPacket函数，通过拦截屏幕的点击、滑动等各种事件，进而分发给原生代码进行响应（ps：Android事件分发）。
如果你看过了解原生Android、iOS的事件分发机制，那么Flutter的事件分发，其实是在Android和iOS上加了壳，即Flutter的事件分发是在原生Android、iOS的的事件分发上进行包装的（Android - C - Dart，iOS- C -Dart）。其中，C是Flutter的底层engine，负责Flutter上层和原生Android、iOS系统的交互。
事件分发到Dart的入口类是GestureBinding类，此类位于gestures/binding.dart文件中，与手势识别相关的都位于gestures包中，如下图所示。
- converter.dart将物理坐标_dispatchPointerDataPacket收到的物理数据PointerDataPacket转换成PointerEvent， 类似于安卓在ViewRootImpl.java将InputEventReceiver收到的InputEvent转换为MotionEvent。
- recognizer.dart的GestureRecognizer是所有手势识别的基类。
- rendering/binding.dart的RendererBinding类关联了render树和Flutter引擎，等价于安卓的Surface。
- view.dart的RenderView是render树的根节点，等价于安卓的DecorView。
Flutter的事件分发基类是GestureBinding，打开GestureBinding类，它的成员函数包括dispatchEvent、handleEvent和hitTes等，主要是从事件队列里按照先入先出方式处理PointerEvent，源码如下。
mixin GestureBinding on BindingBase implements HitTestable, HitTestDispatcher, HitTestTarget {
@override
void initInstances() {
super.initInstances();
_instance = this;
ui.window.onPointerDataPacket = _handlePointerDataPacket;
}其中，WidgetsFlutterBinding.ensureInitialized()函数的作用就是初始化各个binging。
Flutter 事件分发
和Android、iOS类似，Flutter的事件分发的入口在runApp函数，相关的代码如下。
void runApp(Widget app) {
WidgetsFlutterBinding.ensureInitialized()
..attachRootWidget(app)
..scheduleWarmUpFrame();
}
class WidgetsFlutterBinding extends BindingBase with GestureBinding, ServicesBinding, SchedulerBinding, PaintingBinding, SemanticsBinding, RendererBinding, WidgetsBinding {
static WidgetsBinding ensureInitialized() {
if (WidgetsBinding.instance == null)
WidgetsFlutterBinding();
return WidgetsBinding.instance;
}
}
void attachRootWidget(Widget rootWidget) {
_renderViewElement = RenderObjectToWidgetAdapter<RenderBox>(
container: renderView,
debugShortDescription: \'[root]\',
child: rootWidget
).attachToRenderTree(buildOwner, renderViewElement);
}WidgetsFlutterBinding.ensureInitialized()函数的作用是初始化各个binging。事实上，Flutter 中的 WidgetsFlutterBinding的 Binding可以分为GestureBinding、ServicesBinding、SchedulerBinding、PaintingBinding、SemanticsBinding、RendererBinding、WidgetsBinding 等 7 种 Binding，它们都有自己在功能上的划分。其中，GestureBinding就是处理事件分发的，attachRootWidget就是设置根节点， 可以看到真正的根节点是renderview， 也是Flutter事件分发的起点。
下面我们来重点看一下GestureBinding类。
GestureBinding
和Android事件处理的流程一样，首先，系统会拦截用户的事件，然后在使用GestureBinding的_handlePointerEvent进行事件命中处理。原生事件到达Dart层之后调用的第一个方法是_handlePointerDataPacket，它的源码如下。
void _handlePointerDataPacket(ui.PointerDataPacket packet) {
_pendingPointerEvents.addAll(PointerEventConverter.expand(packet.data, window.devicePixelRatio));
if (!locked)
_flushPointerEventQueue();
}_handlePointerDataPacket方法有一个PointerEventConverter类，作用是将原生传来的手势数据全部转化为Dart对应的对象保存数据，然后保存到集合中进行储存。接下来来我们看一下_flushPointerEventQueue方法，源码如下。
void _flushPointerEventQueue() {
assert(!locked);
while (_pendingPointerEvents.isNotEmpty)
_handlePointerEvent(_pendingPointerEvents.removeFirst());
}_flushPointerEventQueue方法的作用就是循环处理每个手指的的事件，并进行处理，源码如下。
void _handlePointerEvent(PointerEvent event) {
assert(!locked);
HitTestResult hitTestResult;
//如果是手指按下的话
if (event is PointerDownEvent || event is PointerSignalEvent) {
assert(!_hitTests.containsKey(event.pointer));
hitTestResult = HitTestResult();
//得到碰撞的控件组
hitTest(hitTestResult, event.position);
if (event is PointerDownEvent) {
_hitTests[event.pointer] = hitTestResult;
}
assert(() {
if (debugPrintHitTestResults)
debugPrint(\'$event: $hitTestResult\');
return true;
}());
}
//手指抬起
else if (event is PointerUpEvent || event is PointerCancelEvent) {
hitTestResult = _hitTests.remove(event.pointer);
}
//缓存点击的事件，接下来发生滑动的时候直接复用原来的碰撞控件组
else if (event.down) {
// Because events that occur with the pointer down (like
// PointerMoveEvents) should be dispatched to the same place that their
// initial PointerDownEvent was, we want to re-use the path we found when
// the pointer went down, rather than do hit detection each time we get
// such an event.
