Touch事件如何传递到Activity

关于Android的事件分发，我们都了解在Activity中的dispatchTouchEvent方法是分发流程的开始，那么一切的起源又在哪里呢？
我们可以在onTouch事件中，插入语句：

Thread.dumpStack();

可以得到下面的调用栈信息：

Stack trace
at java.lang.Thread.dumpStack(Thread.java:1348)
at com.example.android.MainActivity$1.onTouch(MainActivity.java:54)
at android.view.View.dispatchTouchEvent(View.java:11823)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2616)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2616)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2616)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2616)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2616)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2616)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2616)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2616)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3000)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2616)
at com.android.internal.policy.DecorView.superDispatchTouchEvent(DecorView.java:573)
at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1977)
at android.app.Activity.dispatchTouchEvent(Activity.java:3440)
at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:69)
at com.android.internal.policy.DecorView.dispatchTouchEvent(DecorView.java:529)
at android.view.View.dispatchPointerEvent(View.java:12085)
at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4986)
at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4781)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4279)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4349)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4307)
at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4449)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4315)
at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4506)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4279)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4349)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4307)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4315)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4279)
at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6874)
at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:6848)
at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:6803)
at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6986)
at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:196)
at android.os.MessageQueue.nativePollOnce(Native Method)
at android.os.MessageQueue.next(MessageQueue.java:332)
at android.os.Looper.loop(Looper.java:150)
at android.app.ActivityThread.main(ActivityThread.java:6831)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:825)

下面我们一步步分析调用栈信息

at android.app.ActivityThread.main(ActivityThread.java:6831)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:825)

ZygoteInit初始化当前APP的进程，方法调用顺序：
ZygoteInit.main ->
ZygoteInit.forkSystemServer ->
ZygoteInit.handleSystemServerProcess ->
ZygoteInit.zygoteInit ->
RuntimeInit.applicationInit ->
RuntimeInit.findStaticMain ->
new MethodAndArgsCaller ->
MethodAndArgsCaller.run
其中MethodAndArgsCaller是RuntimeInit的内部类，利用反射invoke获取到了当前应用程序的ActivityThread，进程中所有对Activity的操作都是调用ActivityThread中的相关方法。

at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:196)
at android.os.MessageQueue.nativePollOnce(Native Method)
at android.os.MessageQueue.next(MessageQueue.java:332)
at android.os.Looper.loop(Looper.java:150)

Looper.loop方法是在ActivityThread的main方法中被调用，Android消息机制采用的是Handler机制，通过将消息封装到Message中，再发送到消息所在Handler对应的MessageQueue中，且Looper不断调用MessageQueue的next()方法进行消息的处理。
以根据上面的调用信息，当我们触摸屏幕时，nativePollOnce()将会收到消息，并且将事件发送给InputEventReceiver的dispatchInputEvent()方法，此时FrameWork层便开始了Touch事件的传递。

下面看一下InputEventReceiver类

/**
 * Provides a low-level mechanism for an application to receive input events.
 * @hide
 */
public abstract class InputEventReceiver {
    ...
    private static native long   nativeInit(WeakReference<InputEventReceiver> receiver,
            InputChannel inputChannel, MessageQueue messageQueue);
    private static native void nativeDispose(long receiverPtr);

    public InputEventReceiver(InputChannel inputChannel, Looper looper) {
        mReceiverPtr = nativeInit(new WeakReference<InputEventReceiver>(this),
                inputChannel, mMessageQueue);
    }

    // Called from native code.
    @SuppressWarnings("unused")
    private void dispatchInputEvent(int seq, InputEvent event, int displayId) {
        mSeqMap.put(event.getSequenceNumber(), seq);
        onInputEvent(event, displayId);
    }
}

通过注释可知，InputEventReceiver提供了应用程序接受输入事件的低级机制。
在其构造方法中，可以看到有native方法nativeInit的调用，这里系统 native 层就会将这个InputEventReceiver实例记录下来，每当有事件到达时就会通过inputChannel管道派发到这个实例上，当然还有注销的方法：dipose()。
dispatchInputEvent方法上被注释了Called from native code，说明这个是从native层调用过来的，根据我们看到的整个调用栈信息可知，
nativePollOnce方法内部应该就有对dispatchInputEvent的调用。

final class WindowInputEventReceiver extends InputEventReceiver {
    public WindowInputEventReceiver(InputChannel inputChannel, Looper looper) {
           ...
    }

