ViewGroup的事件分发机制
引言
上一次我在View的事件分发机制里完整的分析了View对于触屏点击事件的分发过程，接下来继续探索之旅，紧接着分析ViewGroup的事件分发机制，ViewGroup其实就是一组View的集合，它也是继承于View的，它本身也可以包含View和ViewGroup，方便起见我们还是延用上一次的布局，不过这一次我们给根布局也设置了点击事件和触摸事件：
public class MainActivity extends Activity implements OnClickListener,OnTouchListener{
private RelativeLayout re_Layout;
private Button btn;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btn=(Button) findViewById(R.id.btn);
re_Layout=(RelativeLayout) findViewById(R.id.re_layout);
btn.setOnClickListener(this);
btn.setOnTouchListener(this);
re_Layout.setOnClickListener(this);
re_Layout.setOnTouchListener(this);
}
@Override
public void onClick(View v) {
Log.d("TAG", "OnClick--"+v);
}
@Override
public boolean onTouch(View v, MotionEvent event) {
Log.d("TAG", "onTouch--"+event.getAction()+"--"+v);
return false;
}
}效果图也依旧没有变，现在我们先点击一下Button,查看Log输出：
可以很清楚的看到这里和上一节分析得情况一样：当点击事件发生时onTouch()方法是优先于onClick()方法执行的，并且如果onTouch()返回False既不消耗点击事件那么如果控件设置了setOnClickListener最终是会执行到onClick()方法的，可是我很好奇ViewGroup的点击事件和View的到底有什么区别，点击事件事件的分发到底是从ViewGroup开始还是从View开始的呢，于是我点击了Button以外的空白区域，捕捉到如下信息(注:我的根布局就是RelativeLayout)：
说明根布局也就是Viewroup也是可以响应点击事件的，但是我们点击View的时候为什么没有ViewGroup的Log输出，这是不是说明android事件分发是先传到View的，当View消耗的这个事件它的ViewGroup就无法接收这个事件了呢，为了彻底的谈清楚原因，我们先重写一个ViewGroup，然后重写这个ViewGroup里面的onInterceptTouchEvent(MotionEvent ev)、dispatchTouchEvent(MotionEvent event)还有onTouchEvent(MotionEvent event)这三个方法通过Log输出信息来判断：
public class MyLayout extends RelativeLayout{
public MyLayout(Context context,AttributeSet attrs) {
super(context,attrs);
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
Log.d("TAG", ev.getAction()+" action"+"MyLayout onInterceptTouchEvent");
return super.onInterceptTouchEvent(ev);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
Log.d("TAG", event.getAction()+" action"+"MyLayout onTouchEvent");
return super.onTouchEvent(event);
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
Log.d("TAG", ev.getAction()+" action"+"MyLayout dispatchTouchEvent");
return super.dispatchTouchEvent(ev);
}同理我们还需要重写一个Button：
public class MyButton extends Button{
public MyButton(Context context,AttributeSet attrs){
super(context,attrs);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
Log.d("TAG", event.getAction()+" action"+"MyButton onTouchEvent");
return super.onTouchEvent(event);
}
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
Log.d("TAG", ev.getAction()+" action"+"MyButton dispatchTouchEvent");
return super.dispatchTouchEvent(ev);
}
//这里注意View是没有onInterceptTouchEvent方法的效果图是一样的这里就不再贴了，为了验证刚才的想法我们直接点击一下界面上的Button，Log输出如下：
发现了什么，我们点击的是Button,然而这个事件最开始是传到了我们的根布局MyLayout，并且还按照：
dispatchTouchEvent、onInterceptTouchEvent、dispatchTouchEvent的顺序执行，紧接着执行View的onTouch()和onTouchEvent()方法，还有一点很奇怪的事只有最开始ACTION_DOWN的时候调用了ViewGroup的onInterceptTouchEvent方法，在后面的ACTION_UP事件派发过程中却没有调用，这里给出一个合理的猜想：一旦一个View开始处理这个触摸事件，那么接下来的ACTION_MOVE和ACTION_UP事件都会交给它去处理，就好比你在公司里面做事，分到你做的事你已经做了一些，那么接下来的事你的完完整整的做好，那么如果做到一半不做了会怎么样(即View不消耗ACTION_DOWN事件)？我们可以大胆的假设如果上级交给你做的事没有做好，那么上级_在短期内肯定不敢交代事情给你做了(后续的ACTION_MOVE、ACTION_DOWN事件这个View都接收不到了)，那么究竟如何我们还是从源码看起。
对源码的分析
我们已经知道当一个点击操作发生时事件是先传给ViewGroup处理的并且首先执行的是ViewGroup的dispatchTouchEvent,那么我们就先来看看它的源码：
/** * {@inheritDoc} */
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
// If the event targets the accessibility focused view and this is it, start
// normal event dispatch. Maybe a descendant is what will handle the click.
