征服面试官系列：View的事件冲突，原理你了解吗？有怎样的解决方案？
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前面了解了【view的事件分发】，这里我们继续研究一下view的事件分发和处理，从而找到事件冲突的原因及其解决方案。
一、概念认知
1）事件类型：
在view的onTouchEvent(方法中)处理MotionEvent()的事件有：
DOWN: 手指触摸屏幕的事件
UP: 手指离开屏幕是的事件
MOVE: 手指在屏幕上移动时的事件
CANCEL: 当事件被拦截的时候就会触发
2）事件分发的类与方法
从源码角度来分析，事件分发的流程：
activity--->dispatchTouchEvent()
PhoneWindow --->superDispatchTouchEvent()
DecorView --->superDispatchTouchEvent()
ViewGroup--->dispatchTouchEvent()
view--->dispatchTouchEvent()
view--->onTouchEvent()
3）事件冲突：
只有一个事件，但处理的对象有多个，而真正处理的对象并不是期待的对象，这样就发生了冲突；
也就是说，一个事件可以由多个view处理时会出现事件冲突（如滚动嵌套），而一个view对一个事件有多个处理方式时也会出现冲突（如touch 和click 事件）。
二、事件分发的流程
在学习事件分发之前，有些状态或标记是通过位运算符来运算的，所以下面先简单了解一下位运算符：
& 比较位的值都是1的时候，该位的结果才是1
| 比较的位只要有一位是1 ，该位的结果就是1
~ 将当前位进行0 1 的交换，即0 换成1 ，1 换成0
2.1)、DOWN 事件的处理与分发流程：
ViewGroup.java中dispatchTouchEvent(MotionEvent ev) 的源码中
2.1.1）重置mGroupFlags
执行的结果 mGroupFlags &= mGroupFlags & (~FLAG_DISALLOW_INTERCEPT);
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();
}
//会重置mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
2.1.2）事件是否拦截
用上一步重置的mGroupFlags的值替换下面的表达式：
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
替换后的结果是：
final boolean disallowIntercept = ( mGroupFlags & (~FLAG_DISALLOW_INTERCEPT)& FLAG_DISALLOW_INTERCEPT) != 0;
由此可以判断disallowIntercept 为false; 就会调用 onInterceptTouchEvent(ev) 来判断是否被拦截，这个方法可在自定义view中被重写的。
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;
}
2.1.3）事件处理与分发
1) 当被拦截的时候，即 onInterceptTouchEvent(ev)方法返回为true
A) 直接跳过下面的 if (!canceled && !intercepted)条件 ；
由于首次事件处理，那么mFirstTouchTarget==null成立，然后就进入if (mFirstTouchTarget == null) 里面执行，这时候就会进入到dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS);
注意：第二个参数canceled 是false ,第三个参数是null
if (!canceled && !intercepted) {
//此处忽略
}
// 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 {
//此处忽略
}
}
B) 进入dispatchTransformedTouchEvent()方法:
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
final int oldAction = event.getAction();
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
//此处条件不成立，暂时忽略
}
// Perform any necessary transformations and dispatch.
if (child == null) {
handled = super.dispatchTouchEvent(transformedEvent);
} else {
//此处条件不成立，暂时忽略
}
// Done.
transformedEvent.recycle();
return handled;
}
说明：
由于传入的第三个参数是null ,就会直接进入该方法的最底部，调用super.dispatchTouchEvent(transformedEvent); 而当前viewGroup的super是view；这样就很显然调用了view的dispatchTouchEvent()方法.
