一篇文章读懂android事件消费、事件分发、事件拦截
Android 源码分析事件分发机制、事件消费、事件拦截
1.前言
事件分发这个东西嘛，大家一直都在讲，但总有人觉得吃不透。为什么呢？因为事件分发是多维的，有好多条思维分岔路口，而文章基本上只能用一维的方式从左到右，从上到下进行表达，所以基本不可能让普通智力的人从入门到精通。我们所要做的，就是踏踏实实打开源码，自己多琢磨，多整理。读源码切记要从宏观的角度去看问题，不然往往会深陷细节出不来了，我们自己想想也知道，别人的框架可能10几个人，耗时几个月或者一年完成的，你能一两天、几个星期读个源码就都掌握么。要摸细节的话，你要带着问题有目标的去读，常读常新，每次有收获就行
下面内容请配合源码食用！不然基本上索然无味
读过上一篇文章，相信你对事件的消费、分发和拦截在宏观上，都有了一定的了解，这篇文章我们从源码分析，加深了解。
ViewGroup
- 我们先走正常流程，假设一开始没被拦截，为了方便下面的描述直接贴出viewgroup#dispatchTouchEvent的代码
@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.
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();
}
// Check for interception.
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;
if (!canceled && !intercepted) {
// If the event is targeting accessibility 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.
final ArrayList<View> 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);
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;
}
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
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);
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.
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;
}
我们知道一个事件组都是从down事件开始的，所以我们从触摸事件的起点事件开始分析
Down
一开始进入ACTION_DOWN判断，首先会重置一些状态变量的值，也就是每次down事件开始，都会重置上一次事件的变量，现在我们假设事件DOWN没被拦截，我们走到这句 final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;因为一开始mGroupFlags等于0，0&任何值都是0；0！=0，所有disallowIntercept是false， false就走入intercepted = onInterceptTouchEvent(ev);这句，因为一开始没被拦截，所有intercepted也就是onInterceptTouchEvent(ev)的默认返回值false，一开始肯定也是没有取消的，所以canceled也为false，newTouchTaget一开始初始化为null，alreadyDispatchedToNewTouchTarget = false，进入if (!canceled && !intercepted) {，childrenCount就是子view的数量，因为dispatchTouchEvent方法是从根向上开始走的，所以只要页面有view就肯定不为0，直接进入buildTouchDispatchChildList()对子view排序，排序完成赋值给ArrayList<View> preorderedList数组，我们点进buildTouchDispatchChildList看看是怎么排序的
for (int i = 0; i < childrenCount; i++) {
// add next child (in child order) to end of list
final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
final View nextChild = mChildren[childIndex];
final float currentZ = nextChild.getZ();
// insert ahead of any Views with greater Z
int insertIndex = i;
while (insertIndex > 0 && mPreSortedChildren.get(insertIndex - 1).getZ() > currentZ) {
insertIndex--;
}
mPreSortedChildren.add(insertIndex, nextChild);
}
如果插入的view的z坐标的数值大于当前view的z值，就插mPreSortedChildren，然后insertIndex--一直循环插入完为止。
简单的来说，这个排序完，就是z轴的值越大越排在数组的后面（我们知道，从z方向看，父view一直是在子view的下面的，子view一直是往x轴正方向叠加的）
image.png
image.png
我们接着看，for (int i = childrenCount - 1; i >= 0; i--) {，又从数组最末端开始for循环取出对应的view，所以我们最先拿出最顶部的子view，也就是我们点击事件的时候最先接触的那个子view。顶部向底部一层层拿到对应的view后，然后通过canViewReceivePointerEvents方法判断这个view是不是VISIBLE显示状态，如果是VISIBLE，再通过isTransformedTouchPointInView判断这个view的x、y值和当前点击的位置是否是对应的，如果这个view就是你当前点击的，就返回true
