导语：
在我们使用Android做一些控件的滑动和点击时，各种冲突事件、点击事件无响应等一些touch事件无响应困扰着我们，今天我将从源码角度分析android的事件分发机制
1.简单看下例子(搞清楚onTouch和onClick的关系)：
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
testViewGroup =(TestViewGroup)findViewById(R.id.testViewGroup);
testViewGroup.setOnTouchListener(new View.OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
Log.i(TAG,"MainActivity +++ onTouch");
return true;
}
});
testViewGroup.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Log.i(TAG,"MainActivity +++ onClick");
}
});
}
当onTouch返回值为true和false两种不同的情况
请想下输出的onTouch事件还有onClick事件的打印结果？
当为true，结果为：
1.png
当为false，结果为：
2.png
看到这里是否有疑问？接下来带大家一步一步分析：
首先点开View.java文件 找到:dispatchTouchEvent()方法
public boolean dispatchTouchEvent(MotionEvent event) {
// If the event should be handled by accessibility focus first.
if (event.isTargetAccessibilityFocus()) {
// We don't have focus or no virtual descendant has it, do not handle the event.
if (!isAccessibilityFocusedViewOrHost()) {
return false;
}
// We have focus and got the event, then use normal event dispatch.
event.setTargetAccessibilityFocus(false);
}
//关键值，用于判断onTouchEvent()是否该执行
boolean result = false;
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
final int actionMasked = event.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Defensive cleanup for new gesture
stopNestedScroll();
}
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//判断onTouch返回值是否为true，如果为true，resulet也为true
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
//为true 不执行onTouchEvent()
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;
}
li.mOnTouchListener中存放的是该View的onTouch监听，所以onTouch返回true时， result = true; 不执行onTouchEvent(event)，所以打印时没有onTouchEvent的log日志，当onTouch返回false时，接下来进入onTouchEvent()方法，打印了onTouchEvent的log日志。
接下来找到onTouchEvent()方法
public boolean onTouchEvent(MotionEvent event) {
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
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);
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return 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)) {
//这里执行onClick正真的点击事件的方法
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;
}
return false;
}
只要看关键代码，找到 performClick()方法。
在找到performClick()方法
public boolean performClick() {
final boolean result;
final ListenerInfo li = mListenerInfo;
//如果监听类中的li.mOnClickListener不为空，则执行它的onClick方法
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;
}
这里就是最后的主角登场了onClick()方法在这里执行。
是不是之前的疑惑迎刃而解了。
其实前面的打印截图留有一丝伏笔：
再看截图：
1.png
2.png
*点击事件是怎么从MainActivity的 dispatchTouchEvent传给TestViewGroup的dispatchTouchEvent?
*TestViewGroup的dispatchTouchEvent又是怎么传给onInterceptEvent?
带着疑问我们进入第二节。
2.触摸事件如何传递：
前言：
从之前的截图可以知道事件先从MainActivity的dispatchTouchEvent开始的。具体为什么是从MainActivity的dispatchTouchEvent开始这篇文章不会涉及到，将会在以后的文章中详细解释。
1.MainActivity的 dispatchTouchEvent传给TestViewGroup的dispatchTouchEvent
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
Log.i(TAG,"MainActivity +++ dispatchTouchEvent");
return super.dispatchTouchEvent(ev);
}
点开super.dispatchTouchEvent(ev);
将会进入Activity的dispatchTouchEvent方法：
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
我们只看getWindow().superDispatchTouchEvent(ev)方法。原来dispatchTouchEvent执行的是getWindow().superDispatchTouchEvent(ev)，这个方法是干什么的呢？就是将事件传递给Tree View布局控件。继续点进入superDispatchTouchEvent方法发现只是Window抽象接口。window的实现类其实是PhoneWindow，至于为什么，有了解的朋友应该知道，不知道的朋友可以去查下，下篇文章会讲到。通过文件查找找到PhoneWindow的superDispatchTouchEvent()方法.
