Android事件分发机制与嵌套导致触摸事件冲突的解决方案
实现滑动的常用方法
- 通过
scrollTo()、scrollBy()来进行滑动 - 使用Scroller来进行滑动
@Override
public boolean onTouchEvent(MotionEvent event) {
case ACTION_UP:
scroller.startScroll(getScrollX(), 0, dx, 0);
invalidate();
break;
}
return super.onTouchEvent(event);
}
@Override
public void computeScroll() {
if (scroller.computeScrollOffset()) {
scrollTo(scroller.getCurrX(), scroller.getCurrY());
invalidate();
}
}
上面的代码基本上就是使用Scroller的基本套路,首先调用startScroll(int startX, int startY, int dx, int dy),然后重写computeScroll()方法获取Scroller计算出来的X、Y坐标后调用scrollTo(int x, int y)进行滑动。
- 使用属性动画来进行滑动
事件分发源码流程
事件分发的流程图:
首先我们带着几个问题去看源码,不然会被源码的很多细枝末节给干扰到。
1. MotionAction的起点是什么地方？
触屏事件最先传递的是Activity,因此事件分发的起点是Activity的dispatchTouchEvent方法。
/**
* 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 (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}
从这段代码能够看到Activity会调用getWindow()的superDispatchTouchEvent((MotionEvent ev)方法,这里我们知道Android的Window对象只有一个实现类PhoneWindow,所以我们再看PhonwView的superDispatchTouchEvent((MotionEvent ev)方法。
// This is the top-level view of the window, containing the window decor.
private DecorView mDecor;
@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}
这里可以看到实际上交给了DecorView的superDispatchTouchEvent((MotionEvent ev)方法来处理,比较了解DecorView的可以知道它实际上是一个封装的FrameLayout,也就是一个一个Top-level的ViewGroup,从这里就是我们熟悉的View事件分发机制的开始。
2. ViewGroup是如何将事件分发给子View的？
我们先忽略ViewGroup的拦截事件的调用,先考虑最简单的情况。
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(
preorderedList, children, childIndex);
...
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
...
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;
}
...
}
从代码中我们能够看到将事件分发给子View的方式就是执行一次遍历执行dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)方法,如果这个方法返回了true的会就会立刻停止继续遍历,触屏事件也将会由这个子View进行消费。我们可以详细的看下dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)到底做了什么?
/**
* 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;
...
// 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();
}
// 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;
}
从代码能够看到如果child不等于null,将会调用child的dispatchTouchEventdispatchTouchEvent(MotionEvent event)方法。
继续进行childView的事件分发流程。
这里我们还要关注一下遍历时addTouchTarget(child, idBitsToAssign);这个方法具体的作用是什么?
// First touch target in the linked list of touch targets.
private TouchTarget mFirstTouchTarget;
/**
* 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;
}
从代码我们能够看到这个又一个TouchTarget的链表,我们在dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)返回true之后,会给mFirstTouchTarget进行赋值,此时mFirstTouchTarget将会不为null。
当然,在执行dispatchTransformedTouchEvent前会过滤掉一些不可能接收到触屏事件的子View。
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
从代码我们能够看到主要是判断了两点
1. 点击的Point是否在子View的区域内。
2. 子View是否正在执行动画。
如果符合上述的任意一点就会直接continue,进行循环的下一个项。
3. 什么情况下会触发ViewGroup的onInterceptTouchEvent((MotionEvent ev)方法？
// 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;
}
从代码我们能够看到执行 onInterceptTouchEvent(ev) 的条件这么几个,我们一个一个来看代表什么。
-
actionMasked == MotionEvent.ACTION_DOWN|| mFirstTouchTarget != nullactionMasked == MotionEvent.ACTION_DOWN
这个很好理解如果触屏事件是ACTION_DONW的话条件就能成立。mFirstTouchTarget != null
从上面的源码分析我们可以知道,mFirstTouchTarget!=null就代表着触屏事件之前已经被子View给消费过了,剩余的事件序列将会全部交由那个View来处理。
-
!disallowIntercept
这里我们可以看代码
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
这里有个标志FLAG_DISALLOW_INTERCEPT,如果这个标志不为0的话,disallowIntercept就可以为true,此时!disallowIntercept将会不成立,那么如果让这个值为true呢?这里就牵扯到一个方法,requestDisallowInterceptTouchEvent(boolean disallowIntercept),通过这个方法我们就能够让父View不区拦截子View的事件,因此这个方法也是我们解决事件冲突的方式之一。
4. 能否让ViewGroup的不拦截所有的触屏事件？
这个问题其实是否定的,从代码我们就能够看到
// 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();
}
/**
* Resets all touch state in preparation for a new cycle.