    @Override
    public void onInputEvent(InputEvent event, int displayId) {
        enqueueInputEvent(event, this, 0, true);
    }

    @Override
    public void onBatchedInputEventPending() {
        ...
    }

    @Override
    public void dispose() {
        ...
    }
}

InputEventReceiver类是一个抽象类，根据栈信息，可以看到其子类WindowInputEventReceiver重写了onInputEvent方法，WindowInputEventReceiver是ViewRootImpl的内部类。

at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6874)
at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:6848)
at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:6803)
at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6986)

onInputEvent方法调用了ViewRootImpl类的enqueueInputEvent方法。且该方法第三个参数为true，表示该事件需要立即处理

void enqueueInputEvent(InputEvent event,
            InputEventReceiver receiver, int flags, boolean processImmediately) {
    adjustInputEventForCompatibility(event);
    //将event事件，receiver，flags包装成一个QueuedInputEvent
    //QueuedInputEvent表示一个在队列中等待处理的输入事件，这个类有个next属性可以指向下一个事件
    QueuedInputEvent q = obtainQueuedInputEvent(event, receiver, flags);
    //获得等待队列的最后一个输入事件（Pending的意思是等待的，Tail的意思是尾部)
    QueuedInputEvent last = mPendingInputEventTail;
    //下面的意思就是将事件加入到队列中
    if (last == null) {
        //如果没有最后一个，就说明队列是空的,那么第一个是该事件，最后一个也是该事件
        mPendingInputEventHead = q;
        mPendingInputEventTail = q;
    } else {
        //如果有最后一个，那么就将该事件设置成最后一个
        last.mNext = q;
        mPendingInputEventTail = q;
    }
    //队列数量加1
    mPendingInputEventCount += 1;
    //事件跟踪机制。。。不需要管
    Trace.traceCounter(Trace.TRACE_TAG_INPUT, mPendingInputEventQueueLengthCounterName,
                mPendingInputEventCount);
    //如果事件需要立即处理，则执行doProcessInputEvents()，
    //WindowInputEventReceiver中enqueueInputEvent(event, this, 0, true);传入的是true
    if (processImmediately) {
        doProcessInputEvents();
    } else {
        scheduleProcessInputEvents();
    }
}

由于入参processImmediately的值为true，则会继续执行到doProcessInputEvents方法。

void doProcessInputEvents() {
    // 处理队列中所有的输入事件
    while (mPendingInputEventHead != null) {
        //下面这段代码是取出事件队列中的第一个，若有第二个，将其置为第一个
        QueuedInputEvent q = mPendingInputEventHead;
        mPendingInputEventHead = q.mNext;
        if (mPendingInputEventHead == null) {
            mPendingInputEventTail = null;
        }
        q.mNext = null;
        //队列数量减1
        mPendingInputEventCount -= 1;
        //跟踪事件，不需要管
        Trace.traceCounter(Trace.TRACE_TAG_INPUT, mPendingInputEventQueueLengthCounterName,
                    mPendingInputEventCount);
        //下面的代码是获得当前事件的发生时间，以及此事件与上一个事件间隔间隔时间
        //通过Choreographer，协调动画、输入和绘图的时间
        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);
    }
    // 处理完了所有的输入事件，将处理事件等待标记设为false
    if (mProcessInputEventsScheduled) {
        mProcessInputEventsScheduled = false;
        mHandler.removeMessages(MSG_PROCESS_INPUT_EVENTS);
    }
}

接下来走到了deliverInputEvent方法

    private void deliverInputEvent(QueuedInputEvent q) {
        //跟踪机制，不用管
        Trace.asyncTraceBegin(Trace.TRACE_TAG_VIEW, "deliverInputEvent",
                q.mEvent.getSequenceNumber());
        //一致性验证，不用管，一致性验证就是比如说判断ACTION_DOWN和ACTION_UP是否成对出现
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onInputEvent(q.mEvent, 0);
        }

