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本文链接：https://www.jianshu.com/p/fda04a3709ab
https://www.jianshu.com/p/76f94c6452c0文章中最后讲到了ViewRootImpl的setView函数调用，设置View和窗口关联（通过调用mWindowSession的addToDisplay），以及初始化UI draw方面的工作（硬件加速，enableHardwareAcceleration）和初始化接收事件的相关逻辑。本篇主要讲android的应用层事件接收和处理的过程。
本篇的切入点是，android的Native层将消息封装成InputEvent传送到Java层开始（至于如何形成Native层消息，并传送给Java层的，这不是应用开发所关心的，不是本篇的重点）。
事件从ViewRootImpl传递到Activity的根视图DecorView的过程
ViewRootImpl中mWindowSession添加窗口调用addToDisplay的时候，addToDisplay函数有一个参数是InputChannel对象，它代表着ViewRootImpl和WindowManagerService的事件输入关联，WindowManagerService管理的窗口接收到事件之后，会通知到处于焦点下的窗口的ViewRootImpl的InputChannel。
事件最终到InputEventReceiver对象进行接收和处理WindowInputEventReceiver继承InputEventReceiver，InputEventReceiver接收到事件之后调用onInputEvent，由于对象实例是WindowInputEventReceiver对象，所以最终调用了WindowInputEventReceiver的onInputEvent函数，onInputEvent函数里面调用了enqueueInputEvent
//创建mInputEventReceiver对象代码片段，具体参考ViewRootImpl的setView
if (mInputChannel != null) {
if (mInputQueueCallback != null) {
mInputQueue = new InputQueue();
mInputQueueCallback.onInputQueueCreated(mInputQueue);
}
mInputEventReceiver = new WindowInputEventReceiver(mInputChannel,
Looper.myLooper());
}
final class WindowInputEventReceiver extends InputEventReceiver {
public WindowInputEventReceiver(InputChannel inputChannel, Looper looper) {
super(inputChannel, looper);
}
@Override
public void onInputEvent(InputEvent event) {
enqueueInputEvent(event, this, 0, true);
}
}
WindowInputEventReceiver mInputEventReceiver;
我们来看看enqueueInputEvent的处理过程ViewRootImpl中有两个成员变量mPendingInputEventHead，mPendingInputEventTail代表输入事件的队列的表头和表尾
QueuedInputEvent mPendingInputEventHead;
QueuedInputEvent mPendingInputEventTail;
void enqueueInputEvent(InputEvent event,
InputEventReceiver receiver, int flags, boolean processImmediately) {
QueuedInputEvent q = obtainQueuedInputEvent(event, receiver, flags);
if (last == null) {
mPendingInputEventHead = q;
mPendingInputEventTail = q;
} else {
last.mNext = q;
mPendingInputEventTail = q;
}
mPendingInputEventCount += 1;
if (processImmediately) {
doProcessInputEvents();
} else {
scheduleProcessInputEvents();
}
}
可以看到将事件event封装成QueuedInputEvent对象之后，将QueuedInputEvent事件q插入mPendingInputEventTail最后无论是调用doProcessInputEvents还是scheduleProcessInputEvents，最终都是调用到doProcessInputEvents函数
doProcessInputEvents为一个while循环，从mPendingInputEventHead表头逐个浏览取出InputEvent,调用deliverInputEvent进行处理。
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);
}
}
那么deliverInputEvent又是如何处理的呢，它比较简单，直接调用InputStage对象stage进行处理，而InputStage是在setView中构造的，如下：
private void deliverInputEvent(QueuedInputEvent q) {
InputStage stage;
if (q.shouldSendToSynthesizer()) {
stage = mSyntheticInputStage;
} else {
stage = q.shouldSkipIme() ? mFirstPostImeInputStage : mFirstInputStage;
}
if (stage != null) {
stage.deliver(q);
} else {
finishInputEvent(q);
}
}
//代码片段如下：
// Set up the input pipeline.
