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Android事件分发机制浅析（2）
叁叁肆2018-09-26 11:00本文来自网易云社区
作者：孙有军
上面的两次执行中每次都调用了onInterceptTouchEvent事件，这个到底又是啥？我们去看看他的返回值是什么？
public boolean onInterceptTouchEvent(MotionEvent ev)
{
return false;
}
可以看到默认返回false，注释长的吓人，那我们就来改写一下他的返回值，这个函数是ViewGroup才有的，说明与布局容器有关. round 3
我们将Layout的onInterceptTouchEvent的返回值改为true。重新执行。执行顺序如下：
05-05 14:59:17.829 15157-15157/com.sunny.event E/Event: Layout dispatchTouchEvent ACTION_DOWN05-05 14:59:17.830 15157-15157/com.sunny.event E/Event: Layout onInterceptTouchEvent ACTION_DOWN05-05 14:59:17.830 15157-15157/com.sunny.event E/Event: Layout onTouchEvent ACTION_DOWN
从日志可以发现，只有最外层的控件能够执行事件，TextView已经收不到任何事件。
小结
父控件onInterceptTouchEvent返回true会拦截子控件的事件
追根溯源
我们从代码的层面来看看他是怎么执行的，当屏幕接收到点击事件时会首先传递到Activity的dispatchTouchEvent：
/**
* Called to process touch screen events. You can override this to
* intercept all touch screen events before they are dispatched to the
* window. Be sure to call this implementation for touch screen events
* that should be handled normally.
*
* @param ev The touch screen event.
*
* @return boolean Return true if this event was consumed.
*/public boolean dispatchTouchEvent(MotionEvent ev) { if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
} if (getWindow().superDispatchTouchEvent(ev)) { return true;
} return onTouchEvent(ev);
}
在这里执行了三步，
第一告诉用户ACTION_DOWN，用户可以复写onUserInteraction来处理点击开始
调用了getWindow().superDispatchTouchEvent(ev)，这里的getWindow得到是PhoneWindow对象，因此执行的PhoneWindow的superDispatchTouchEvent函数，
调用了Activity的onTouchEvent事件
PhoneWindow的superDispatchTouchEvent又调用了DecorView的superDispatchTouchEvent函数，每一个Activity都有一个PhoneWindow，每一个PhoneWindow都有一个DecorView，DecoView继承自FrameLayout，这里又调用了super.dispatchTouchEvent(event)，FrameLayout里面是没有改函数的，所以最终执行的是ViewGroup的dispatchTouchEvent函数。
这里我们先穿插一点界面的知识，以我测试手机为例，DecorView中有两个child，分别是ViewStub和LinerLayout，LinerLayout中又包含了FrameLayout，FrameLayout中包含了一个ActionBarOverlayLayout，ActionBarOverlayLayout里又包含了两个child，分别是ActionBarContainer与ContentFrameLayout。
接下来我们去看看ViewGroup的dispatchTouchEvent函数：
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
.........
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) {
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;
}
............
// 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 (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 = buildOrderedChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = customOrder
? getChildDrawingOrder(childrenCount, i) : i;
final View child = (preorderedList == null)
? children[childIndex] : preorderedList.get(childIndex);
.......
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;
}
}
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;
}
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