先说一些结论吧：
1、只有ViewGroup有onInterceptTouchEvent方法，但并不是每个触摸事件都走该方法，只有ACTION_DOWN事件以及ACTION_DOWN事件分发后，在该ViewGroup内找到能消费事件的子view，后续事件才会走该方法判断。同时它又会受disallowIntercept 影响，当子view调用了parent.requestDisallowInterceptTouchEvent方法后，该ViewGroup同样不会走onInterceptTouchEvent方法。
2、一般自定义ViewGroup会重写onInterceptTouchEvent，返回true，代表自身能消费事件，不再继续下发，走自己的onTouchEvent消费事件；返回false，代表不拦截事件，分发给子view处理。
3、一般自定义View还会重写onTouchEvent，该方法中通常会处理触摸的四种事件（不考虑多指）ACTION_DOWN、ACTION_MOVE、ACTION_UP、ACTION_CANCEL。当手指按在ViewGroup上时，事件就会从最外层或者说最顶层的ViewGroup依次向它们子View分发。而这一系列View中到底哪个能消费触摸事件是在ACTION_DOWN向下分发的过程中确立的！当ACTION_DOWN事件分发到View的onTouchEvent中时：
返回false，代表自己处理不了，让父View继续处理，后续事件走父View的onTouchEvent；
返回true，代表报告父View自己能够消费事件（将自身赋给父View的mFirstTouchTarget变量，具体看下方源码），后续事件看到这个mFirstTouchTarget，就都让它处理，这种情况下父View的onTouchEvent中无法接收ACTION_DOWN事件，但也有例外，如果后续事件如ACTION_MOVE，父View在OniterceptTouchEvnet中拦截了，那么，后续事件依然会给父View的onTouchEvent处理。
4、dispatchTouchEvent方法中处理了事件的分发逻辑，每种事件到底是拦截还是分发，亦或是处理，都和它息息相关。
它的返回值是给它的父View看的，return true，代表向父View报告自己能够消费事件；return false，让父View处理事件，然后父View会走自己的super.dispatchTouchEvent->onTouchEvent...；这一过程和onTouchEvent有点类似，都是在ACTION_DOWN事件分发中判定的。
5、一般自定义ViewGroup不建议重写dispatchTouchEvent方法，因为无论我们直接return true或false, 该ViewGroup的onTouchEvent就被屏蔽了，当然我们可以主动调用onTouchEvent，或super.dispatchTouchEvent执行向子view分发事件的逻辑，但这样有点复杂了，得不偿失。
6、对于mFirstTouchTarget和newTouchTarget未能理解透彻，暂时只是自己猜想（处理多指触控），如果哪位老铁知道的话，还望指点一下。
ViewGroup的dispatchTouchEvent，很重要！
@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.
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.
//当事件按下的时候，会先清除mFirstTouchTarget.
cancelAndClearTouchTargets(ev);
resetTouchState();
}
// Check for interception.
final boolean intercepted;
//down事件或上一次down事件中找到了消费触摸事件的子View，后续的事件都会判断一下子view是否要求不拦截
//换句话说，也就是如果down事件没找到子View，那么后续事件就不会执行onInterceptTouchEvent（）；
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
//正常情况下会调用这个方法, 即是否拦截down事件, 该方法默认返回false.
intercepted = onInterceptTouchEvent(ev);
ev.setAction(action); // restore action in case it was changed
} else {
//子View调用了requestDisallowInterceptTouchEvent（true），屏蔽onInterceptTouchEvent(ev)判断，直接不拦截事件。
intercepted = false;
}
} else {
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
//执行到这的时候代表此次事件为move或up, 且mFirstTouchTarget为空，即没有找到能消耗次事件的子View，所以直接拦截。
intercepted = true;
}
// If intercepted, start normal event dispatch. Also if there is already
// a view that is handling the gesture, do normal event dispatch.
//如果拦截了，或者有找到了能消耗此次事件的子View。
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;
//newTouchTarget在此处声明
TouchTarget newTouchTarget = null;
boolean alreadyDispatchedToNewTouchTarget = false;
//默认情况下, if能执行.
//如果没有取消触摸，并且没有拦截事件。
if (!canceled && !intercepted) {
//我们将事件分发给能获取焦点的view，如果该view不处理该事件，那么我们就继续向该view的子view分发。
// 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;
//down事件进入判断
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;
//假设次ViewGroup中有一个view, 则能进入if.(down)
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 preorderedList = buildTouchDispatchChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
//反序for循环, 假设该viewGroup为relativelayout, 则会拿到最顶上的view.