CharSequence counterSuffix = attrs.getTitle();
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);
mFirstInputStage = nativePreImeStage;
mFirstPostImeInputStage = earlyPostImeStage;
InputState一系列链式调用之后最终会调用到ViewPostImeInputStage对象的onProcess函数
final class ViewPostImeInputStage extends InputStage {
public ViewPostImeInputStage(InputStage next) {
super(next);
}
@Override
protected int onProcess(QueuedInputEvent q) {
if (q.mEvent instanceof KeyEvent) {
return processKeyEvent(q);
} else {
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);
}
}
}
}
根据事件类型，屏幕触摸事件是SOURCE_CLASS_POINTER类型，调用到processPointerEvent,而processPointerEvent中的eventTarget为Activity的DecorView，事件最终传递到DecorView的dispatchPointerEvent函数
private int processPointerEvent(QueuedInputEvent q) {
final MotionEvent event = (MotionEvent)q.mEvent;
mAttachInfo.mUnbufferedDispatchRequested = false;
final View eventTarget =
(event.isFromSource(InputDevice.SOURCE_MOUSE) && mCapturingView != null) ?
mCapturingView : mView;
mAttachInfo.mHandlingPointerEvent = true;
boolean handled = eventTarget.dispatchPointerEvent(event);
maybeUpdatePointerIcon(event);
mAttachInfo.mHandlingPointerEvent = false;
if (mAttachInfo.mUnbufferedDispatchRequested && !mUnbufferedInputDispatch) {
mUnbufferedInputDispatch = true;
if (mConsumeBatchedInputScheduled) {
scheduleConsumeBatchedInputImmediately();
}
}
return handled ? FINISH_HANDLED : FORWARD;
}
DecorView继承View，dispatchPointerEvent在View中，view中调用的dispatchTouchEvent，由于继承关系，所以最终调用到了DecorView的dispatchTouchEvent，下面看看DecorView的dispatchTouchEvent
public final boolean dispatchPointerEvent(MotionEvent event) {
if (event.isTouchEvent()) {
return dispatchTouchEvent(event);
} else {
return dispatchGenericMotionEvent(event);
}
}
//DecorView的dispatchTouchEvent
public boolean dispatchTouchEvent(MotionEvent ev) {
final Window.Callback cb = mWindow.getCallback();
return cb != null && !mWindow.isDestroyed() && mFeatureId < 0
? cb.dispatchTouchEvent(ev) : super.dispatchTouchEvent(ev);
}
DecorView中的mWindow是PhoneWindow实例对象，所以dispatchTouchEvent中调用了Window的Callback，而Callback在前文中说过（https://www.jianshu.com/p/76f94c6452c0），它是Activity的attach中设置给PhoneWindow的，所以就调用到了Activity的dispatchTouchEvent，来看看Activity的dispatchTouchEvent
//Activity的dispatchTouchEvent
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
可以看到Activity的dispatchTouchEvent又调用getWindow()的superDispatchTouchEvent(ev)进行事件传递，而getWindow()返回的也是PhoneWindow，所以又将事件传递回了PhoneWindow，来看看Phonewindow的superDispatchTouchEvent调用过程。
//PhoneWindow的superDispatchTouchEvent
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
//mDecor的superDispatchTouchEvent
public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}
PhoneWindow的superDispatchTouchEvent又调用了DecorView的superDispatchTouchEvent，而DecorView的superDispatchTouchEvent调用了dispatchTouchEvent，由于DecorView是继承FrameLayout的，也就是继承了ViewGroup，所以兜了一圈，终于又将事件传递到了Activity的根View即DecorView的dispatchTouchEvent。
下面我们来总结下事件传递的第一步，即事件从WindowManagerService传递到Activity的根视图DecorView的过程
1：ViewRootImpl通过mInputEventReceiver对象接收到InputEvent事件输入，mInputEventReceiver调用onInputEvent处理事件。
2：mInputEventReceiver是WindowInputEventReceiver对象，同时继承InputEventReceiver，所以它的onInputEvent事件处理最终调用到ViewRootImpl的enqueueInputEvent，把事件插入到mPendingInputEventTail表尾中，并调用doProcessInputEvents进行处理事件。
3：doProcessInputEvents处理事件最终调用到了DecorView的dispatchPointerEvent，将事件传递到DecorView。
4：DecorView调用PhoneWindow的Callback又将事件传递到Activity的dispatchTouchEvent。
5：Activity的dispatchTouchEvent又返回调用PhoneWindow的superDispatchTouchEvent将事件传递给PhoneWindow，而PhoneWindow最后将事件传递到DecorView即Activity的根视图的dispatchTouchEvent，dispatchTouchEvent之后就是我们熟悉的View树的递归调用，将事件传递给焦点View，这在后面详解。
事件从Activity的根视图DecorView传递给子View的过程
要说清楚这个先得说下递归和View的继承问题。比如阶乘的递归算法
int factorial（int n）{
if(n<=1) return 1;
else return factorial(n-1);
}
比如n=5时。递归调用是factorial（5） = 5factorial（4）=54factorial（3）=543factorial（2）=5432factorial（1）。由于调用factorial（1）时满足if(n<=1) return 1。所有factorial（5）=54321。
在来举一个继承的例子，有类A和类B，类B继承类A
class A {
public void event(){
