android事件分发机制的实现原理
作者：骑着毛驴追宝马
android中的事件处理，以及解决滑动冲突问题都离不开事件分发机制，android中的事件流，即MotionEvent都会经历一个从分发，拦截到处理的一个过程。即dispatchTouchEvent(),onInterceptEvent()到onTouchEvent()的一个过程，在dispatchTouchEvent()负责了事件的分发过程，在dispatchTouchEvent()中会调用onInterceptEvent()与onTouchEvent()，如果onInterceptEvent()返回true，那么会调用到当前view的onTouchEvent()方法，如果不拦截，事件就会下发到子view的dispatchTouchEvent()中进行同样的操作。本文将带领大家从源码角度来分析android是如何进行事件分发的。
android中的事件分发流程最先从activity的dispatchTouchEvent()开始:
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWidow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
这里调用了getWindow().superDispatchTouchEvent(ev)，这里可以看出activity将MotionEvent传寄给了Window。而Window是一个抽象类，superDispatchTouchEvent()也是一个抽象方法，这里用到的是window的子类phoneWindow。
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
从这里可以看出，event事件被传到了DecorView,也就是我们的顶层view.我们继续跟踪：
public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}
这里调用到了父类的dispatchTouchEvent()方法，而DecorView是继承自FrameLayout，FrameLayout继承了ViewGroup,所以这里会调用到ViewGroup的dispatchTouchEvent()方法。
所以整个事件流从activity开始，传递到window，最后再到我们的view(viewGroup也是继承自view)中，而view才是我们整个事件处理的核心阶段。
我们来看一下viewGroup的dispatchTouchEvent()中的实现：
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Throw away all previous state when starting a new touch gesture.
// The framework may have dropped the up or cancel event for the previous gesture
// due to an app switch, ANR, or some other state change.
cancelAndClearTouchTargets(ev);
resetTouchState();
}
这是dispatchTouchEvent()开始时截取的一段代码，我们来看一下，首先，当我们手指按下view时，会调用到resetTouchState()方法，在resetTouchState()中：
private void resetTouchState() {
clearTouchTargets();
resetCancelNextUpFlag(this);
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
mNestedScrollAxes = SCROLL_AXIS_NONE;
}
我们继续跟踪clearTouchTargets()方法：
private void clearTouchTargets() {
TouchTarget target = mFirstTouchTarget;
if (target != null) {
do {
TouchTarget next = target.next;
target.recycle();
target = next;
} while (target != null);
mFirstTouchTarget = null;
}
}
在clearTouchTargets()方法中，我们最终将mFirstTouchTarget赋值为null，我们继续回到dispatchTouchEvent()中，接着执行了下段代码：
// 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 {
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
intercepted = true;
}
当view被按下或mFirstTouchTarget != null 的时候，从前面可以知道，当每次view被按下时，也就是重新开始一次事件流的处理时，mFirstTouchTarget都会被设置成null，一会我们看mFirstTouchTarget是什么时候被赋值的。
从disallowIntercept属性我们大概能猜到是用来判断是否需要坐拦截处理，而我们知道可以通过调用父view的requestDisallowInterceptTouchEvent(true)可以让我们的父view不能对事件进行拦截，我们先来看看requestDisallowInterceptTouchEvent()方法中的实现：
@Override
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {
if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {
// We're already in this state, assume our ancestors are too
return;
}
if (disallowIntercept) {
mGroupFlags |= FLAG_DISALLOW_INTERCEPT;
} else {
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
}
// Pass it up to our parent
if (mParent != null) {
mParent.requestDisallowInterceptTouchEvent(disallowIntercept);
}
}
这里也是通过设置标志位做判断处理，所以这里是通过改变mGroupFlags标志，然后在dispatchTouchEvent()刚发中变更disallowIntercept的值判断是否拦截，当为true时，即需要拦截，这个时候便会跳过onInterceptTouchEvent()拦截判断，并标记为不拦截，即intercepted = false，我们继续看viewGroup的onInterceptTouchEvent()处理：
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;
}
即默认情况下，只有在ACTION_DOWN时，viewGroup才会表现为拦截。
我们继续往下看：
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;
}
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);
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();
}
这段代码首先会通过一个循环去遍历所有的子view，最终会调用到dispatchTransformedTouchEvent()方法，我们继续看dispatchTransformedTouchEvent()的实现：
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
// Canceling motions is a special case. We don't need to perform any transformations
// or filtering. The important part is the action, not the contents.
final int oldAction = event.getAction();
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
event.setAction(MotionEvent.ACTION_CANCEL);
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}
event.setAction(oldAction);
return handled;
}
// Calculate the number of pointers to deliver.
final int oldPointerIdBits = event.getPointerIdBits();
final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;
// If for some reason we ended up in an inconsistent state where it looks like we
// might produce a motion event with no pointers in it, then drop the event.
if (newPointerIdBits == 0) {
return false;
}
// If the number of pointers is the same and we don't need to perform any fancy
// irreversible transformations, then we can reuse the motion event for this
// dispatch as long as we are careful to revert any changes we make.