hitTestResult = _hitTests[event.pointer];
}
assert(() {
if (debugPrintMouseHoverEvents && event is PointerHoverEvent)
debugPrint(\'$event\');
return true;
}());
if (hitTestResult != null ||
event is PointerHoverEvent ||
event is PointerAddedEvent ||
event is PointerRemovedEvent) {
dispatchEvent(event, hitTestResult);
}
}这个方法的主要目的就是得到HitTestResult，就是根据按下的坐标位置找出view树中哪些控件在点击的范围内，手指在移动和抬起的时候都复用当前的事件，区别在于不同的手指有不同的索引值。接下来，看一下用户的触摸行为，hitTest首先会进入RendererBinding处理，打开RendererBinding类的hitTest方法，如下所示。
RenderView get renderView => _pipelineOwner.rootNode as RenderView;
void hitTest(HitTestResult result, Offset position) {
assert(renderView != null);
renderView.hitTest(result, position: position);
super.hitTest(result, position);
}其中，RenderView可以理解为Flutter 视图树的根View，在Flutter中也叫做Widget ，一个Widget 对应一个Element 。在Flutter中，渲染会三棵树，即Widget 树、Element 树和RenderObject 树。我们进行页面布局分析时，就可以看到它们，如下所示。
关于Widget 树、Element 树和RenderObject 树，可以查看Flutter渲染之Widget、Element 和 RenderObject的介绍。
然后，我们打开renderView.hitTest方法，对应的代码如下所示。
bool hitTest(HitTestResult result, { Offset position }) {
if (child != null)
child.hitTest(BoxHitTestResult.wrap(result), position: position);
result.add(HitTestEntry(this));
return true;
}可以看到，根视图是先从子view开始放进集合，放完子view再放自己，这和前端JS点击事件冒泡的原理是一样的。并且，只有满足条件子视图才会放到 入RenderBox 的这个方法中。
bool hitTest(BoxHitTestResult result, { @required Offset position }) {
//所点击的范围是否在当前控件的范围内
if (_size.contains(position)) {
//先添加孩子中的事件后选人
if (hitTestChildren(result, position: position) || hitTestSelf(position)) {
result.add(BoxHitTestEntry(this, position));
return true;
}
}
return false;
}接下来，看一下Stack小部件hitTestChildren的实现，源码如下。
@override
bool hitTestChildren(BoxHitTestResult result, { Offset position }) {
return defaultHitTestChildren(result, position: position);
}
bool defaultHitTestChildren(BoxHitTestResult result, { Offset position }) {
// the x, y parameters have the top left of the node\'s box as the origin
ChildType child = lastChild;
while (child != null) {
final ParentDataType childParentData = child.parentData;
final bool isHit = result.addWithPaintOffset(
offset: childParentData.offset,
position: position,
hitTest: (BoxHitTestResult result, Offset transformed) {
assert(transformed == position - childParentData.offset);
return child.hitTest(result, position: transformed);
},
);
if (isHit)
return true;
child = childParentData.previousSibling;
}
return false;
}这个方法的作用就是判断包含Padding的视图是否在点击范围内，如果命中，则阻止其他事件继续冒泡。看到此处，我们大体可以看出，Flutter的事件处理主要是判断点击的坐标知否在控件范围内，如果在范围内直接响应，如果不在继续向上冒泡，并且事件是从叶子开始的，也即Web中的事件冒泡。
完成命中处理后，接下来回到事件处理的主流程，即事件派发dispatchEvent，代码位于gestrues/binding里面，源码如下。
void dispatchEvent(PointerEvent event, HitTestResult hitTestResult) {
assert(!locked);
// No hit test information implies that this is a hover or pointer
// add/remove event.这种情况出在指针悬停屏幕上方，微微接触或不接触，是手机敏感而言
if (hitTestResult == null) {
assert(event is PointerHoverEvent || event is PointerAddedEvent || event is PointerRemovedEvent);
try {
pointerRouter.route(event);
} catch (exception, stack) {
FlutterError.reportError(FlutterErrorDetailsForPointerEventDispatcher(
exception: exception,
stack: stack,
library: \'gesture library\',
context: ErrorDescription(\'while dispatching a non-hit-tested pointer event\'),
event: event,
hitTestEntry: null,
