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ps：源码是基于 android api 27 来剖析的
后面写了好几篇 View 事件的散发，但更多的偏差于总结论断并写 demo 来演示验证论断；这一篇咱们来具体的剖析 View 事件散发中的 ViewGroup 中的 dispatchTouchEvent 办法源码。
首先咱们先找到 View 事件散发的源头，那就是 Activity 中的 dispatchTouchEvent办法：
public boolean dispatchTouchEvent(MotionEvent ev) {
//1、 if (ev.getAction() == MotionEvent.ACTION_DOWN) { onUserInteraction(); } //2、 if (getWindow().superDispatchTouchEvent(ev)) { return true; } //3、 return onTouchEvent(ev);}
正文1 是 down 事件，onUserInteraction 是空的办法，不用理睬，正文2 如果 if 语句的值是 false，那么就执行正文3 的返回语句，也就是 Activity 的 onTouchEvent 办法，即 Activity 解决触摸事件；那么正文2 是什么呢？正文2 是事件的散发，它首先分发给 Window，getWindow 办法就是获取一个 Window，Window 的实现类是 PhoneWindow，所以咱们看看 PhoneWindow 的 superDispatchTouchEvent 办法；
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
//4、 return mDecor.superDispatchTouchEvent(event);}
看正文4，mDecor 是一个 DecorView 类对象，父类是 FrameLayout，FrameLayout 继承于 ViewGroup；DecorView 是咱们常常用到 setContentView 办法时的底层根布局，咱们往下查看 DecorView 的 superDispatchTouchEvent 办法；
public boolean superDispatchTouchEvent(MotionEvent event) {
//5、 return super.dispatchTouchEvent(event);}
咱们看正文5，最终调用了父类的 dispatchTouchEvent 办法，这时候终于见到咱们的配角 ViewGroup 了，ViewGroup 的 dispatchTouchEvent 办法如下所示：
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
//6、 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); } //7、 boolean handled = false; //8、 if (onFilterTouchEventForSecurity(ev)) { final int action = ev.getAction(); //9、 final int actionMasked = action & MotionEvent.ACTION_MASK; //10、 // 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(); } //11、 // Check for interception. final boolean intercepted; //12、 if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { //13、 final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { //14、 intercepted = onInterceptTouchEvent(ev); //15、 ev.setAction(action); // restore action in case it was changed } else { intercepted = false; } //16、 } 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); } //17、 // Check for cancelation. final boolean canceled = resetCancelNextUpFlag(this) || actionMasked == MotionEvent.ACTION_CANCEL; //18、 // Update list of touch targets for pointer down, if needed. final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0; //19、 TouchTarget newTouchTarget = null; //20、 boolean alreadyDispatchedToNewTouchTarget = false; //21、 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; //21A、 if (actionMasked == MotionEvent.ACTION_DOWN || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN) || actionMasked == MotionEvent.ACTION_HOVER_MOVE) { //22、 final int actionIndex = ev.getActionIndex(); // always 0 for down //23、 final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex) : TouchTarget.ALL_POINTER_IDS; //24、 // Clean up earlier touch targets for this pointer id in case they // have become out of sync. removePointersFromTouchTargets(idBitsToAssign); final int childrenCount = mChildrenCount; //25、 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(); //26、 final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled(); //27、 final View[] children = mChildren; for (int i = childrenCount - 1; i >= 0; i--) { //28、 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; } //29、 if (!canViewReceivePointerEvents(child) || !isTransformedTouchPointInView(x, y, child, null)) { ev.setTargetAccessibilityFocus(false); continue; } //30、 newTouchTarget = getTouchTarget(child); //31、 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; } //32、 resetCancelNextUpFlag(child); //33、 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(); } //34、 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; } } } //35、 // Dispatch to touch targets. if (mFirstTouchTarget == null) { //36、 // No touch targets so treat this as an ordinary view. handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS); //37、 } else { //38、 // 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 { //39、 final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted; //40、 if (dispatchTransformedTouchEvent(ev, cancelChild, target.child, target.pointerIdBits)) { handled = true; } //41、 if (cancelChild) { //42、 if (predecessor == null) { mFirstTouchTarget = next; //43、 } else { predecessor.next = next; } target.recycle(); target = next; continue; } } //44、 predecessor = target; target = next; } } //45、 // Update list of touch targets for pointer up or cancel, if needed. if (canceled || actionMasked == MotionEvent.ACTION_UP || actionMasked == MotionEvent.ACTION_HOVER_MOVE) { resetTouchState(); //46、 } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) { final int actionIndex = ev.getActionIndex(); final int idBitsToRemove = 1 << ev.getPointerId(actionIndex); removePointersFromTouchTargets(idBitsToRemove); } } //47、 if (!handled && mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1); } return handled;}
为了不便浏览，咱们把代码一段一段的拿进去剖析，有些不重要的代码我会进行省略。
//6、 if (mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onTouchEvent(ev, 1); } ...... //7、 boolean handled = false;正文6 它示意的是验证事件是否间断；正文7 示意的是这个变量用来记录这个事件是否被解决过。
//8、 if (onFilterTouchEventForSecurity(ev)) { ...... }正文8 这里是为了过滤掉一些不合理的事件，比如说以后的 View 的窗口被遮挡了，比方再具体一点弹出一个 Dialog；如果没有被挡住，那么就执行 if 括号外面的语句。
//9、 final int actionMasked = action & MotionEvent.ACTION_MASK; //10、 // 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(); }正文9 示意重置后面为0 ，只留下后八位，用于判断相等时候，能够进步性能；正文10 示意判断是不是 down 事件，如果是的话，就要做初始化操作，cancelAndClearTouchTargets 办法会调用 resetCancelNextUpFlag 办法，而 resetCancelNextUpFlag 办法是为了清空 mPrivateFlags 的 PFLAG_CANCEL_NEXT_UP_EVEN 标记，作用是将下一个事件变成 Cancel；resetTouchState 办法清空了 mGroupFlags 的 FLAG_DISALLOW_INTERCEPT 标记,如果设置了FLAG_DISALLOW_INTERCEPT，ViewGroup 对触摸事件进行拦挡；resetTouchState 办法调用了 clearTouchTargets 办法，目标是为了清空 mFirstTouchTarget 链表，并设置 mFirstTouchTarget 为 null，mFirstTouchTarget 是"承受触摸事件的 View "所组成的单链表。
//11、 // Check for interception. final boolean intercepted; //12、 if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { //13、 final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { //14、 intercepted = onInterceptTouchEvent(ev); //15、 ev.setAction(action); // restore action in case it was changed } else { intercepted = false; } //16、 } else { // There are no touch targets and this action is not an initial down // so this view group continues to intercept touches. intercepted = true; }正文11 示意查看是否拦挡；正文12 示意如果为 down 事件，或者 mFirstTouchTarget 不为null（那么事件间接给到本人），先不拦挡；正文13 示意查看是否设置了禁止拦挡的标记，即子元素是否调用了 getParent().requestDisallowInterceptTouchEvent(true) 语句；正文14 示意 ViewGroup 的 onInterceptTouchEvent 办法是否要拦挡事件，它默认不执行拦挡，除非它的子类（留神是子类不是子元素）重写它的 onInterceptTouchEvent 办法并返回 true；正文15 示意从新复原 Action，免得 Action 在下面的步骤被扭转了；正文16 示意事件曾经初始化过了，而且没有子 View 被调配解决，这就阐明了 ViewGroup 曾经拦挡了过这个事件了，那么 ViewGroup 第二次就不会是否须要拦挡了的。
//17、 // Check for cancelation. final boolean canceled = resetCancelNextUpFlag(this) || actionMasked == MotionEvent.ACTION_CANCEL; //18、 // Update list of touch targets for pointer down, if needed. final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0; //19、 TouchTarget newTouchTarget = null; //20、 boolean alreadyDispatchedToNewTouchTarget = false;正文17 示意查看 viewFlag 是否被标记了 PFLAG_CANCEL_NEXT_UP_EVENT，那么下一步应该是 Cancel 事件，或者假如以后的 Action 为勾销，那么以后事件就是勾销了；正文18 示意以后的 ViewGroup 是不是反对把 MotionEvent 传送到不同的 View 当中，比方咱们把两个手指放到了屏幕上，是否要将第二个手指的事件传送上面去；正文19 示意新的触摸对象；正文20 示意是否把事件调配给了新的触摸。
//21、 if (!canceled && !intercepted) { //21A、 if (actionMasked == MotionEvent.ACTION_DOWN || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN) || actionMasked == MotionEvent.ACTION_HOVER_MOVE) { //22、 final int actionIndex = ev.getActionIndex(); // always 0 for down //23、 final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex) : TouchTarget.ALL_POINTER_IDS; //24、 // Clean up earlier touch targets for this pointer id in case they // have become out of sync. removePointersFromTouchTargets(idBitsToAssign); ...... } }正文21 示意如果事件不是勾销事件，也没有进行拦挡；正文21A 示意如果是个新的 down 事件，或者是有新的触摸点，又或者是光标来回挪动事件；正文22 示意这个事件的索引，即第几个事件，如果是 up 事件就是1；正文23 示意获取调配的 id 的 bit 数量；正文24 示意革除 Targets 中相应的 pointer 中的 ids，防止它们的指标变得不同步。