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;
}
//过滤不能接收事件或者不在手指触摸范围内的子view
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
//从touchTarget链表中寻找该child相关的target。第一次down是找不到的
**newTouchTarget = getTouchTarget(child);**
if (newTouchTarget != null) {
//多指触控，找到了target。
// 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);
//类似递归调用dispatchTransformedTouchEvent, 向子view分发事件， 只要没被拦截，该事件一直分发，返回子view 是否有能力消费触摸事件。
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
//找到了最终子view，且该子view能消费down事件
// 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 {
//记录子view index
mLastTouchDownIndex = childIndex;
}
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
//该child能消费触摸事件，生成newTouchTarget，同时赋给mFirstTouchTarget。
**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();
}
//处理多指触控，如第二根手指点击了子view范围之外，newTouchTarget无法找到，将第一根手指按下时的touchTarget给它。
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. 上述代码只是处理的down事件，后续事件将交给touchTarget处理。
if (mFirstTouchTarget == null) {
//如果down事件中未找到mFirstTouchTarget （没有消费事件的子view或onInterceptTouchEvent返回true），或者找到了mFirstTouchTarget，但后续的某一事件被它前置事件
//所拦截，mFirstTouchTarget 重新被置空。 则此次事件走的是自身的(super)dispatchTouchEvent->onTouchEvent
//这里体现了类似于向上递归分发处理事件的过程（即有些文章提到的事件分发过程是一个U形，从上往下，在从下往上，这里体现了从下往上的过程），上面处理ACTION_DOWN逻辑中调用子view的dispatchTouchEvent
//但没找到mFirstTouchTarget，那么就调用自己的super.dispatchTouchEvent。
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) {
//遍历target链表
final TouchTarget next = target.next;
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
//down事件中找到了mFirstTouchTarget，且mFirstTouchTarget == newTouchTarget，多指触控
handled = true;
} else {
//是否被前置事件拦截
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
//move事件分发处理。
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
if (cancelChild) {
if (predecessor == null) {
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
//回收
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;
}
cancelAndClearTouchTargets
/**
* Cancels and clears all touch targets.
*/
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;
}
//清除mFirstTouchTarget
for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
resetCancelNextUpFlag(target.child);
dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits);
}
//关注这句话.
clearTouchTargets();
if (syntheticEvent) {
event.recycle();
}
}
}
clearTouchTargets
每次down事件会清除target
/**
* Clears all touch targets.
*/
private void clearTouchTargets() {
TouchTarget target = mFirstTouchTarget;
if (target != null) {
do {
TouchTarget next = target.next;
target.recycle();
target = next;
} while (target != null);
//这句话是核心,清除target
mFirstTouchTarget = null;
}
}
dispatchTransformedTouchEvent
将viewGroup接收到事件的坐标，转化为在坐标在子view坐标体系中的坐标，如果参数child为空，则调用该ViewGroup父类中的dispatchTouchEvent->onTouch->onTouchEvent->onClickListener. 如果子view不为空， 则调用子view的dispatchTouchEvent，这是一个递归向下调用dispatchTouchEvent的过程
/**
* Transforms a motion event into the coordinate space of a particular child view,
* filters out irrelevant pointer ids, and overrides its action if necessary.
* If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
*/
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
// Canceling motions is a special case. We don't need to perform any transformations
// or filtering. The important part is the action, not the contents.
final int oldAction = event.getAction();
//分发cancel事件
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
event.setAction(MotionEvent.ACTION_CANCEL);
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}
event.setAction(oldAction);
return handled;
}
// Calculate the number of pointers to deliver.
final int oldPointerIdBits = event.getPointerIdBits();
final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;
// If for some reason we ended up in an inconsistent state where it looks like we
// might produce a motion event with no pointers in it, then drop the event.
if (newPointerIdBits == 0) {
return false;
}
// If the number of pointers is the same and we don't need to perform any fancy
// irreversible transformations, then we can reuse the motion event for this
// dispatch as long as we are careful to revert any changes we make.
// Otherwise we need to make a copy.
final MotionEvent transformedEvent;
if (newPointerIdBits == oldPointerIdBits) {
if (child == null || child.hasIdentityMatrix()) {
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
event.offsetLocation(offsetX, offsetY);
handled = child.dispatchTouchEvent(event);
event.offsetLocation(-offsetX, -offsetY);
}
return handled;
}
transformedEvent = MotionEvent.obtain(event);
} else {
transformedEvent = event.split(newPointerIdBits);
}
// Perform any necessary transformations and dispatch.
if (child == null) {
handled = super.dispatchTouchEvent(transformedEvent);
} else {
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;
}
参考：
https://www.jianshu.com/p/e99b5e8bd67b
http://blog.csdn.net/yanbober/article/details/45912661
https://www.jianshu.com/p/e75dd6fba1b7