informationCollector: () sync* {
yield DiagnosticsProperty<PointerEvent>(\'Event\', event, style: DiagnosticsTreeStyle.errorProperty);
},
));
}
return;
}
for (HitTestEntry entry in hitTestResult.path) {
try {
entry.target.handleEvent(event.transformed(entry.transform), entry);
} catch (exception, stack) {
FlutterError.reportError(FlutterErrorDetailsForPointerEventDispatcher(
exception: exception,
stack: stack,
library: \'gesture library\',
context: ErrorDescription(\'while dispatching a pointer event\'),
event: event,
hitTestEntry: entry,
informationCollector: () sync* {
yield DiagnosticsProperty<PointerEvent>(\'Event\', event, style: DiagnosticsTreeStyle.errorProperty);
yield DiagnosticsProperty<HitTestTarget>(\'Target\', entry.target, style: DiagnosticsTreeStyle.errorProperty);
},
));
}
}
}此方法最根本的作用是循环事件分发，并以冒泡的形式从底部到分发事件，当事件被命中时，即由当前子节点处理事件，这和Android的事件分发的逻辑是一样的。下面以GestureDetector和Listener来举例事件分发的不同。如果用Listener的话，Listener的组件最终对应的RenderObject是RenderPointerListener，它的监测当前点击是否命中的方法如下。
bool hitTest(BoxHitTestResult result, { Offset position }) {
bool hitTarget = false;
if (size.contains(position)) {
hitTarget = hitTestChildren(result, position: position) || hitTestSelf(position);
if (hitTarget || behavior == HitTestBehavior.translucent)
result.add(BoxHitTestEntry(this, position));
}
return hitTarget;
}
@override
bool hitTestSelf(Offset position) => behavior == HitTestBehavior.opaque;使用Listener嵌套的子组件默认情况下是命中的，很多子部件例如Text、Image等，它们的hitTestSelf返回True，假如我们为Text嵌套了Listener，那么事件分发的时候设计的代码如下所示。
void handleEvent(PointerEvent event, HitTestEntry entry) {
assert(debugHandleEvent(event, entry));
if (onPointerDown != null && event is PointerDownEvent)
return onPointerDown(event);
if (onPointerMove != null && event is PointerMoveEvent)
return onPointerMove(event);
if (onPointerUp != null && event is PointerUpEvent)
return onPointerUp(event);
if (onPointerCancel != null && event is PointerCancelEvent)
return onPointerCancel(event);
if (onPointerSignal != null && event is PointerSignalEvent)
return onPointerSignal(event);
}如果使用的是GestureDetector的话，build方法会为我们添加很多处理手势的方法类，如TapGestureRecognizer，通过处理手势识别后，最终返回的是RawGestureDetector，涉及的代码如下。
final Map<Type, GestureRecognizerFactory> gestures = <Type, GestureRecognizerFactory>{};
if (
onTapDown != null ||
onTapUp != null ||
onTap != null ||
onTapCancel != null ||
onSecondaryTapDown != null ||
onSecondaryTapUp != null ||
onSecondaryTapCancel != null
) {
gestures[TapGestureRecognizer] = GestureRecognizerFactoryWithHandlers<TapGestureRecognizer>(
() => TapGestureRecognizer(debugOwner: this),
(TapGestureRecognizer instance) {
instance
..onTapDown = onTapDown
..onTapUp = onTapUp
..onTap = onTap
..onTapCancel = onTapCancel
..onSecondaryTapDown = onSecondaryTapDown
..onSecondaryTapUp = onSecondaryTapUp
..onSecondaryTapCancel = onSecondaryTapCancel;
},
);
}
if (onDoubleTap != null) {
gestures[DoubleTapGestureRecognizer] = GestureRecognizerFactoryWithHandlers<DoubleTapGestureRecognizer>(
() => DoubleTapGestureRecognizer(debugOwner: this),
(DoubleTapGestureRecognizer instance) {
instance
..onDoubleTap = onDoubleTap;
},
);
}
if (onLongPress != null ||
onLongPressUp != null ||
onLongPressStart != null ||
onLongPressMoveUpdate != null ||
onLongPressEnd != null) {
