上篇文章已经分析了Android的Touch事件分发。如果没看的建议先看一下。Android View的Touch事件分发。

接下来我们开始写几种场景，得出一个初步的执行顺序，然后我们按照这个顺序开始分析。

首先我们自定义一个ViewGroup和一个View，然后重写相关事件进行打印:

场景一:正常返回super，TouchView设置click和onTouchListener事件(onTouch返回false)

TouchViewGroup.png

TouchView.png

布局.png

TouchView设置事件.png

这时候我们点击一下TouchView，触发事件:

点击一下.png

可以看到触发的DOWN MOVE UP事件顺序都为:

ViewGroup.dispatchTouchEvent -> ViewGroup.onInterceptTouchEvent -> View.dispatchTouchEvent -> View.onTouch -> View.onTouchEven

只是在UP事件的时候最后多了一个click事件。

场景二：在场景一的基础上取消TouchView的onClick事件

TouchView取消click事件.png

这时候发现除了，执行的顺序变为了：

ViewGroup.dispatchTouchEvent -> ViewGroup.onInterceptTouchEvent -> View.dispatchTouchEvent -> View.onTouch -> View.onTouchEven->ViewGroup.onTouchEven

并且只有DOWN事件，其他事件就没有了。

场景三:在场景二的基础上TouchViewGroup的onInterceptTouchEvent里面返回true

这个时候就只有DOWN事件，并且顺序为:

ViewGroup.dispatchTouchEvent -> ViewGroup.onInterceptTouchEvent -> ViewGroup.onTouchEvent

接下来我们通过源码来分析:

首先从ViewGroup的dispatchTouchEvent入手

@Override

public boolean dispatchTouchEvent(MotionEvent ev) {

//...

boolean handled = false;

//...

//1.取消之前的手势

// 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();

}

//2.判断是否拦截

// Check for interception.

final boolean intercepted;

if (actionMasked == MotionEvent.ACTION_DOWN

|| mFirstTouchTarget != null) { //DOWN

//父类是否拦截 getParent().requestDisallowInterceptTouchEvent();来改变值

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;

}

//....

//3.0 如果是不取消不拦截为down，并且dispatchTransformedTouchEvent返回为true的时候会为 mFirstTouchTarget赋值

// 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;

//3.1 如果不取消并且不拦截的情况下，

if (!canceled && !intercepted) {

if (actionMasked == MotionEvent.ACTION_DOWN

|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)

|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {// 3.2 DOWN的时候

//...

if (newTouchTarget == null && childrenCount != 0) {

//...

final View[] children = mChildren;

for (int i = childrenCount - 1; i >= 0; i--) {//3.3 反序for循环，为了先拿到上层的view

//...

//3.4 拿到child

final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);

//...

//3.5 根据child给newTouchTarget赋值 DOWN的时候因为 mFirstTouchTarget==null 所以进不去 返回的是null

newTouchTarget = getTouchTarget(child);

}

//...

//3.6. 执行操作 是执行自己的dispatchTouchEvent还是child的dispatchTouchEvent

if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {

//...

//3.7 子View如果返回true添加一个newTouchTarget 并且为mFirstTouchTarget赋值

newTouchTarget = addTouchTarget(child, idBitsToAssign);

//....

}

}

}

}

//...

// Dispatch to touch targets.

if (mFirstTouchTarget == null) {//执行自身的dispatchTouchEvent

// No touch targets so treat this as an ordinary view.

handled = dispatchTransformedTouchEvent(ev, canceled, null,

TouchTarget.ALL_POINTER_IDS);

} else {// mFirstTouchTarget已经赋值

// 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) {//执行完3.7操作的

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;

}

}

return handled;

}

/**

* 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;

}

for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {

resetCancelNextUpFlag(target.child);

dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits);

}

clearTouchTargets();

if (syntheticEvent) {

event.recycle();

}

}

}

//清楚所有的TouchTarget

/**

* 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);

mFirstTouchTarget = null;

}

}

//根据childVie得到TouchTarget

/**

* Gets the touch target for specified child view.

* Returns null if not found.