        InputStage stage;
        if (q.shouldSendToSynthesizer()) {
            stage = mSyntheticInputStage;
        } else {
            stage = q.shouldSkipIme() ? mFirstPostImeInputStage : mFirstInputStage;
        }

        if (q.mEvent instanceof KeyEvent) {
            mUnhandledKeyManager.preDispatch((KeyEvent) q.mEvent);
        }

        if (stage != null) {
            handleWindowFocusChanged();
            stage.deliver(q);
        } else {
            finishInputEvent(q);
        }
    }

这里出现了InputStage，它是处理输入事件的一个阶段，可以将事件完成或者转送到下一个阶段。
InputStage可以分很多种，如下示例：

mSyntheticInputStage = new SyntheticInputStage();
InputStage viewPostImeStage = new ViewPostImeInputStage(mSyntheticInputStage);
InputStage nativePostImeStage = new NativePostImeInputStage(viewPostImeStage, "aq:native-post-ime:" + counterSuffix);
InputStage earlyPostImeStage = new EarlyPostImeInputStage(nativePostImeStage);
InputStage imeStage = new ImeInputStage(earlyPostImeStage,"aq:ime:" + counterSuffix);
InputStage viewPreImeStage = new ViewPreImeInputStage(imeStage);
InputStage nativePreImeStage = new NativePreImeInputStage(viewPreImeStage, "aq:native-pre-ime:" + counterSuffix);

Android在这里使用了设计模式中的责任链模式，多个InputStage连成一条链，并沿着这条链传递输入事件，直到有一个InputStage处理了该输入事件

at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4986)
at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4781)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4279)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4349)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4307)
at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4449)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4315)
at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4506)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4279)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4349)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4307)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4315)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4279)

触摸事件是由ViewPostImeInputStage处理，事件最终传递给了ViewPostImeInputStage中的onProcess()然后传递给processPointerEvent()方法

protected int onProcess(QueuedInputEvent q) {
    if (q.mEvent instanceof KeyEvent) {
        //按键事件，比如回退键
        return processKeyEvent(q);
    } else {
        final int source = q.mEvent.getSource();
        if ((source & InputDevice.SOURCE_CLASS_POINTER) != 0) {
            //普通的触摸点事件
            return processPointerEvent(q);
        } else if ((source & InputDevice.SOURCE_CLASS_TRACKBALL) != 0) {
            //轨迹球事件
            return processTrackballEvent(q);
        } else {
            //滚轮事件，比如外接蓝牙鼠标时，可以触发滚轮事件
            return processGenericMotionEvent(q);
        }
    }
}

private int processPointerEvent(QueuedInputEvent q) {
    ...
    boolean handled = mView.dispatchPointerEvent(event);
    ...
}

终于有一个看起来熟悉的东西了，这里的mView是什么

at android.app.Activity.dispatchTouchEvent(Activity.java:3440)
at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:69)
at com.android.internal.policy.DecorView.dispatchTouchEvent(DecorView.java:529)
at android.view.View.dispatchPointerEvent(View.java:12085)

我们看一下View类中的dispatchPointerEvent方法

public final boolean dispatchPointerEvent(MotionEvent event) {
    if (event.isTouchEvent()) {
        return dispatchTouchEvent(event);
    } else {
        return dispatchGenericMotionEvent(event);
    }
}

通过调用栈，可以发现实际上mView就是DecorView，接下来就是走DecorView的dispatchTouchEvent方法

public boolean dispatchTouchEvent(MotionEvent ev) {
    final Window.Callback cb = mWindow.getCallback();
    return cb != null && !mWindow.isDestroyed() && mFeatureId < 0
                ? cb.dispatchTouchEvent(ev) : super.dispatchTouchEvent(ev);
}

mWindow就是与Activity关联的PhoneWindow对象，由于DecorView是PhoneWindow创建的，并且通过setWindow()方法，DecorView对象持有了PhoneWindow对象的引用。通过getCallback()方法，就获得了Window.Callback对象，Window.Callback包含了窗口的各种回调接口，Activity就实现了该接口。根据return后的判断，当调用cb.dispatchTouchEvent(ev)时，其实调用的就是Activity中的dispatchTouchEvent()方法

至此，Touch事件是如何传递到Activity的dispatchTouchEvent方法中的流程就全部分析完了，至于接下来的详细的事件分发流程，可以在Android事件分发机制中了解到。