if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
ev.setTargetAccessibilityFocus(false);
}
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;
}
// If intercepted, start normal event dispatch. Also if there is already
// a view that is handling the gesture, do normal event dispatch.
if (intercepted || mFirstTouchTarget != null) {
ev.setTargetAccessibilityFocus(false);
}
// 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 the event is targeting accessiiblity focus we give it to the
// view that has accessibility focus and if it does not handle it
// we clear the flag and dispatch the event to all children as usual.
// We are looking up the accessibility focused host to avoid keeping
// state since these events are very rare.
View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
? findChildWithAccessibilityFocus() : null;
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 ArrayList<View> preorderedList = buildOrderedChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = customOrder
? getChildDrawingOrder(childrenCount, i) : i;
final View child = (preorderedList == null)
? children[childIndex] : preorderedList.get(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();
}
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;
}
相当长，还是一点点来看，源码这种东西看不懂肯定会觉得很枯燥，所以能弄懂的尽量弄懂，最开始只是一些对View是否可以获得焦点的判断、设置标志位以及初始化一些布尔值，并且在ACTION_DOWN事件产生的时候清楚以外的状态并且准备开始新一轮的手势操作，不重要：
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
// If the event targets the accessibility focused view and this is it, start
// normal event dispatch. Maybe a descendant is what will handle the click.
if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
ev.setTargetAccessibilityFocus(false);
}
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();
}先看这一段：
// 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;
}这里首先设立了一个布尔值interception去判断当前ViewGroup是否要拦截View的点击事件，if条件语句中的内容是当产生ACTION_DOWN按下事件或者mFirstTouchTarget != null的时候去判断是否要拦截当前事件，这里主要关注mFirstTouchTarget != null这个点，我们找一找哪个方法跟这个mFirstTouchTarget变量有关，还真给我找到了，看下面：
private void clearTouchTargets() {
TouchTarget target = mFirstTouchTarget;
if (target != null) {
do {
TouchTarget next = target.next;
target.recycle();
target = next;
} while (target != null);
mFirstTouchTarget = null;
}
}/** * Cancels and clears all touch targets. */
private void cancelAndClearTouchTargets(MotionEvent event) {
if (mFirstTouchTarget != null) {
boolean syntheticEvent = false;
if (event == null) {
final long now = SystemClock.uptimeMillis();
event = MotionEvent.obtain(now, now,
MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0);
event.setSource(InputDevice.SOURCE_TOUCHSCREEN);
syntheticEvent = true;
}
for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
resetCancelNextUpFlag(target.child);
dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits);
}
clearTouchTargets();
if (syntheticEvent) {
event.recycle();
}
}
}这两段代码结合起来，在加上在ACTION_DOWN初始时候是调用了cancelAndClearTouchTargets(MotionEvent event)这个方法的，所以我们可以推荐起初这个mFirstTouchTarget 的值是null的，那么mFirstTouchTarget是在哪里赋值的呢，我们在dispatchTouchEvent(MotionEvent ev)接着往下看：
newTouchTarget = addTouchTarget(child, idBitsToAssign);我们看到newTouchTarget是在这里赋值的，看一下addTouchTarget方法：
/** * Adds a touch target for specified child to the beginning of the list. * Assumes the target child is not already present. */
private TouchTarget addTouchTarget(View child, int pointerIdBits) {
TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
target.next = mFirstTouchTarget;
mFirstTouchTarget = target;
return target;
}从该方法的内部结构可以看出，mFirstTouchTarget其实是一中单链表结构，如果找到了处理该点击事件的子View那么mFirstTouchTarget就会被赋值并且会指向子元素。
这一下弄清楚了回到刚才的那段代码：
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;
} final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;这个布尔值是判断子元素是否调用了requestDisallowInterceptTouchEvent这个方法，如果调用了这个布尔值就为True，这里看一眼这个方法的代码：
/** * {@inheritDoc} */
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);
}
}你可以在子View中调用这个方法来让ViewGroup不拦截除了ACTION_DOWN以外的点击事件，这里为什么说是ACTION_DOWN以外呢，因为ViewGroup在分发事件的时候最开始是会重置FLAG_DISALLOW_INTERCEPT这个标志位的，所以无论你有没有在子View中设置requestDisallowInterceptTouchEvent方法都不会影响到ViewGroup去拦截ACTION_DOWN事件的，接着往下看：
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;
}如果子View没有设置requestDisallowInterceptTouchEvent方法那么就调用ViewGroup的onInterceptTouchEvent(ev)方法，我们找到这个方法：
/**
* Implement this method to intercept all touch screen motion events. This
* allows you to watch events as they are dispatched to your children, and
* take ownership of the current gesture at any point.