// Otherwise we need to make a copy.
final MotionEvent transformedEvent;
if (newPointerIdBits == oldPointerIdBits) {
if (child == null || child.hasIdentityMatrix()) {
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
event.offsetLocation(offsetX, offsetY);
handled = child.dispatchTouchEvent(event);
event.offsetLocation(-offsetX, -offsetY);
}
return handled;
}
transformedEvent = MotionEvent.obtain(event);
} else {
transformedEvent = event.split(newPointerIdBits);
}
// Perform any necessary transformations and dispatch.
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;
}
这段代码就比较明显了，如果child不为null，始终会调用到child.dispatchTouchEvent();否则调用super.dispatchTouchEvent();
如果child不为null时，事件就会向下传递，如果子view处理了事件，即dispatchTransformedTouchEvent()即返回true。继续向下执行到addTouchTarget()方法，我们继续看addTouchTarget()方法的执行结果：
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
target.next = mFirstTouchTarget;
mFirstTouchTarget = target;
return target;
}
这个时候我们发现mFirstTouchTarget又出现了，这时候会给mFirstTouchTarget重新赋值，即mFirstTouchTarget不为null。也就是说，如果事件被当前view或子view消费了，那么在接下来的ACTION_MOVE或ACTION_UP事件中，mFirstTouchTarget就不为null。但如果我们继承了该viewGroup，并在onInterceptTouchEvent()的ACTION_MOVE中拦截了事件，那么后续事件将不会下发，将由该viewGroup直接处理，从下面代码我们可以得到：
// 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;
}
当存在子view并且事件被子view消费时，即在ACTION_DOWN阶段mFirstTouchTarget会被赋值，即在接下来的ACTION_MOVE事件中，由于intercepted为true，所以将ACTION_CANCEL 事件传递过去，从dispatchTransformedTouchEvent()中可以看到：
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
event.setAction(MotionEvent.ACTION_CANCEL);
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}
event.setAction(oldAction);
return handled;
}
并将mFirstTouchTarget 最终赋值为 next，而此时mFirstTouchTarget位于TouchTarget链表尾部，所以mFirstTouchTarget会赋值为null，那么接下来的事件将不会进入到onInterceptTouchEvent()中。也就会直接交由该view处理。
如果我们没有进行事件的拦截，而是交由子view去处理，由于ViewGroup的onInterceptTouchEvent()默认并不会拦截除了ACTION_DOWN以外的事件，所以后续事件将继续交由子view去处理，如果存在子view且事件位于子view内部区域的话。
所以无论是否进行拦截，事件流都会交由view的dispatchTouchEvent()中进行处理，我们接下来跟踪一下view中的dispatchTouchEvent()处理过程：
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Defensive cleanup for new gesture
stopNestedScroll();
}
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//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;
}
}
当被按下时，即ACTION_DOWN时，view会停止内部的滚动，如果view没有被覆盖或遮挡时，首先会进行mListenerInfo是否为空的判断，我们看下mListenerInfo是在哪里初始化的：
ListenerInfo getListenerInfo() {
if (mListenerInfo != null) {
return mListenerInfo;
}
mListenerInfo = new ListenerInfo();
return mListenerInfo;
}
这里可以看出，mListenerInfo一般不会是null，知道在我们使用它时调用过这段代码，而当view被加入window中的时候，会调用下面这段代码，从注释中也可以看出来：
/**
* Add a listener for attach state changes.
*
* This listener will be called whenever this view is attached or detached
* from a window. Remove the listener using
* {@link #removeOnAttachStateChangeListener(OnAttachStateChangeListener)}.
