Android事件分发机制全面解析
16lz
2022-08-01
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：
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 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 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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