如果不通过判断直接continue，进入循环进入下一个view，通过验证判断则进入if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {，在
dispatchTransformedTouchEvent方法分发事件，我们点进入看看
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();
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;
}
因为child是有值的，所以进入handled = child.dispatchTouchEvent(event);，
调用子view的dispatchTouchEvent方法，如果child是ViewGroup，就相当于重新执行上面的一大波步骤；如果child是View，则类似于父View拦截事件的过程，会执行View本身的Touch或Click方法，我们这里假设child是view不是ViewGroup，我们知道view类的dispatchTouchEvent方法里，如果onTouchEvent(event)返回true就代表view消费了这件事件，所以 handled = child.dispatchTouchEvent(transformedEvent);
handled就是true，所以dispatchTransformedTouchEvent就是true，返回到之前代码，我们接着看如果这个view消费了这个down事件，这个if里接下来会做什么处理，
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;
}
在循环最后我们看到有一个break，也就是如果有一个子view处理了事件，整个for循环就break跳出了，所以这个点击事件直接交给子view来处理了，不再循环，它的父view就接受不到这个事件了，当然前面如果一直没有进入if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {这个循环就会一直沿着
z轴的负方向去找这个子view的父view、父亲的父view、、、、直到找到对应的父view来处理。
我们再看看在跳出dispatchTransformedTouchEvent的这个if之前，有两个赋值操作newTouchTarget = addTouchTarget() ===> newTouchTarget =mFirstTouchTarget；alreadyDispatchedToNewTouchTarget = true;
这两个赋值完才breadk的，break跳出循环后，我们接着看进入mFirstTouchTarget ！= null这个判断（因为前面赋值了）
while (target != null) {因为target第一次等于mFirstTouchTarget不为空，第二次等于target.next.mFirstTouchTarget是为空的，所以这个while循环只走一次，
我们再往下看，因为之前我们对alreadyDispatchedToNewTouchTarget和newTouchTarget赋值了，所以进入这个if
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
将handled赋值为true，最后再return handled，也就是整个dispatchTouchEvent方法return true，到此，整个Down事件处理结束。这个是正常的down事件，是没有被拦截的，
假如被拦截呢？，我们再回到ViewGroup#dispatchTouchEvent看看，
假如第一次进来被拦截的话，也就是如果我们如果在ViewPager里重写onInterceptTouchEvent并且返回true的时候，intercepted = onInterceptTouchEvent(ev);返回true；也就是Viewpager里拦截了事件，通过前面，我们知道事件分发是从父容器分发到子容器的，之前的if就不进去了，直接被拦截在Viewpager层了，后面的都不会分发到子view了，所以我们越过之前那个if (!canceled && !intercepted) {，接下去看if (mFirstTouchTarget == null) {
因为第一次进入，也没有进入过f (!canceled && !intercepted) {，所以mFirstTouchTarget没被赋值是为null的，所以进入if (mFirstTouchTarget == null) {这个if判断，所以会直接也会进入dispatchTransformedTouchEvent，只不过传进去的这个child默认是null了，我们再点进去dispatchTransformedTouchEvent看看
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;
}
我们直接走进child == null分支，直接进入 handled = super.dispatchTouchEvent(transformedEvent);所以也调用到View类的dispatchTouchEvent方法里去分发事件，请注意！虽然最后代码跑到了View中，但这个View是ViewGroup的父类！也就是说最终执行的Touch或Click方法依然是外层ViewGroup中重写的Touch或Click方法！请区分ViewGroup、View、父View与子View的区别~。
事件DOWN在之前已经是被最顶部的view拿到了，再看看MOVE事件是怎么处理的。
Move
Move也是进入ViewGroup的dispatchTouchEvent方法，一开始还是会进入