C) dispatchTouchEvent()方法
说明：
1、设置了OnTouchListener监听者，会回调到监听者的onTouch() 方法中处理，
**
onTouch()方法返回true ,表示当前view已经消费了该事件， onTouch()方法返回false, 则会进入view的onTouchEvent()中2、没有设置OnTouchListener监听者，则会进入view的onTouchEvent()中
public boolean dispatchTouchEvent(MotionEvent event) {
boolean result = false;
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
}
return result;
}
D) View.java 的onTouchEvent()，该方法是真正的事件处理的地方，下面只取了down事件的处理
//onTouchEvent()
public boolean onTouchEvent(MotionEvent event) {
//此时是点击事件，所有下面的clickable是true
final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
switch (action) {
case MotionEvent.ACTION_DOWN:
// Walk up the hierarchy to determine if we're inside a scrolling container.
boolean isInScrollingContainer = isInScrollingContainer();
// For views inside a scrolling container, delay the pressed feedback for
// a short period in case this is a scroll.
if (isInScrollingContainer) {
//此处处理的是带滚动的容器中的down事件
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true, x, y);
checkForLongClick(
ViewConfiguration.getLongPressTimeout(),
x,
y,
TOUCH_GESTURE_CLASSIFIED__CLASSIFICATION__LONG_PRESS);
}
break;
}
//此处返回的是true ,表示已经消费了该down事件
return true;
}
return false;
}
2) 当未被拦截的时候：即 onInterceptTouchEvent(ev)方法返回为false
A) 没有被拦截时，上面拦截状态下跳过的if条件就会执行
说明：
1、mChildrenCount 是指当前viewgroup的直接childview的数量，不包括childview的childview
2、在循环遍历childView的时候，if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign))这个条件中再次给childview进行事件分发；
此处的第二个参数是false ,第三个参数child不为null ,这一步和down事件传入的不同
3、当第2点中if语句成立，也就是有child处理了事件的时候，将该处理事件的view加入到addTouchTarget(child, idBitsToAssign);事件视图链表中（这一点的巧妙之处在于，当move ，up事件发生时，就不用再次查找，直接从该链表中找即可）
if (!canceled && !intercepted) {
if (actionMasked == MotionEvent.ACTION_DOWN || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
final int childrenCount = mChildrenCount;
if (newTouchTarget == null && childrenCount != 0) {
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 (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;
}
}
}
B) 再次进入dispatchTransformedTouchEvent（）方法,就会执行下面代码中的else分支，这个时候会调用的child.dispatchTouchEvent(),而这个时候的child就是我们参数中第三个传递进来的view ,其实就是递归遍历当前view下面的所有的节点childview ,直到找到事件处理的地方。
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
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);
}
}
至此，down事件的分发与处理已经完成，那么接下来看看move事件的处理与分发。
2.2)、MOVE 事件的分发与处理
完成了上面的DOWN的分发与处理后，再次分发MOVE事件的时候，还是从ViewGroup的dispatchTouchEvent()方法开始，只是这个时候if (mFirstTouchTarget == null)条件就不成立了，会进入else分支中：
在执行while循环是， if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) 这个条件其实就是处理已经分发过的，避免再次分发
1、如果已经分发过的view ,那么就直接返回了
2、如果没有分发过 ,再次进入dispatchTransformedTouchEvent（）方法中,此处需要关注他的第二个参数cancelChild ,关键看一下intercepted的值
final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted;
如果intercepted 在上面是被拦截的，cancelChild 即为true,否则就为false.
{
// 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;
/**
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
target.next = mFirstTouchTarget;
mFirstTouchTarget = target;
return target;
}
*/
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;
}
3、 cancelChild 为true
下面的代码和donw事件的分发和处理是基本上一样的，如果没有childview节点就交给view的dispatchTouchEvent()来处理，如果有就执行递归调用child.dispatchTouchEvent(event)
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;
}
//下面的代码省略
}
4、cancelChild 为false
这种情况就和down的事件处理与分发就一样了。直到调用到view的onTouchEvent()方法中
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
// 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;
}
5、view的onTouchEvent() 处理move事件
public boolean onTouchEvent(MotionEvent event) {
switch (action) {
case MotionEvent.ACTION_MOVE:
if (clickable) {
drawableHotspotChanged(x, y);
}
final int motionClassification = event.getClassification();
final boolean ambiguousGesture =
motionClassification == MotionEvent.CLASSIFICATION_AMBIGUOUS_GESTURE;
int touchSlop = mTouchSlop;
if (ambiguousGesture && hasPendingLongPressCallback()) {
if (!pointInView(x, y, touchSlop)) {
// The default action here is to cancel long press. But instead, we
// just extend the timeout here, in case the classification
// stays ambiguous.