gestures[LongPressGestureRecognizer] = GestureRecognizerFactoryWithHandlers<LongPressGestureRecognizer>(
() => LongPressGestureRecognizer(debugOwner: this),
(LongPressGestureRecognizer instance) {
instance
..onLongPress = onLongPress
..onLongPressStart = onLongPressStart
..onLongPressMoveUpdate = onLongPressMoveUpdate
..onLongPressEnd =onLongPressEnd
..onLongPressUp = onLongPressUp;
},
);
}
if (onVerticalDragDown != null ||
onVerticalDragStart != null ||
onVerticalDragUpdate != null ||
onVerticalDragEnd != null ||
onVerticalDragCancel != null) {
gestures[VerticalDragGestureRecognizer] = GestureRecognizerFactoryWithHandlers<VerticalDragGestureRecognizer>(
() => VerticalDragGestureRecognizer(debugOwner: this),
(VerticalDragGestureRecognizer instance) {
instance
..onDown = onVerticalDragDown
..onStart = onVerticalDragStart
..onUpdate = onVerticalDragUpdate
..onEnd = onVerticalDragEnd
..onCancel = onVerticalDragCancel
..dragStartBehavior = dragStartBehavior;
},
);
}
if (onHorizontalDragDown != null ||
onHorizontalDragStart != null ||
onHorizontalDragUpdate != null ||
onHorizontalDragEnd != null ||
onHorizontalDragCancel != null) {
gestures[HorizontalDragGestureRecognizer] = GestureRecognizerFactoryWithHandlers<HorizontalDragGestureRecognizer>(
() => HorizontalDragGestureRecognizer(debugOwner: this),
(HorizontalDragGestureRecognizer instance) {
instance
..onDown = onHorizontalDragDown
..onStart = onHorizontalDragStart
..onUpdate = onHorizontalDragUpdate
..onEnd = onHorizontalDragEnd
..onCancel = onHorizontalDragCancel
..dragStartBehavior = dragStartBehavior;
},
);
}
if (onPanDown != null ||
onPanStart != null ||
onPanUpdate != null ||
onPanEnd != null ||
onPanCancel != null) {
gestures[PanGestureRecognizer] = GestureRecognizerFactoryWithHandlers<PanGestureRecognizer>(
() => PanGestureRecognizer(debugOwner: this),
(PanGestureRecognizer instance) {
instance
..onDown = onPanDown
..onStart = onPanStart
..onUpdate = onPanUpdate
..onEnd = onPanEnd
..onCancel = onPanCancel
..dragStartBehavior = dragStartBehavior;
},
);
}
if (onScaleStart != null || onScaleUpdate != null || onScaleEnd != null) {
gestures[ScaleGestureRecognizer] = GestureRecognizerFactoryWithHandlers<ScaleGestureRecognizer>(
() => ScaleGestureRecognizer(debugOwner: this),
(ScaleGestureRecognizer instance) {
instance
..onStart = onScaleStart
..onUpdate = onScaleUpdate
..onEnd = onScaleEnd;
},
);
}
if (onForcePressStart != null ||
onForcePressPeak != null ||
onForcePressUpdate != null ||
onForcePressEnd != null) {
gestures[ForcePressGestureRecognizer] = GestureRecognizerFactoryWithHandlers<ForcePressGestureRecognizer>(
() => ForcePressGestureRecognizer(debugOwner: this),
(ForcePressGestureRecognizer instance) {
instance
..onStart = onForcePressStart
..onPeak = onForcePressPeak
..onUpdate = onForcePressUpdate
..onEnd = onForcePressEnd;
},
);
}
return RawGestureDetector(
gestures: gestures,
behavior: behavior,
excludeFromSemantics: excludeFromSemantics,
child: child,
);并且，RawGestureDetector默认使用的也是Listener，它注册了手指按下的方法，分发的时候Down事件是sdk默认处理的。
void _handlePointerDown(PointerDownEvent event) {
assert(_recognizers != null);
for (GestureRecognizer recognizer in _recognizers.values)
recognizer.addPointer(event);