@Override
public boolean superDispatchTrackballEvent(MotionEvent event) {
return mDecor.superDispatchTrackballEvent(event);
}
发现它还是调用的自己的内部类DecorView的mDecor.superDispatchTouchEvent(event)，顺便提下DecorView是整个视图组成的根视图，继续往下走：
public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}
发现它并未有实现这个superDispatchTouchEvent()方法而是调用父类的dispatchTouchEvent方法。继续点进去，发现直接进入到ViewGroup的dispatchTouchEvent方法，并没有进入DecorView的父类FrameLayout,而是直接抛给FrameLayout的父类ViewGroup,所有就是将事件传递Tree View最近的并且实现了ViewGroup的控件，TestViewGroup继承自ViewGroup，并且是最上层。
所有这里就可以解释触摸事件是怎么从MainActivity的 dispatchTouchEvent传给TestViewGroup的dispatchTouchEvent。
2.分析ViewGroup的dispatchTouchEvent方法
dispatchTouchEvent做了些什么，看代码：
@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.当按下的时候
// 第一次步初始状态
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.
//清除标记
cancelAndClearTouchTargets(ev);
//恢复标志位
resetTouchState();
}
// Check for interception.
//第二步判断是否拦截
// 第二次move触发时 压根不会遍历子控件
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;
//在该if中做真正事件分发相关操作
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.
//child重新排序
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);
//preorderedList.get(childIndex)等价于
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;
}
/**
* 判断不能被接收的View 条件：clickable invisiable 点击事件 不在view范围中
* 还有正在动画中
*/
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
/**
* 绝对接收到事件
*/
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;
}
/**
* 真正做事件分发
* child 不为空
*/
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.
//如果被拦截，则直接进入这里
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;
}
我们只分析重要部分。
首先第一步初始化状态：当手指第一次按下的时候它会先进入cancelAndClearTouchTargets(ev)方法
/**
* Cancels and clears all touch targets.
* 清除标志
*/
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();
}
}
}
你可以看到如果mFirstTouchTarget不为null的时候执行会执行括号里的方法，最后会执行clearTouchTargets(),进入clearTouchTargets()可以看到就是执行清空操作
private void clearTouchTargets() {
TouchTarget target = mFirstTouchTarget;
//进行while循环清除存在的target标记
if (target != null) {
do {
TouchTarget next = target.next;
target.recycle();
target = next;
} while (target != null);
//将第一次标记清空
mFirstTouchTarget = null;
}
}
回到dispatchTouchEvent()方法，接下来看第二方法 resetTouchState()：
private void resetTouchState() {
clearTouchTargets();
resetCancelNextUpFlag(this);
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
mNestedScrollAxes = SCROLL_AXIS_NONE;
}
发现该方法中还进行了一次clearTouchTargets()方法，防止没有清空的标记，之后将所有的接触状态制为最初始状态，准备重新开始新的事件。
接下来第二步：判断是否被拦截。
// Check for interception. //是否拦截
// 第二次move触发时 压根不会遍历子控件
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;
}
在这块代码中，会先判断disallowIntercept是否为false，如果为false就会进入到拦截方法onInterceptTouchEvent(ev)
补充：
mGroupFlags的值可以通过requestDisallowInterceptTouchEvent()设置。如果在TestViewGroup或者其子View中设置了requestDisallowInterceptTouchEvent(true)则TestViewGroup就不会执行onInterceptTouchEvent()方法
@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);
}
}
接下来继续分析：disallowIntercept为false 则会进入onInterceptTouchEvent（）方法。onInterceptTouchEvent()方法我们在TestViewGroup中重写实现，如果重现后的onInterceptTouchEvent放回为true，则intercepted = true，则不会进入到真正的事件分发中，也就是事件不能传递给子View。
分析onInterceptTouchEvent为true的情况。
接下来判断这里：
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;
}
}
因为我们开始的初始化状态已经把mFirstTouchTarget置空了。所以这里进入dispatchTransformedTouchEvent(ev,canceled,null,TouchTarget.ALL_POINTER_IDS);。点击进去这个方法。
// Perform any necessary transformations and dispatch.