*
* <p>Using this function takes some care, as it has a fairly complicated
* interaction with {@link View#onTouchEvent(MotionEvent)
* View.onTouchEvent(MotionEvent)}, and using it requires implementing
* that method as well as this one in the correct way. Events will be
* received in the following order:
*
* <ol>
* <li> You will receive the down event here.
* <li> The down event will be handled either by a child of this view
* group, or given to your own onTouchEvent() method to handle; this means
* you should implement onTouchEvent() to return true, so you will
* continue to see the rest of the gesture (instead of looking for
* a parent view to handle it). Also, by returning true from
* onTouchEvent(), you will not receive any following
* events in onInterceptTouchEvent() and all touch processing must
* happen in onTouchEvent() like normal.
* <li> For as long as you return false from this function, each following
* event (up to and including the final up) will be delivered first here
* and then to the target's onTouchEvent().
* <li> If you return true from here, you will not receive any
* following events: the target view will receive the same event but
* with the action {@link MotionEvent#ACTION_CANCEL}, and all further
* events will be delivered to your onTouchEvent() method and no longer
* appear here.
* </ol>
*
* @param ev The motion event being dispatched down the hierarchy.
* @return Return true to steal motion events from the children and have
* them dispatched to this ViewGroup through onTouchEvent().
* The current target will receive an ACTION_CANCEL event, and no further
* messages will be delivered here.
*/
public boolean onInterceptTouchEvent(MotionEvent ev) {
return false;
}唔，注释相当长，但是有用的就一个返回值，这里返回False，说明ViewGrup不拦截点击事件，事件可以继续往下传递，这个方法的最后，如果当前界面除了一个ViewGroup没有任何子View，那么此时ViewGroup也会拦截点击事件，就好比一个公司人手不够，公司领导需要亲力亲为一样。
接着：
// If intercepted, start normal event dispatch. Also if there is already // a view that is handling the gesture, do normal event dispatch. if (intercepted || mFirstTouchTarget != null) { ev.setTargetAccessibilityFocus(false);
}
如果确定拦截或者已经有子View着手处理这个点击事件，那么就开始正常的事件分发流程。
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;这里是通过标志位和ACTION_CANCLE来检查是否cancle，再下去：
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;首先可以看见获取一个boolean变量标记split来标记，默认是true，作用是是否把事件分发给多个子View，这个同样在ViewGroup中提供了public的方法设置，如下：
public void setMotionEventSplittingEnabled(boolean split) {
// TODO Applications really shouldn't change this setting mid-touch event,
// but perhaps this should handle that case and send ACTION_CANCELs to any child views
// with gestures in progress when this is changed.
if (split) {
mGroupFlags |= FLAG_SPLIT_MOTION_EVENTS;
} else {
mGroupFlags &= ~FLAG_SPLIT_MOTION_EVENTS;
}
}这一段摘自：Android触摸屏事件派发机制详解与源码分析二(ViewGroup篇)
if (!canceled && !intercepted) {如果没有取消当前动作并且ViewGroup未拦截事件那么事件就传递到接收了该点击事件的View，接下来是一大段代码预警：
// If the event is targeting accessiiblity focus we give it to the
// view that has accessibility focus and if it does not handle it
// we clear the flag and dispatch the event to all children as usual.