log.d("class A event");
}
public void dispatchEvent(){
event();
}
}
class B extends A{
private A[] mChildren;
public void event() {
log.d("class B event");
}
public void dispatchEvent() {
if(mChildren != null) {
for(int i = 0; i < mChildren.length();++i){
mChildren[i].dispatchEvent();
}
event();
}
}
}
比如上面类B的实例b，它的mChildren有3个实例对象，分别是b1，b2和a，那么调用b的成员函数dispatchEvent。for循环就会依次递归调用到b1，b2和a的dispatchEvent，即进入到子类的dispatchEvent中，那么依次输出就是b1的dispatchEvent（）-->event() 输出：class B event，然后b2也是输出：class B event，最后到a，a的类型为类A，dispatchEvent()中最终调用到自身的event()所以输出class A event。最后b对象调用自身event()输出class B event。
所以输出依次为：class B event class B event class A event class B event。
上面例子中类A相当于类View，类B相当于ViewGroup，如果明白了上述的调用过程，基本上明白了View的事件的传递的百分六七十。实际的ViewGroup中可能还有onInterceptTouchEvent事件拦截，以及事件有ACTION_DOWN和ACTION_UP，ACTION_MOVE等不同类型引入了mFirstTouchTarget这个链表对象处理差异。
public boolean onInterceptTouchEvent(MotionEvent ev) {
if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
&& ev.getAction() == MotionEvent.ACTION_DOWN
&& ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
&& isOnScrollbarThumb(ev.getX(), ev.getY())) {
return true;
}
return false;
}
// First touch target in the linked list of touch targets.
private TouchTarget mFirstTouchTarget;
现在直接从ViewGroup的dispatchTouchEvent说起。dispatchTouchEvent代码比较长，截取部分关键代码说明
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
boolean handled = false;
//安全检查，认为返回true，忽略
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
//step1:如果是ACTION_DOWN事件，表明是一个事件序列的开始，reset状态和clear mFirstTouchTarget
// Handle an initial down.
if (actionMasked == MotionEvent.ACTION_DOWN) {
cancelAndClearTouchTargets(ev);
resetTouchState();
}
//step2:检查是否进行事件拦截
// Check for interception.
final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
intercepted = onInterceptTouchEvent(ev);
ev.setAction(action); // restore action in case it was changed
} else {
intercepted = false;
}
} else {
intercepted = true;
}
//step3:检查是否是cancel状态，如果为True，则不进入
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
// Update list of touch targets for pointer down, if needed.
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
TouchTarget newTouchTarget = null;
boolean alreadyDispatchedToNewTouchTarget = false;
if (!canceled && !intercepted) {
View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
? findChildWithAccessibilityFocus() : null;
//step4:检查是否DOWN类事件，是才进入，不是则表示UP或者MOVE类，根据保存的mFirstTouchTarget进
//行事件传递
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
final int actionIndex = ev.getActionIndex(); // always 0 for down
final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
: TouchTarget.ALL_POINTER_IDS;
// Clean up earlier touch targets for this pointer id in case they
// have become out of sync.
removePointersFromTouchTargets(idBitsToAssign);
//step5:注释写的很清楚Find a child that can receive the event.
//find的条件就是事件的坐标是否在View上和View是否能接收事件
final int childrenCount = mChildrenCount;
if (newTouchTarget == null && childrenCount != 0) {
final float x = ev.getX(actionIndex);
final float y = ev.getY(actionIndex);
// Find a child that can receive the event.
// Scan children from front to back.
final ArrayList<View> preorderedList = buildTouchDispatchChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(
preorderedList, children, childIndex);
// If there is a view that has accessibility focus we want it
// to get the event first and if not handled we will perform a
// normal dispatch. We may do a double iteration but this is
// safer given the timeframe.
if (childWithAccessibilityFocus != null) {
if (childWithAccessibilityFocus != child) {
continue;
}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
//find的条件就是事件的坐标是否在View上和View是否能接收事件，这样就避免了逐个dispathch的问题
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
newTouchTarget = getTouchTarget(child);
if (newTouchTarget != null) {
// Child is already receiving touch within its bounds.
// Give it the new pointer in addition to the ones it is handling.
