阅读提示
开篇强调，事件与手势动作(点击、滑动等)是有区别，例如：一个滑动事件以ACTION_DOWN事件开始，经历多个ACTION_MOVE事件，最后以ACTION_UP事件结束。而分发机制讲述的主体是事件，没这点的意识的话，理解起来估计就没完没了。还有要理解super关键字并不是父ViewGroup（大多时候ViewGroup中的super很有可能指的就是View这个java类）。
关键函数
说到点击事件的分发怎么能不提到这三个函数：
public boolean dispatchTouchEvent(MotionEvent ev)
public boolean onInterceptTouchEvent(MotionEvent ev)
public boolean onTouchEvent(MotionEvent ev)
主体内容
dispatchTouchEvent来源
其中最难理解的也就是dispatchTouchEvent了，而且dispachTouchEvent描述了整个事件的分发与处理流程。
当点击屏幕时，事件最先传递给Activity的dispatchTouchEvent(MotionEvent ev),下面我们开始分析源码：
源码：Activity#dispatchTouchEvent
/**
* Called to process touch screen events. You can override this to
* intercept all touch screen events before they are dispatched to the
* window. Be sure to call this implementation for touch screen events
* that should be handled normally.
*
* @param ev The touch screen event.
*
* @return boolean Return true if this event was consumed.
*/
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
......
/**
* Called whenever a key, touch, or trackball event is dispatched to the
* activity. Implement this method if you wish to know that the user has
* interacted with the device in some way while your activity is running.
* This callback and {@link #onUserLeaveHint} are intended to help
* activities manage status bar notifications intelligently; specifically,
* for helping activities determine the proper time to cancel a notfication.
*
* <p>All calls to your activity's {@link #onUserLeaveHint} callback will
* be accompanied by calls to {@link #onUserInteraction}. This
* ensures that your activity will be told of relevant user activity such
* as pulling down the notification pane and touching an item there.
*
* <p>Note that this callback will be invoked for the touch down action
* that begins a touch gesture, but may not be invoked for the touch-moved
* and touch-up actions that follow.
*
* @see #onUserLeaveHint()
*/
public void onUserInteraction() {
}
显然，从内容上看onUserInteraction();并不影响事件的传递，它只是可以在一个新手势开始(ACTION_DOWN事件发生)时，在事件传递开始前做最优先的事情。
下面会执行`getWindow().superDispatchTouchEvent(ev)`操作，那么`getWindow()`又是啥？
......
mWindow = new PhoneWindow(this, window, activityConfigCallback);
......
/**
* Retrieve the current {@link android.view.Window} for the activity.
* This can be used to directly access parts of the Window API that
* are not available through Activity/Screen.
*
* @return Window The current window, or null if the activity is not
* visual.
*/
public Window getWindow() {
return mWindow;
}
再进入AndroidXRef，找到PhoneWindow.java:
// This is the top-level view of the window, containing the window decor.
private DecorView mDecor;
....
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
咋样？是不是想到了传说种的Activity->PhoneWindow->DecorView。且慢，我们的目的时“事件分发机制”。那么重新找到DecorView.java:
public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}
而DecorView extends FrameLayout，现在知道为啥有的文章说“所有的布局都在一个FrameLayout里面”了吧？其实就是DecorView,FrameLayout extends ViewGroup，简单梳理就会知道这个dispatchTouchEvent(event)就是在ViewGroup种实现的。
最终还是看ViewGroup怎么实现这个dispatchTouchEvent(event)；ViewGroup#dispatchTouchEvent(MotionEvent ev),最终返回true的话，Activity#dispatchTouchEvent(MotionEvent ev)函数就直接返回了,Activity#onTouchEvent(ev)也不会被调用，如果最终返回为false的话则Activity#onTouchEvent(ev)执行，为什么要加“最终”二字。请听我娓娓道来，前方高能。
源码解析
dispatchTouchEvent源码
没错，源码（为了便于理解，这段源码有一点点的删除，阅读的时候可直接跳过这一段源码，因为我会在后面具体分析）：ViewGroup#dispatchTouchEvent(MotionEvent ev):
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;
}
.....
