android-----事件分发机制
我们首先从一个简单的例子开始分析
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
System.out.println("执行onClick");
}
});
button.setOnTouchListener(new View.OnTouchListener() {
@Override
public boolean onTouch(View view, MotionEvent event) {
System.out.println("执行onTouch的"+event.getAction());
return false;
}
});
这段代码经常出现在我们绑定按钮监听事件的情况下，但是你能确定当同时为按钮绑定onClick和onTouch事件的情况下会首先执行哪个事件么？我们来看看输出结果：
04-29 22:52:43.411: I/System.out(845): 执行onTouch的0
04-29 22:52:43.481: I/System.out(845): 执行onTouch的1
04-29 22:52:43.572: I/System.out(845): 执行onClick
也许你的手会抖几下，那么你的输出结果将会多几个执行onTouch的2的值；
但是不管怎么说，首先还是会先执行的onTouch方法，随后onTouch执行结束才会执行onClick方法，除此之外，我们还发现onTouch方法是有返回值的，上面实例中，返回值是false，假如我们将该返回值设置为true的话，程序的输出结果是：
04-29 23:25:15.082: I/System.out(966): 执行onTouch的0
04-29 23:25:15.161: I/System.out(966): 执行onTouch的1
不管你怎么点击按钮，都不会执行onClick方法，这更加印证了onTouch优先于onClick方法执行，当onTouch返回值设置为true的时候我们可以理解为onTouch方法将事件消费掉了，不再会传递下去；
平常的应用就是这样啦，接下来从源码的角度分析一下造成这种情况的原因：
首先分析之前应该明白的一点就是触摸屏事件首先传递到的是ViewGroup的，他是我们平常的各种布局的父类或者间接父类，这点可以从android官方文档中看到：
可以看出，LinearLayout以及各种布局文件都是他的子类，ViewGroup是一组View的集合，他包含很多子View和子ViewGroup，因此在探讨ViewGroup的事件分发机制之前我们首先应该看看View的事件分发机制；
首先应该明白的是每当触摸一个控件的时候都会调用该控件的dispatchTouchEvent方法，以上面的Button类为例，发现Button类中不存在dispatchTouchEvent方法，那么就到他的父类TextView中寻找，发现TextView中也不存在这个方法，随后到TextView的父类View中去寻找，终于找到啦！
public boolean dispatchTouchEvent(MotionEvent event) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
return true;
}
if (onTouchEvent(event)) {
return true;
}
}
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
return false;
}
这段源码的核心代码在第9行，他会首先查看下li是否为null以及li中的mOnTouchListener是否为null，(mViewFlags & ENABLED_MASK)==ENABLE判断当前控件是否是ENABLE的，那么这里的li到底是什么呢？第8行看到，li是等于 mListenerInfo的ListenerInfo对象，那么mListenerInfo是在哪里赋值的呢？
ListenerInfo getListenerInfo() {
if (mListenerInfo != null) {
return mListenerInfo;
}
mListenerInfo = new ListenerInfo();
return mListenerInfo;
}
可以看到是调用getListenerInfo方法来生成的，那么li中的属性mOnTouchListener是在哪里赋值的呢？
public void setOnTouchListener(OnTouchListener l) {
getListenerInfo().mOnTouchListener = l;
}
发现就是将我们平常使用setOnTouchListener的参数设置为了li的mOnTouchListener属性，那么当li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED为true的时候会执行最后一个条件判断语句，也就是我们在setOnTouchListener里面经常重写的那个方法onTouch啦，这个方法会有一个返回值，当返回值为true的时候执行if语句块中的内容，直接返回true，那么dispatchTouchEvent方法执行结束，也就是整个事件分发已经结束了，所以上面例子中当我们将onTouch方法的返回值设置为true的时候，并不会执行onClick方法，因为程序在这个if语句块中已经返回了；
那么我们有理由相信第14行的if语句块中的onTouchEvent方法肯定与onClick方法有关系，来看看源码：
public boolean onTouchEvent(MotionEvent event) {
final int viewFlags = mViewFlags;
if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (event.getAction() == 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));
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
switch (event.getAction()) {
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);
}
if (!mHasPerformedLongPress) {
// 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();
}
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();
}
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true);
checkForLongClick(0);
}
break;
case MotionEvent.ACTION_CANCEL:
setPressed(false);
removeTapCallback();
removeLongPressCallback();
break;
case MotionEvent.ACTION_MOVE:
final int x = (int) event.getX();
final int y = (int) event.getY();
// 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;
}
return false;
}
这个方法有点长，我们仅看核心部分，首先第4行的if语句的作用在于可能当前View是disabled的，但是仍然是可点击的，这种情况下我们同样需要消费掉这个Touch事件，只不过不对这个Touch事件做任何事情而已，只是返回true而已；第20行，如果当前控件是可点击的话，则进入switch里面判断当前事件的类型，我们进入MotionEvent.ACTION_UP里面查看将会发现经过层层if判断，最终将会执行第54行的performClick方法，进入这个方法里面看看执行了些什么操作：
public boolean performClick() {
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
return true;
}
return false;
}
第5行判断li以及他的属性mOnClickListener是否为null，聪明的你一定会猜到mOnClickListener的赋值操作和前面的mOnTouchListener将是一样的
public void setOnClickListener(OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}
没错吧，最后执行他的onClick方法就行啦，当然平时我们是重写这个方法的；
一切都变的那么明了了，其实如果说onClick方法执行在onTouch方法之后具体有多后呢？刚刚就已经看出来了，在我们的手指即将离开控件的时候，即MotionEvent.ACTION_UP情况下才会执行onClick方法；
好啦，View的事件分发过程就已经介绍结束啦，我们做个总结一下马上看看事件是怎么从ViewGroup传递到View的：
