Step1. 从ViewRootImpl到DecorView
1.1 ViewRootImpl.dispatchInputEvent
public void dispatchInputEvent(InputEvent event, InputEventReceiver receiver) {
SomeArgs args = SomeArgs.obtain();
args.arg1 = event;
args.arg2 = receiver;
Message msg = mHandler.obtainMessage(MSG_DISPATCH_INPUT_EVENT, args);
msg.setAsynchronous(true);
mHandler.sendMessage(msg);
}
ViewRootImpl中有一个内部类ViewRootHandler，继承自Handler，此时的消息会交由ViewRootHandler处理，核心代码如下：
case MSG_DISPATCH_INPUT_EVENT: {
SomeArgs args = (SomeArgs)msg.obj;
InputEvent event = (InputEvent)args.arg1;
InputEventReceiver receiver = (InputEventReceiver)args.arg2;
enqueueInputEvent(event, receiver, 0, true);
args.recycle();
} break;
注意：此时调用enqueueInputEvent，最后一个参数为true，表明要立即处理当前事件。
1.2 ViewRootImpl.enqueueInputEvent
void enqueueInputEvent(InputEvent event,
InputEventReceiver receiver, int flags, boolean processImmediately) {
adjustInputEventForCompatibility(event);
QueuedInputEvent q = obtainQueuedInputEvent(event, receiver, flags);
QueuedInputEvent last = mPendingInputEventTail;
if (last == null) {
mPendingInputEventHead = q;
mPendingInputEventTail = q;
} else {
last.mNext = q;
mPendingInputEventTail = q;
}
mPendingInputEventCount += 1;
Trace.traceCounter(Trace.TRACE_TAG_INPUT, mPendingInputEventQueueLengthCounterName,
mPendingInputEventCount);
if (processImmediately) {
doProcessInputEvents();
} else {
scheduleProcessInputEvents();
}
}
按照先后顺序将QueuedInputEvent 加入到当前队列中，然后调用doProcessInputEvents。
1.3 ViewRootImpl.doProcessInputEvents
void doProcessInputEvents() {
// Deliver all pending input events in the queue.
while (mPendingInputEventHead != null) {
QueuedInputEvent q = mPendingInputEventHead;
mPendingInputEventHead = q.mNext;
if (mPendingInputEventHead == null) {
mPendingInputEventTail = null;
}
q.mNext = null;
mPendingInputEventCount -= 1;
Trace.traceCounter(Trace.TRACE_TAG_INPUT, mPendingInputEventQueueLengthCounterName,
mPendingInputEventCount);
long eventTime = q.mEvent.getEventTimeNano();
long oldestEventTime = eventTime;
if (q.mEvent instanceof MotionEvent) {
MotionEvent me = (MotionEvent)q.mEvent;
if (me.getHistorySize() > 0) {
oldestEventTime = me.getHistoricalEventTimeNano(0);
}
}
mChoreographer.mFrameInfo.updateInputEventTime(eventTime, oldestEventTime);
deliverInputEvent(q);
}
// We are done processing all input events that we can process right now
// so we can clear the pending flag immediately.
if (mProcessInputEventsScheduled) {
mProcessInputEventsScheduled = false;
mHandler.removeMessages(MSG_PROCESS_INPUT_EVENTS);
}
}
循环从pending队列中取出未处理的事件，然后调用deliverInputEvent处理。
1.4 ViewRootImpl.deliverInputEvent
private void deliverInputEvent(QueuedInputEvent q) {
Trace.asyncTraceBegin(Trace.TRACE_TAG_VIEW, "deliverInputEvent",
q.mEvent.getSequenceNumber());
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onInputEvent(q.mEvent, 0);
}
InputStage stage;
if (q.shouldSendToSynthesizer()) {
stage = mSyntheticInputStage;
} else {
stage = q.shouldSkipIme() ? mFirstPostImeInputStage : mFirstInputStage;
}
if (stage != null) {
stage.deliver(q);
} else {
finishInputEvent(q);
}
}
private void finishInputEvent(QueuedInputEvent q) {
Trace.asyncTraceEnd(Trace.TRACE_TAG_VIEW, "deliverInputEvent",
q.mEvent.getSequenceNumber());
if (q.mReceiver != null) {
boolean handled = (q.mFlags & QueuedInputEvent.FLAG_FINISHED_HANDLED) != 0;
q.mReceiver.finishInputEvent(q.mEvent, handled);
} else {
q.mEvent.recycleIfNeededAfterDispatch();
}
recycleQueuedInputEvent(q);
}
在deliverInputEvent中，调用了InputStage的deliver方法继续处理事件。
在deliverInputEvent函数开始是，开始跟踪事件：
Trace.asyncTraceBegin(Trace.TRACE_TAG_VIEW, "deliverInputEvent",
q.mEvent.getSequenceNumber());
在finishInputEvent中，结束跟踪事件：
Trace.asyncTraceEnd(Trace.TRACE_TAG_VIEW, "deliverInputEvent",
q.mEvent.getSequenceNumber());
此处的“deliverInputEvent”，有什么含义呢？很明显，Trace在此处跟踪的是一个Pointer事件的处理过程。通过systrace抓取数据时，可以看到“deliverInputEvent”的具体耗时。
1.5 InputStage.deliver
public final void deliver(QueuedInputEvent q) {
if ((q.mFlags & QueuedInputEvent.FLAG_FINISHED) != 0) {
forward(q);
} else if (shouldDropInputEvent(q)) {
finish(q, false);
} else {
apply(q, onProcess(q));
}
}
InputStage采用职责链的模式处理事件，正常情况下，事件处理会执行第7行的分支。InputStage有多个子类，此处onProcess在InputStage的子类ViewPostImeInputStage中实现。
1.6 ViewPostImeInputStage.onProcess
@Override
protected int onProcess(QueuedInputEvent q) {
if (q.mEvent instanceof KeyEvent) {
return processKeyEvent(q);
} else {
// If delivering a new non-key event, make sure the window is
// now allowed to start updating.