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
//第一段-----------------------------------------------------------START
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;
}
//第一段-----------------------------------------------------------END
//第二段-----------------------------------------------------------START
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 = 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();
}
}
}
//第二段-----------------------------------------------------------END
//第三段-----------------------------------------------------------START
// 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;
}
}
predecessor = target;
target = next;
}
}
//第三段-----------------------------------------------------------END
return handled;
}
年轻人，我知道你不想看源码，我也很无奈。
第一段
if (actionMasked == MotionEvent.ACTION_DOWN) {
cancelAndClearTouchTargets(ev);
resetTouchState();
}
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;
}
从备注与函数名上理解，开始只是做一些重置初始工作（99.9%情况下，我们要对Google的工程师足够的相信，不然你会越陷越深）。
然后，会在判断里面看到onInterceptTouchEvent(ev),嗯，终于看到第二个函数了。那么什么情况下会调用这个函数呢？disallowIntercept从字面和后面的赋值理解，这是通过mGroupFlags字段，不允许ViewGroup拦截，通过语句判断如果ACTION_DOWN不予许拦截成立的话(!disallowIntercept == false),所以intercepted = false,（分析时避免情况复杂化，我们要猜一些变量或者函数的意义，比如:!canceled应该就时动作没被取消，正常情况下该条件是成立的）所以，会进入执行第二段。
第二段
接下来分析一下这个"mChildrenCount"：
// Child views of this ViewGroup
private View[] mChildren;
// Number of valid children in the mChildren array, the rest should be null or not
// considered as children
private int mChildrenCount;
现在，大概可以看出什么意思了吧？mChildren是指ViewGroup里面的所有子View,而mChildrenCount指的是能所有有效的ViewGroup里子View的数量，好了，我们接着执行第二段。可以看出，如果没有有效的子View，则会直接进入第三段。如果有有效的子View，且满足传递的要求（这一点可以通过备注看出，各种函数名也表面），则会在该子View执行dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)，再看看这个函数名:
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits){
//各种判断各种处理以后都会成为这种结构
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}
return hanled;
}
啥意思？子View不为空，则子View继续分发事件，否则执行super（赌一包辣条，这里的super就是View）的dispatchTouchEvent,所以接下来看看View的dispatchTouchEvent函数：
源码：View#dispatchTouchEvent
/**
* Pass the touch screen motion event down to the target view, or this
* view if it is the target.
*
* @param event The motion event to be dispatched.
* @return True if the event was handled by the view, false otherwise.
*/
public boolean dispatchTouchEvent(MotionEvent event) {
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;
}
}
if (!result && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
// Clean up after nested scrolls if this is the end of a gesture;
// also cancel it if we tried an ACTION_DOWN but we didn't want the rest
// of the gesture.
if (actionMasked == MotionEvent.ACTION_UP ||
actionMasked == MotionEvent.ACTION_CANCEL ||
(actionMasked == MotionEvent.ACTION_DOWN && !result)) {
stopNestedScroll();
}
return result;
}
熟悉不？你有没有OnTouchListener啊，要是有的话，你onTouch返回了啥东东啊？给我看看，我好决定怎么处理onTouchEvent，这里就清楚onTouch和onTouchEvent了吧！！！关于传递--->处理的流程就到这里了。接下来看看返回值对后续的过程影响。
很明显，onTouch与onTouchEvent任意一个返回true(从代码可见得：onTouch返回true后，onTouchEvent是没有机会执行的)，都将成为其所在的ViewGroup的dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)的返回值，如果返回false则，会使得mFirstTouchTarget == null，直接执行第三段的handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS);。明显进入[onTouch], onTouchEvent过程handle的返回值将直接依赖这两个（或没有设置onTouch监听就是一个）的返回值，从这里我们知道父ViewGroup中的[onTouch], onTouchEvent也调用了。如果返回true，将会走进addTouchTarget(child, idBitsToAssign)：
源码：ViewGroup#addTouchTarget
/**
* 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;
}
第三段
mFirstTouchTarget == null 时，上面已经分析。
但是在上述条件不成立的情况下这里mFirstTouchTarget被赋值后。这里有一段不是很理解，还请各位指教，但是能确定handle = true成为必然。
// 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;
}
}
predecessor = target;
target = next;
}
大致流程就是这样分析的，未完待续。。。。。。
小结
这篇文章是看完《Android开发艺术探索》后自己跟着源码一步步来走的，看完书以后其实只是一个轮廓，然后自己看源码后映像深刻了些，但总是感觉不放心，编程路上一位导师曾经跟我说过：当你觉得你懂了得时候，你就该写下来。当你觉得你能写明白得时候，去找一个人然后把他教会。这时候你才是真正得懂。
遗憾由于水平有限，连写都没能写得明白，但是写得过程中确实也有了不少得收获，但也不该只满足于此。
有人说对于源码得追究其实不该太过钻牛角，然而还是希望尽量满足自己得好奇心，希望得到大家指导，待续。。。