// We are looking up the accessibility focused host to avoid keeping
// state since these events are very rare.
View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
? findChildWithAccessibilityFocus() : null;
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 ArrayList<View> preorderedList = buildOrderedChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = customOrder
? getChildDrawingOrder(childrenCount, i) : i;
final View child = (preorderedList == null)
? children[childIndex] : preorderedList.get(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;
}嗯，这段代码的逻辑比较清晰，大体上就是遍历ViewGroup的所有子元素，然后判断子元素是否能够接收到点击事件，接收的依据有两种：第一种是判断子元素是否在播放动画，第二种是判断点击事件的坐标是否落在子元素的区域内，从这里可以看出来：
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;
}如果子元素满足了这两个条件，点击事件就会交给它处理，接下来这段代码里面有一个很重要的方法：
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;
}这里if判断里面这个dispatchTransformedTouchEvent是将Touch事件传递给特定的子View，它实际上在内部是调用了子元素的disPatchTouchEvent方法，找一下它的源码：
/** * Transforms a motion event into the coordinate space of a particular child view, * filters out irrelevant pointer ids, and overrides its action if necessary. * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead. */
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;
}看这一段内容：
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}如果传递的child为null就调用父类的dispatchTouchEvent否则就调用子类的dispatchTouchEvent，而上面的代码中child不为null，所以执行子元素的dispatchTouchEvent，如果子元素的dispatchTouchEvent返回的是True，那么含有dispatchTransformedTouchEvent这个方法内部的for循环就不会继续下去，直接跳到这里：
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;这个地方前面说过是给mFirstTouchTarget赋值的地方，如果情况改变，前面那个dispatchTransformedTouchEvent方法中child返回的是False，那么如果当前ViewGroup会把点击事件传递给下一个子元素进行处理，执行for循环查找下一个子元素，此时mFirstTouchTarget依然未被赋值为null，那么这时候继续查看接下来的代码：
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;
}
这一段代码表示当前没有找到可以接收点击事件的View并且我们的mFirstTouchTarget!=null那么就把最开始的TouchTarget赋值给newTouchTarget，最后：
// 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);执行到这里的话有两种情况，一种是ViewGroup里面没有找到子View，另一种就是找到了处理这次点击事件的子View但是这个子View的disPatchTouchEvent返回了False，我们通过前面的分析知道disPatchTouchEvent中是先执行onTouch()方法的，而一般onTouch()方法返回的是False，此时disPatchTouchEvent方法的返回值由onTouchEvent方法决定，出现这种情况说明onTouchEvent返回了False，在以上两种情况下，ViewGroup会自己处理这个点击事件，注意这里这个方法里的child传入的是null，我们前面就知道了传入null会执行handled = super.dispatchTouchEvent(event);也就是说此时交由ViewGroup处理这个事件。而ViewGroup也是View的子类，它里面是没有重写View的onTouchEvent方法的，所以它自身处理点击事件的流程和我们在View的事件分发机制解析里面分析得是一样的，至此ViewGroup的分发事件分析完毕。
总结
这次我们只分析了点击Button时的Log输出，下面给出点击空白处的Log输出，可以自己检验一下分析成果：
一些结论：
ViewGroup默认不拦截任何事件，Android源码中ViewGroup的onInterceptTouchEvent方法默认返回falseView的onTouchEvent默认都会消耗事件(返回true)，除非是不可点击的(clickable和longClicjable同时为false)，View的longClickable属性默认是false的View的enable属性不会影响onTouchEvent的返回值，哪怕该View为disable的，只要它的clickable和longClickable其中一个为true，那么它的onTouchEvent就返回true- 事件传递过程是由外向内的，即事件总是先传给父元素，然后再由父元素分发给子
VIew,子View可以通过requestDisallowInterceptTouchEvent来干预父元素的分发过程，但是影响不到ACTION_DOWN事件
参考：
Android触摸屏事件派发机制详解与源码分析二(ViewGroup篇)
Android事件分发机制完全解析，带你从源码的角度彻底理解(下)