*/

private TouchTarget getTouchTarget(@NonNull View child) {

// DOWN的时候因为 mFirstTouchTarget==null 所以进不去 返回的是null

for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {

if (target.child == child) {

return target;

}

}

return null;

}

/**

* 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) {

//伪代码

final boolean handled;

if (child == null) {//执行View.dispatchTouchEvent 也就是自己的dispatchTouchEvent

handled = super.dispatchTouchEvent(event);

} else {//执行child的dispatchTouchEvent

handled = child.dispatchTouchEvent(event);

}

return handled;

}

//添加TouchTarget 并且给mFirstTouchTarget赋值

/**

* Adds a touch target for specified child to the beginning of the list.

* Assumes the target child is not already present.

*/

private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {

final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);

target.next = mFirstTouchTarget;

mFirstTouchTarget = target;

return target;

}

当DOWN的时候，从注释和方法名可以看出，会调用cancelAndClearTouchTargets,然后在调用clearTouchTargets使mFirstTouchTarget = null用来废弃上一次的触摸手势。

接着判断父类需不需要拦截，先通过(mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0来判断，在这里可以通过getParent().requestDisallowInterceptTouchEvent(boolean disallowIntercept)来改变值，如果上面为判断为false再通过onInterceptTouchEvent的返回值来确定,这个函数默认情况下返回false。

检测是否为取消事件，如果不是取消、不拦截并且为 DOWN事件的时候，就会对childView一个反序的for循环来遍历，并且执行dispatchTransformedTouchEvent操作，这个操作用来执行dispatchTouchEvent,如果childView是null的话将执行View.dispatchTouchEvent,也就是自己的dispatchTouchEvent,反之执行childView的dispatchTouchEvent，如果执行dispatchTransformedTouchEvent返回的值是true那么将会调用addTouchTarget()为这个childView生成一个TouchTarget并且执行mFirstTouchTarget = target将之赋值于mFirstTouchTarget ，然后跳出for循环遍历。

判断操作，首先判断mFirstTouchTarget是否为null,如果是DOWN事件，不拦截不取消并且dispatchTransformedTouchEvent返回了true，那么将会不进入这个判断，如果不是，那么将会在这执行自身的dispatchTouchEvent函数并且将返回值赋于handled返回。进入else语句，在里面将其mFirstTouchTarget进行next遍历,里面的if语句则是DOWN事件下的dispatchTransformedTouchEvent返回true的情况，直接将其赋值，然后返回，里面的else语句则是，调用dispatchTransformedTouchEvent,然后将其返回值返回。

到这里，ViewGroups事件分发源码的流程就分析了，我们根据这个来说说上面的场景。

场景一：我们在TouchViewGroup的dispatchTouchEvent正常返回super,DOWN事件先触发TouchViewGroup的dispatchTouchEvent，然后就执行onInterceptTouchEvent是否拦截，onInterceptTouchEvent返回的是super，也就是false,所以就会通过dispatchTransformedTouchEvent来执行TouchView的dispatchTouchEvent，后面就是View的Touch事件分发了，View流程将会按照dispatchTouchEvent->onTouchListener - > onTouchEvent的顺序执行，因为设置了点击事件，所以在这里就返回了true,这个时候就会通过addTouchTarget()给mFirstTouchTarget赋值，下面就直接返回了true。然后在MOVE和UP事件的时候，也是首先执行dispatchTouchEvent，调用super然后调用onInterceptTouchEvent询问是否拦截，还是false,但是这里因为不是DOWN事件，所以就不会进入判断对其childView反遍历，因为在DOWN的时候mFirstTouchTarget赋值了，所以这时候进入第4步的else语句里面，这时候就对其遍历执行dispatchTransformedTouchEvent，也就是dispatchTouchEvent，然后将其返回。

场景2:我们取消了点击事件，那么在DOWN的时候就不会给mFirstTouchTarget赋值，这个时候将会进入第4步的if判断里面，直接调用dispatchTransformedTouchEvent,所以事件就不会有拦截，最终返回false，所以后续将不会接受到任何事件

场景3:我们在TouchViewGroup的时候是在onInterceptTouchEvent返回true，所以我们intercepted=true，这时候就不会给mFirstTouchTarget赋值，这个时候就调用自身的dispatchTransformedTouchEvent，同样的返回false，后续将不会接受到事件。

通过源码的角度我们也知道了为什么会这么执行，初步有点模糊，我们需要通过项目慢慢的来完善对它的认知。希望对大家有所帮助。