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;
}
因为之前经过DOWN事件处理，所以mFirstTouchTarget不为null，所以MOVE事件也能够被拦截的原因就在于这个if判断。接着看，现在是MOVE事件也没有被取消，我们还是假定没被拦截的情况，所以直接进入 if (!canceled && !intercepted) {判断，因为我们现在是Move事件，所以也没有preorderedList循环子view分发事件的操作。我们接着往下，直接进入mFirstTouchTarget ！= null分支，这里注意了，虽然mFirstTouchTarget没有被重置，但alreadyDispatchedToNewTouchTarget在进入之前进入循环之前会被置为false;所以进入里面的else的分支，进入dispatchTransformedTouchEvent方法，因为intercepted == false，所以dispatchTransformedTouchEvent传入的cancelChild也为false，所以我们次再进入dispatchTransformedTouchEvent方法看看，因为经过Down事件child也是不为null的，所以进入handled = child.dispatchTouchEvent(transformedEvent);MOVE事件就是在这里进行分发与回调的！所以dispatchTransformedTouchEvent就是true，返回到之前代码，我们接着看如果这个view消费了这个down事件，这个if里，执行 handled = true;
最后再return handled，也就是整个dispatchTouchEvent方法return true，到此，整个Move事件也处理结束。
虽然MOVE和DOWN都是交由View本身的onTouchEvent()方法执行，但是但因为动作类型MotionEvent不同，也是不同操作的，这里就讲一个点，如果是ACTION_UP类型，则执行performClick()方法。在performClick方法里，会监听onClick()方法（如果你通过setOnClickListener为当前View设置了Click监听）。
getParent().requestDisallowInterceptTouchEvent(true)
大家都用过这个方法，我们知道在子view里调用父view的requestDisallowInterceptTouchEvent(true)方法，方法里传入true，这样就能通知到父view不要在事件流里拦截我了，我们看到源码是具体怎么实现的。
@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);
}
}
我们知道在调用requestDisallowInterceptTouchEvent方法的时候，入参传入true的时候，会执行这句代码mGroupFlags |= FLAG_DISALLOW_INTERCEPT;把mGroupFlags赋值为mGroupFlags |FLAG_DISALLOW_INTERCEPT，并且递归调用，只要父View不为null， 就把父View的FLAG同样设置。
之前在viewgroup的dispatchTouchEvent方法里有这么一段代码
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;
}
我们看到这句final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;现在mGroupFlags为mGroupFlags |FLAG_DISALLOW_INTERCEPT，所以现在mGroupFlags和FLAG_DISALLOW_INTERCEPT一做 & 操作就是等于FLAG_DISALLOW_INTERCEPT，所以disallowIntercept是为true，就不会走
intercepted = onInterceptTouchEvent(ev);也就是就算父view内部重写了onInterceptTouchEvent(ev);我这边通过改变mGroupFlags标志位，直接把intercepted设置为false，直接不设置拦截。
通过一个事件组，分析完ViewGroup里的dispatchTouchEvent方法，我们得出的结论其实就是上一篇文章里讲的重点：
- 1、事件都是以事件组的形式存在的，开始于DOWN事件结束于UP事件或者ACTION_CANCEL事件。
- 2、在一个事件组中，你最终每执行一个事件（down、move）就会走一次onTouchEvent。
- 3、事件分发都是从顶部的子view开始，向下传递的，如果第一个view的onTouchEvent对这个DOWN事件没有响应，就一直向下传递，直到遇到第一个对这个DOWN事件作出响应的View，这个向下的DOWN事件就会结束，并接下来的move事件也交于这个view处理。
- 4、在事件从顶部的子view开始分发之前，Android会从这个Activity里面最底部的那个根view，向上去一级一级的询问：你要不要拦截这组事件？拦截的意思就是事件不交给子view了，我自己来处理。具体在实现上，就是通过调用viewGroup的onInterceptTouchEvent方法返回true来实现的。
- 5、还有一个就是requestDisallowInterceptTouchEvent，子view里调用父view的requestDisallowInterceptTouchEvent(true)方法，方法里传入true，这样就能通知到父view不要在事件流里拦截我了，源码实际是通过改变标志位mGroupFlags来实现的。
最后有空的可以看下ScrollView嵌套ScrollView的滑动冲突这篇文章，带着源码requestDisallowInterceptTouchEvent的思路，看看滑动冲突的解决，趁热带铁，效果更佳。
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