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
resetCancelNextUpFlag(child);
//step6:传递事件到children View，成功则将View保存在mFirstTouchTarget列表中
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
// Child wants to receive touch within its bounds.
mLastTouchDownTime = ev.getDownTime();
if (preorderedList != null) {
// childIndex points into presorted list, find original index
for (int j = 0; j < childrenCount; j++) {
if (children[childIndex] == mChildren[j]) {
mLastTouchDownIndex = j;
break;
}
}
} else {
mLastTouchDownIndex = childIndex;
}
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
// The accessibility focus didn't handle the event, so clear
// the flag and do a normal dispatch to all children.
ev.setTargetAccessibilityFocus(false);
}
if (preorderedList != null) preorderedList.clear();
}
}
}
//step7:根据step4到step6的处理结果。mFirstTouchTarget是否为空，和根据UP，MOVE等事件进行不同的处理
// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else {
// Dispatch to touch targets, excluding the new touch target if we already
// dispatched to it. Cancel touch targets if necessary
TouchTarget predecessor = null;
TouchTarget target = mFirstTouchTarget;
while (target != null) {
final TouchTarget next = target.next;
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
if (cancelChild) {
if (predecessor == null) {
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
}
//step8:UP等事件处理重置状态，准备下一次事件序列流程
// Update list of touch targets for pointer up or cancel, if needed.
if (canceled
|| actionMasked == MotionEvent.ACTION_UP
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
resetTouchState();
} else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
final int actionIndex = ev.getActionIndex();
final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
removePointersFromTouchTargets(idBitsToRemove);
}
}
if (!handled && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
}
return handled;
}
上面的注释step1到step8主要是事件的dispatch过程中的关键处理过程，step1到step3，可以作为一部分解读。step1中针对DOWN类事件对一些标志位和列表作重置处理，step3作一些是否cancel判断。step2作事件拦截处理。如果需要拦截直接跳转到step7。这个时候mFirstTouchTarget == null成立，调用dispatchTransformedTouchEvent(ev,canceled,null,TouchTarget.ALL_POINTER_IDS);第三个参数为空表明没有child。最终调用父类View的dispatchTouchEvent。最终将事件分发给自身处理。这也就是事件拦截的原理过程。
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
// Perform any necessary transformations and dispatch.
//child == null为空成立调用父类View的dispatchTouchEvent
if (child == null) {
handled = super.dispatchTouchEvent(transformedEvent);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
transformedEvent.offsetLocation(offsetX, offsetY);
if (! child.hasIdentityMatrix()) {
transformedEvent.transform(child.getInverseMatrix());
}
handled = child.dispatchTouchEvent(transformedEvent);
}
// Done.
transformedEvent.recycle();
return handled;
}
View的dispatchTouchEvent
public boolean dispatchTouchEvent(MotionEvent event) {
boolean result = false;
if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
}
return result;
}
可以看到View的dispatchTouchEvent中先会调用TouchListener设置的回调，比方说的Button等的回调就是这个时候调用。如果回调返回true表示将事件消耗掉了，就直接返回。如果返回false就调用自身的onTouchEvent处理事件。
如果step2不拦截，则进入step4到step6，选择一个或者多个子View作为事件的分发对象，递归调用dispatchTouchEvent。如果子View在dispatchTouchEvent过程中消耗了事件返回true，则父View也直接返回，不处理事件。如果子View未处理事件。则父View处理事件。
至此我们明白的一个ViewGroup分发事件的过程。1：先查看是否拦截，a：如果拦截，调用自身父类的dispatchTouchEvent分发事件，这之中会调用TouchListener设置的回调或者onTouchEvent处理事件，返回处理result。b：如果不拦截，会选择事件坐标所在的View或者ViewGroup进行事件分发(可能有多个View，依次for循环调用)。然后递归重复上面的过程。
最后来总结下Activity的dispatchTouchEvent函数调用，事件的分发和处理过程
1：Activity的dispatchTouchEvent中调用getWindow()的superDispatchTouchEvent将事件分发给PhoneWindow
2：PhoneWindow中调用成员变量mDecor（DecorView对象）的superDispatchTouchEvent将事件分发给DecorView
3：DecorView作为Activity的根视图同时作为一个ViewGroup。调用dispatchTouchEvent传递事件。
4：DecorView的dispatchTouchEvent分发事件，根据onInterceptTouchEvent觉得是否拦截事件，拦截自己最终调用onTouchEvent处理事件。不拦截根据选择事件坐标所在View范围内的View分发事件，子View根据是否是最终子View递归重复该过程。处理之后依次向上层返回。上层View根据处理result，为true继续上传至根布局。为false调用OnTouchEvent处理事件，然后返回结果。
5：DecorView的dispatchTouchEvent最终处理完成，返回到Activity，Activity根据处理结果为true向上返回，为false调用自身的onTouchEvent处理事件，然后接着向上返回到ViewRootImpl。