handleDispatchWindowAnimationStopped();
final int source = q.mEvent.getSource();
if ((source & InputDevice.SOURCE_CLASS_POINTER) != 0) {
return processPointerEvent(q);
} else if ((source & InputDevice.SOURCE_CLASS_TRACKBALL) != 0) {
return processTrackballEvent(q);
} else {
return processGenericMotionEvent(q);
}
}
}
首先检查是不是一个按键事件，如果是，则执行第4行；若不不是，需要继续判断事件类型。如果是屏幕点击，执行第11行，轨迹球事件则执行第13行。接下来分析第11行的函数。
1.7 ViewPostImeInputStage.processPointerEvent
private int processPointerEvent(QueuedInputEvent q) {
final MotionEvent event = (MotionEvent)q.mEvent;
mAttachInfo.mUnbufferedDispatchRequested = false;
boolean handled = mView.dispatchPointerEvent(event);
if (mAttachInfo.mUnbufferedDispatchRequested && !mUnbufferedInputDispatch) {
mUnbufferedInputDispatch = true;
if (mConsumeBatchedInputScheduled) {
scheduleConsumeBatchedInputImmediately();
}
}
return handled ? FINISH_HANDLED : FORWARD;
}
ViewPostImeInputStage是ViewRootImpl的内部类，在第5行，mView的真实类型是DecorView。DecorView继承自FrameLayout，第5行调用的就是View中定义的dispatchPointerEvent方法。
1.8 View.dispatchPointerEvent
public final boolean dispatchPointerEvent(MotionEvent event) {
if (event.isTouchEvent()) {
return dispatchTouchEvent(event);
} else {
return dispatchGenericMotionEvent(event);
}
}
很明显，此时是Touch事件，进入第3行的分支。由于DecorView重写了dispatchTouchEvent方法，接下来分支DecorView中该函数的实现。
1.9 DecorView.dispatchTouchEvent
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
final Callback cb = getCallback();
return cb != null && !isDestroyed() && mFeatureId < 0 ? cb.dispatchTouchEvent(ev)
: super.dispatchTouchEvent(ev);
}
DecorView是PhoneWindow的内部类，PhoneWindow的是Window的子类。Activity实现了Window中的接口Callback,在Activity的attach函数中，给Window对象设置了Callback：mWindow.setCallback(this)，因此，第4行的cb.dispatchTouchEvent就跳转到了Activity中，事件传递到了Activity。
Step2. 从DecorView到每一个View
在Step1中，我们看到了一个Touch事件是如何由ViewRootImpl传递到Activity中的，现在来分析事件是如何从Activity传递到Activity中的每个View中的。
2.1 Activity.dispatchTouchEvent
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
Activity收到事件后，调用了PhoneWindow中的superDispatchTouchEvent方法。
2.2 PhonWindow.superDispatchTouchEvent
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
调用DecorView的superDispatchTouchEvent方法。
2.3 DecorView.superDispatchTouchEvent
public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}
DecorView继承自FrameLayout，是Activity中所有界面元素的根布局，此时，事件传递到了所有布局元素中最外层的FrameLayout中，接下来通过FrameLayout的dispatchTouchEvent方法，将事件一层层的传递下去。
总结：
事件传递的整体流程如下图：
- ViewRootImpl首先接收到事件，然后交给DecorView处理
- DecorView接收到事件后，没有直接处理，而是调用了Activity实现的Callback。为什么要这么设计？理论上来说，DecorView可以直接处理事件。如果DecorView直接处理了事件，这样用户就无法拦截到初始事件。某些情况下，可能会重新Activity中的dispatchTouchEvent事件来实现某些功能。
- 事件传递到Activity，是为了给用户一个拦截、处理事件的机会。在Activity的dispatchTouchEvent中，事件又传递到了PhoneWindow中，PhoneWindow又将事件传递到DecorView中，即最终交由DecorView处理。