*/
private void resetTouchState() {
clearTouchTargets();
resetCancelNextUpFlag(this);
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
mNestedScrollAxes = SCROLL_AXIS_NONE;
}
从代码我们能够看到当触屏事件是ACTION_DOWN时会重置全部的TouchTargets和FLAG_DISALLOW_INTERCEPT,因此触屏事件是
ACTION_DOWN时,父ViewGroup将会必定执行onInterceptTouchEvent(ev)方法。
5. setOnTouchListneer后还会不会触发onTouchEvent((MotionEvent ev)方法？
我们可以分析View的dispatchTouchEvent(MotionEvent event)方法
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;
}
从这段代码我们可以知道如果mOnTouchListener!=null,那么执行mOnTouchListener.onTouch(this, event),如果返回true,那么就是result =true,这时onTouchEvent(event)便不会再执行了,所以onTouchEvent(event)是否还执行需要看mOnTouchListener.onTouch(this, event)的返回值。
6. 某个View一旦开始处理事件,那么同一个事件序列的事件还会交给其它View处理吗？
// 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;
}
// 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;
}
}
从这段我们可以知道如果mFirstTouchTarget != null,那么触摸事件ViewGroup将不会进行拦截,同时将会遍历mFirstTouchTarget执行dispatchTransformedTouchEvent()。
什么时候会执行ViewGroup的onTouchEvent()
当mFirstTouchTarget为null时才会调用ViewGroup的onTouchEvent()方法。也就是说有两种可能
1. ACTION_DOWN时ViewGroup的onInterceptTouchEvent()返回为true。
2. 遍历调用了一遍child的dispatchTouchEvent()触摸事件都没有被消费掉,此时也会执行ViewGroup的onTouchEvent()
嵌套导致滑动事件冲突的解决方法
-
自己处理事件分发的过程解决冲突
这种方式需要自己处理事件的分发与拦截,比较复杂。同时又分为外部处理与内部处理两种。对开发者事件分发机制理解的要求比较的高。
简单举个面试经常会被问的问题,
ScrollView嵌套ListView该如何处理,正常情况下滑动事件会被ScrollView拦截消费,不过一般来说ViewGroup都不会拦截ACTION_DONE事件时调用requestDisallowInterceptTouchEvent(true);让ScrollView不拦截滑动事件,于是滑动事件变能够正常传递给ListView,ListView也就能够正常进行滑动了。@Override public boolean dispatchTouchEvent(MotionEvent ev) { switch (ev.getAction()) { case MotionEvent.ACTION_DOWN: mLastY = ev.getY(); requestDisallowInterceptTouchEvent(true); break; return super.dispatchTouchEvent(ev); }但是这种方式也存在瑕疵,如果ScrollView中存在一个HeadView,当ListView已经滑动到最上方时,如果将互动的事件转交给ScrollView呢?正常的事件分发一旦某个View消费了事件,那么事件序列接下来的事件都会被他消费掉,此时我们可以手动的将事件再传递给
ScrollView不过自己来写的话耦合会非常的大,类似这样的:case MotionEvent.ACTION_MOVE: boolean isMoveUp = mLastY - ev.getY() < 0; if (isMoveUp && firstVisiblePosition == 0) { Log.d("tset", "needParent consume!"); requestDisallowInterceptTouchEvent(false); ((ViewGroup)getParent()).onTouchEvent(ev); return false; } else if (!isMoveUp && lastVisiblePosition == getAdapter().getCount() - 1) { Log.d("tset", "needParent consume!"); requestDisallowInterceptTouchEvent(false); ((ViewGroup)getParent()).onTouchEvent(ev); return false; } break;于是Android又给我们提供了基于NestedScrolling的解决方案。
-
基于NestedScrolling的实现方案
NestedScrolling主要的思想是:Child来做事件的接收者,当Child接收到Move事件时,先调用
dispatchNestedPreScroll()把事件交给Parent来处理,Parent将会收到回调onNestedPreScroll()此时Parent判断是否需要消耗事件,消耗多少距离的滑动,之后Child根据剩余未消耗掉的距离继续执行滑动。 -
基于CoordinatorLayout、Behavior的实现方案(后续再专门写一篇总结)
参考资料
Android Scroller完全解析，关于Scroller你所需知道的一切
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Android嵌套滑动机制
NestedScrolling帮你实现一个简单的嵌套滑动
Android NestedScrolling机制完全解析 带你玩转嵌套滑动
Awesome-Android-Interview