removeLongPressCallback();
long delay = (long) (ViewConfiguration.getLongPressTimeout()
* mAmbiguousGestureMultiplier);
// Subtract the time already spent
delay -= event.getEventTime() - event.getDownTime();
checkForLongClick(
delay,
x,
y,
TOUCH_GESTURE_CLASSIFIED__CLASSIFICATION__LONG_PRESS);
}
touchSlop *= mAmbiguousGestureMultiplier;
}
// Be lenient about moving outside of buttons
if (!pointInView(x, y, touchSlop)) {
// Outside button
// Remove any future long press/tap checks
removeTapCallback();
removeLongPressCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
}
final boolean deepPress =
motionClassification == MotionEvent.CLASSIFICATION_DEEP_PRESS;
if (deepPress && hasPendingLongPressCallback()) {
// process the long click action immediately
removeLongPressCallback();
checkForLongClick(
0 /* send immediately */,
x,
y,
TOUCH_GESTURE_CLASSIFIED__CLASSIFICATION__DEEP_PRESS);
}
break;
}
}
2.3)、UP事件的处理与分发
了解了DOWN ,MOVE 事件的分发与处理后，相信UP事件的处理与分发就简单多了，我们着重看一下view的onTouchEvent()方法对UP事件的处理
public boolean onTouchEvent(MotionEvent event) {
switch (action) {
case MotionEvent.ACTION_MOVE:
mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
if ((viewFlags & TOOLTIP) == TOOLTIP) {
handleTooltipUp();
}
if (!clickable) {
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;
}
boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
// take focus if we don't have it already and we should in
// touch mode.
boolean focusTaken = false;
if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
focusTaken = requestFocus();
}
if (prepressed) {
// The button is being released before we actually
// showed it as pressed. Make it show the pressed
// state now (before scheduling the click) to ensure
// the user sees it.
setPressed(true, x, y);
}
if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
// This is a tap, so remove the longpress check
removeLongPressCallback();
// Only perform take click actions if we were in the pressed state
if (!focusTaken) {
// Use a Runnable and post this rather than calling
// performClick directly. This lets other visual state
// of the view update before click actions start.
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClickInternal();
}
}
}
if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}
if (prepressed) {
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
mIgnoreNextUpEvent = false;
break;
}
}
- 特别注意
performClickInternal(); 实际就是处理我们自己实现的OnClickListener的监听者的。
因此：TouchListener是在down的时候会回调，而clickListener是在up事件处理的，这样当我们对一个view添加了touchListener和clickListener两个监听时，如果想要clickListener有效，就需要在touchListener的onTouch回调方法中返回false ,让事件继续分发，否则就不会分发了。
总结：
当发生一个touch事件(down)的时候，如果事件被拦截，那么就该view处理该事件，否则就开始递归遍历view及其child,找到处理该事件的view ,并将该事件添加到目标view的链表中，下一个事件（move）发生的时候，就直接在目标view的链表中进行分发，最后在view的onTouchEvent()中处理；
当view设置了touchListener时，在down事件发生时就会回调到listener中去，listener返回true ,则事件不再下发，否则的话事件继续下发。
当view设置了clickListener时，在up事件发生时就回调到listener中去，在listener的回调中处理该事件。
注意：
如果view是叶子节点时，那么如果他没有处理down事件，同样也不会处理move,up事件；
在事件分发和处理过程中，viewGroup主要负责分发事件，而view则负责处理事件；
事件如果被拦截，那就代表事件不能再传递给自己的childview了，也就是自己就是最后一个view，事件需要自己处理；
由此可见，事件冲突的处理方式：
内部拦截：在childview中处理事件冲突
外部拦截：在parentview中处理事件冲突。
parentView可以抢夺childview的事件，反之childview不能抢夺parentView的事件；view一旦拿到了事件，那么事件由谁处理就是该view说了算了。特别注意的一点就是：这里所说的childview都是指当前view的直接子view ,而不包括childview的childview.
最后
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转载自:https://juejin.cn/post/6995464264144142349