*
* @param listener Listener to attach
* @see #removeOnAttachStateChangeListener(OnAttachStateChangeListener)
*/
public void addOnAttachStateChangeListener(OnAttachStateChangeListener listener) {
ListenerInfo li = getListenerInfo();
if (li.mOnAttachStateChangeListeners == null) {
li.mOnAttachStateChangeListeners
= new CopyOnWriteArrayList<OnAttachStateChangeListener>();
}
li.mOnAttachStateChangeListeners.add(listener);
}
到这里我们就知道，mListenerInfo一开始就是被初始化好了的，所以li不可能为null，li.mOnTouchListener != null即当设置了TouchListener时不为null，并且view是enabled状态，一般情况view都是enable的。这个时候会调用到onTouch()事件，当onTouch()返回true时，这个时候result会赋值true。而当result为true时，onTouchEvent()将不会被调用。
从这里可以看出，onTouch()会优先onTouchEvent()调用；
当view设置touch监听并返回true时，那么它的onTouchEvent()将被屏蔽。否则会调用onTouchEvent()处理。
那么让我们继续来看看onTouchEvent()中的事件处理：
if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return (((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);
}
首先，当view状态是DISABLED时，只要view是CLICKABLE或LONG_CLICKABLE或CONTEXT_CLICKABLE，都会返回true,而button默认是CLICKABLE的，textview默认不是CLICKABLE的，而view一般默认都不是LONG_CLICKABLE的。
我们继续向下看：
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
如果有代理事件，仍然会返回true.
if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
(viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
switch (action) {
case MotionEvent.ACTION_UP:
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
// take focus if we don't have it already and we should in
// touch mode.
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}
if (prepressed) {
// The button is being released before we actually
// showed it as pressed. Make it show the pressed
// state now (before scheduling the click) to ensure
// the user sees it.
setPressed(true, x, y);
}
if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
// This is a tap, so remove the longpress check
removeLongPressCallback();
// Only perform take click actions if we were in the pressed state
if (!focusTaken) {
// Use a Runnable and post this rather than calling
// performClick directly. This lets other visual state
// of the view update before click actions start.
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClick();
}
}
}
if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}
if (prepressed) {
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_DOWN:
mHasPerformedLongPress = false;
if (performButtonActionOnTouchDown(event)) {
break;
}
// Walk up the hierarchy to determine if we're inside a scrolling container.
boolean isInScrollingContainer = isInScrollingContainer();
// For views inside a scrolling container, delay the pressed feedback for
// a short period in case this is a scroll.
if (isInScrollingContainer) {
mPrivateFlags |= PFLAG_PREPRESSED;
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPendingCheckForTap.x = event.getX();
mPendingCheckForTap.y = event.getY();
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true, x, y);
checkForLongClick(0, x, y);
}
break;
case MotionEvent.ACTION_CANCEL:
setPressed(false);
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
case MotionEvent.ACTION_MOVE:
drawableHotspotChanged(x, y);
// Be lenient about moving outside of buttons
if (!pointInView(x, y, mTouchSlop)) {
// Outside button
removeTapCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
// Remove any future long press/tap checks
removeLongPressCallback();
setPressed(false);
}
}
break;
}
return true;
}
当view是CLICKABLE或LONG_CLICKABLE或CONTEXT_CLICKABLE状态时，当手指抬起时，如果设置了click监听，最终会调用到performClick()，触发click()事件。这点从performClick()方法中可以看出：
public boolean performClick() {
final boolean result;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
return result;
}
从这里我们也可以得出，click事件会在onTouchEvent()中被调用，如果view设置了onTouch()监听并返回true，那么click事件也会被屏蔽掉，不过我们可以在onTouch()中通过调用view的performClick()继续执行click()事件，这个就看我们的业务中的需求了。
从这里我们可以看出，如果事件没有被当前view或子view处理，即返回false，那么事件就会交由外层view继续处理，直到被消费。
如果事件一直没有被处理，会最终传递到Activity的onTouchEvent()中。
到这里我们总结一下：
事件是从Activity->Window->View(ViewGroup)的一个传递流程；
如果事件没有被中途拦截，那么它会一直传到最内层的view控件；
如果事件被某一层拦截，那么事件将不会向下传递，交由该view处理。如果该view消费了事件，那么接下来的事件也会交由该view处理；如果该view没有消费该事件，那么事件会交由外层view处理，...并最终调用到activity的onTouchEvent()中，除非某一层消费了该事件；
一个事件只能交由一个view处理；
DispatchTouchEvent()总是会被调用，而且最先被调用，onInterceptTouchEvent()和onTouchEvent()在DispatchTouchEvent()内部调用；
子view不能干扰ViewGroup对ACTION_DOWN事件的处理；
子view可以通过requestDisallowInterceptTouchEvent(true)控制父view不对事件进行拦截，跳过onInterceptTouchEvent()方法的执行。
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持脚本之家。