// 第一次次 接下来 被拦截
if (child == null) {
handled = super.dispatchTouchEvent(transformedEvent);
} else {
//不拦截的
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;
找到最后这部分代码。这里是事件传递的地方，因为child的值为null，所以进入到
handled = super.dispatchTouchEvent(transformedEvent);
点开dispatchTouchEvent方法，你会发现这里跳到之前我们分析View的dispatchTouchEvent()方法,所以结合最上面的分析如果没做其它的onTouch操作，则会顺利的进入到onTouchEvent(event)方法。到这里是否明白我们实现了拦截方法后事件不会传递到子View而是直接跳转到了onTouchEvent(event)让后结束。最后返回到MainActivity中，当onTouchEvent()返回值会决定MainActivity会不会调用MainActivity自己的onTouchEvent(event)方法。
分析onInterceptTouchEvent为false的情况。
当为false则会进入到ViewGroup.dispatchTouchEvent方法中的判断中：
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);
//判断是否有子View
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.
//<1>child重新排序
final ArrayList<View> preorderedList = buildTouchDispatchChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
//<2>事件分发 遍历子控件
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
//preorderedList.get(childIndex)等价于
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;
}
/**
* 判断不能被接收的View 条件：clickable invisiable 点击事件 不在view范围中
* 还有正在动画中
*/
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
/**
* 绝对接收到事件
*/
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);
/**
* 真正做事件分发
* child 不为空
*/
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;
}
}
}
<1>先会判断是否有子View遍历子View，让后将这些View重新排序，为什么要进行重新排序?是因为多个子View之间会存在重叠的现象，点在重叠位置的时候事件是可以穿透的，谁先接收到事件？谁该执行？在这之前的顺序并不知道。所以就需要先将子View重排序，按照重排序之后的顺序执行
<2>遍历子View，先判断子View能否接收，clickable、invisible、点击事件、不在view范围中这几种情况是不能接收那就直接放回。让后执行能接收到事件的View
newTouchTarget = getTouchTarget(child);
这里是找到直接接收事件的子View，因为第一次点击newTouchTarget=null,所以不执行下面if判断。但如果是手指移动的时候 就可以找到接收事件的子View，就不用继续遍历。
让后在dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)做事件分发，这时候子View child不为空。继续分析dispatchTransformedTouchEvent中的child不为空的方法。
// Perform any necessary transformations and dispatch.
// 第一次次 接下来 被拦截
if (child == null) {
handled = super.dispatchTouchEvent(transformedEvent);
} else {
//不拦截的
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;
为什么这里需要 final float offsetX = mScrollX - child.mLeft;final float offsetY = mScrollY - child.mTop;因为offsetX是子View相对屏幕边的X方向的距离。怎么得到？mScrollX是ViewGroup距离子控件的距离，ViewGroup的边可能在屏幕外面也可能在屏幕里面，child.mLeft是ViewGroup距离屏幕边的距离，当ViewGroup超出屏幕child.mLeft的值是正的相减，当ViewGroup没有超出屏幕child.mLeft的值是负的相加，offsetY同理。
让后执行transformedEvent.transform，这里是得到真正的相对屏幕距离
最后执行子View的dispatchTouchEvent(transformedEvent)。到这里又和上面分析的情况一样了。最后如果子View的onTouchEvent()有返回true，子View则会被添加到newTouchTarget链表中，让后结束遍历；如果子View的onTouchEvent()的返回值都为false则newTouchTarget就没有子View添加，newTouchTarget＝null。
ViewGroup的dispatchTouchEvent后执行到这里：
// 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;
}
}
mFirstTouchTarget到时候就不等于空了，到else里面，这里又进行了一次dispatchTransformedTouchEvent()方法，target.child 不为空则handled＝true最后也返回值也是true，如果target.child为空则根据上面分析的dispatchTransformedTouchEvent（）方法又会到super.dispatchTouchEvent(transformedEvent);让后执行自己的onTouchEvent()方法
最后的到一张图。
3.png
这里分析的是down事件，当滑动的时候，不会再进入到循环遍历里面了，如果之前已经拿到childe，newTouchTarget不为空则直接将事件传递给了相应的子View，不用在做遍历判读了。这就谷歌工程师优化的地方。
结束语：
总算是分析完了，分析时候大致思路就是这样，可能其中有一些没说清楚的，希望大家提给我，我会努力改好的
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