}此方法会向Binding路由器中注册那些需要处理的事件，假如我们只声明了点击事件，那么集合中负责添加的GestureRecognizer的实现类就是TapGestureRecognizer，接下来我们看一下addPointer方法。
void addPointer(PointerDownEvent event) {
_pointerToKind[event.pointer] = event.kind;
if (isPointerAllowed(event)) {
addAllowedPointer(event);
} else {
handleNonAllowedPointer(event);
}
}
bool isPointerAllowed(PointerDownEvent event) {
switch (event.buttons) {
case kPrimaryButton:
if (onTapDown == null &&
onTap == null &&
onTapUp == null &&
onTapCancel == null)
return false;
break;
case kSecondaryButton:
if (onSecondaryTapDown == null &&
onSecondaryTapUp == null &&
onSecondaryTapCancel == null)
return false;
break;
default:
return false;
}
return super.isPointerAllowed(event);
}isPointerAllowed方法的作用就是用来判定当前的手势，默认返回false，如果事件比命中，接下来执行addAllowedPointer方法，如下所示。
void addAllowedPointer(PointerDownEvent event) {
startTrackingPointer(event.pointer, event.transform);
if (state == GestureRecognizerState.ready) {
state = GestureRecognizerState.possible;
primaryPointer = event.pointer;
initialPosition = OffsetPair(local: event.localPosition, global: event.position);
if (deadline != null)
_timer = Timer(deadline, () => didExceedDeadlineWithEvent(event));
}
void startTrackingPointer(int pointer, [Matrix4 transform]) {
GestureBinding.instance.pointerRouter.addRoute(pointer, handleEvent, transform);
_trackedPointers.add(pointer);
assert(!_entries.containsValue(pointer));
_entries[pointer] = _addPointerToArena(pointer);
}这两个方法的主要作用就是用来将当前的handleEvent方法添加到GestureBinding路由器里面去，而_addPointerToArena是就是添加处理事件的具体逻辑。接下来，我们来看一下GestureBinding里面的handleEvent函数的事件分发逻辑。
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);
}
}
}如果手指按下的时候GestureRecognizer的handleEvent方法没有决策出到底哪个控件会成为事件的处理者，那么会执行 gestureArena.close()方法，如下所示。
void close(int pointer) {
final _GestureArena state = _arenas[pointer];
if (state == null)
return; // This arena either never existed or has been resolved.
state.isOpen = false;
assert(_debugLogDiagnostic(pointer, \'Closing\', state));
_tryToResolveArena(pointer, state);
}如果未决策出哪个控件处理事件的时候，state.isOpen此时被标记为false，也即是关闭手势的处理。
void _tryToResolveArena(int pointer, _GestureArena state) {
assert(_arenas[pointer] == state);
assert(!state.isOpen);
if (state.members.length == 1) {
scheduleMicrotask(() => _resolveByDefault(pointer, state));
} else if
(state.members.isEmpty) {
_arenas.remove(pointer);
assert(_debugLogDiagnostic(pointer, \'Arena empty.\'));
}
else if (state.eagerWinner != null) {
assert(_debugLogDiagnostic(pointer, \'Eager winner: ${state.eagerWinner}\'));
_resolveInFavorOf(pointer, state, state.eagerWinner);
}
}如果手势竞争中，有竞争胜出者，则由胜出者执行事件处理，如下所示。
void _resolveInFavorOf(int pointer, _GestureArena state, GestureArenaMember member) {
assert(state == _arenas[pointer]);
assert(state != null);
assert(state.eagerWinner == null || state.eagerWinner == member);
assert(!state.isOpen);
_arenas.remove(pointer);
//其他的命中全部拒绝
for (GestureArenaMember rejectedMember in state.members) {
if (rejectedMember != member)
rejectedMember.rejectGesture(pointer);
}
member.acceptGesture(pointer);
}如果事件处理中没有具体的事件处理对象，将会默认采用最底层的的叶子节点控件作为事件处理者，也就是说最内层的那个控件将消耗事件。也就是说，如果使用GestureRecognizer来识别手势事件时，最终事件会被最内层的GestureRecognizer消耗，这和Android单个控件消耗事件差不多，所以嵌套滚动总是先滚动内层，先被内层消耗，然后再执行外层。
参考： Flutter 事件分发
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