ps：本文系为转载文章，浏览原文可读性会更好，文章开端有原文链接
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。