首先在每个控件View上面触发事件的时候都会首先执行dispatchTouchEvent方法，接着在这个方法里面首先会查看是否有touch事件绑定，有的话直接执行该touch事件的onTouch方法，并且如果onTouch方法返回的是true的话，则dispatchTouchEvent方法直接返回true，执行结束；如果没有绑定touch事件或者onTouch方法返回false的话，则执行onTouchEvent方法，在这个方法switch的MotionEvent.ACTION_UP这个case情况下调用performClick，进而调用onClick方法执行点击事件，执行结束之后返回true,最后dispatchTouchEvent方法返回，事件分发结束；
前面已经提到过事件是从ViewGroup传递到View的，那么这个过程又是怎么的呢？
来，我们看个例子：
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
System.out.println("执行了Button的onClick方法");
}
});
relativeLayout.setOnTouchListener(new View.OnTouchListener() {
@Override
public boolean onTouch(View arg0, MotionEvent arg1) {
System.out.println("执行了RelativeLayout的onTouch方法,action为: "+arg1.getAction());
return false;
}
});
我们分别为Button以及RealtiveLayout设置了Click监听事件
点击button的输出为：
04-30 02:46:45.131: I/System.out(1164): 执行了Button的onClick方法
点击屏幕空白区域的输出为：
04-30 02:52:57.305: I/System.out(1220): 执行了RelativeLayout的onTouch方法,action为: 0
也就是当我们直接点击button按钮的时候是不会执行RelativeLayout的onTouch方法的，在这里Button按钮相当于是View，而RelativeLayout相当于ViewGroup，前面已经提到事件是从ViewGroup传递到View的，事实依据就是：
假如我们认为是从View传递到ViewGroup，那么只要我们找到一种情况就是点击View控件的时候触发的只是ViewGroup绑定的方法而不是View绑定的方法的话，我们就可以认为是ViewGroup消费了事件，因此事件是ViewGroup到View的，接下来举出这样的例子：
我们自己来实现一个RelativeLayout,并且重写onInterceptTouchEvent方法，这个方法的原型是在ViewGroup里面的，原型见下：
public boolean onInterceptTouchEvent(MotionEvent ev) {
return false;
}
就是这么简单，只是返回false而已；
我们自定义的RelativeLayout命名为MyRelativeLayout:
public class MyRelativeLayout extends RelativeLayout{
public MyRelativeLayout(Context context) {
super(context);
}
public MyRelativeLayout(Context context, AttributeSet attrs) {
super(context,attrs);
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
return true;
}
}
可见我们把onInterceptTouchEvent的返回值修改成了true,在将自定义布局引入到布局文件之后，重新执行程序会发现不管我们是点击button按钮还是点击空白区域输出的结果都是：
04-30 03:12:36.901: I/System.out(1310): 执行了RelativeLayout的onTouch方法,action为: 0
这也就是说当我们把onInterceptTouchEvent的返回值修改成了true，可以理解为当前布局也就是ViewGroup消费了事件，事件没有再向下传递了，也就说明了事件是从ViewGroup传递到View的啦！
那么上面我们说过触发View控件的事件首先会触发其dispatchTouchEvent事件，那么其实实际上呢，触发这个方法的本质上就已经可以说成是ViewGroup触发的啦，那么我们进入ViewGroup的源码中看看他的dispatchTouchEvent方法是什么样子的呢？
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
// Handle an initial down.
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();
}
// 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;
}
// Check for cancelation.
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) {
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);
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 View[] children = mChildren;
final boolean customOrder = isChildrenDrawingOrderEnabled();
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = customOrder ?
getChildDrawingOrder(childrenCount, i) : i;
final View child = children[childIndex];
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
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();
mLastTouchDownIndex = childIndex;
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
}
}
if (newTouchTarget == null && mFirstTouchTarget != null) {
// Did not find a child to receive the event.
// Assign the pointer to the least recently added target.
newTouchTarget = mFirstTouchTarget;
while (newTouchTarget.next != null) {
newTouchTarget = newTouchTarget.next;
}
newTouchTarget.pointerIdBits |= idBitsToAssign;
}
}
}
// 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;
}
}
// 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;
}
同样源码好长，我们只看核心：
第46行我们看到if判断的条件是只要canceled和intercepted都为false的话，就会执行if语句块的内容，而intercepted的值正是前面我们通过onInterceptTouchEvent方法返回的值，那么我们有理由相信按钮的点击事件肯定是在这个if语句块中执行的，因为前面我们将onInterceptTouchEvent的返回值改为true之后，if不会执行其语句块，也就意味着直接屏蔽了按钮点击事件了，在if语句块中经过层层计算最后到达第85行执行了dispatchTransformedTouchEvent方法，这个方法也属于ViewGroup，在这个方法里面肯定会出现调用View的dispatchTouchEvent方法，我们可以看看dispatchTransformedTouchEvent的源码：
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;
}
同样挺长的，我们只看重点，在第11行、13行、37行、43行、56行、65行可能会执行不同判断条件下的View的dispatchTouchEvent方法，但是不管怎么说这个方法是从ViewGroup方法里面的
dispatchTouchEvent调用的，接下来的事件分发就是最开始我们介绍的View的事件分发过程啦，至此整个事件分发过程介绍结束！
总结一下整个事件分发流程是这样子的：
ViewGroup(所有布局的父类)通过调用自己的dispatchTouchEvent方法，将事件传递给自己里面的View，View自己调用自己的dispatchTouchEvent方法来进行事件处理，细节方面上面已经总结过啦！