探究事件分发
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为什么掌握事件分发
面对滑动冲突时如何解决，如何自定义可双指扩大缩小的view,都需要掌握事件分发的相关知识
首先在探究前，要明白什么是事件序列
什么是事件序列
一个完整的事件序列包括ACTION_DOWN、ACITON_MOVE、ACTION_UP、ACTION_CANCEL,因为包括多指操作，所有会包含ACTION_POINTER_DOWN、ACTION_POINTER_UP等
还有一点，如何标记被触摸的view,而且还是多指操作，通过TouchTarget
TouchTarget
TouchTarget的作用是用来标记被触摸的view以及触摸id集合，是一个单向链表结构，ViewGroup中的mFirstTouchTarget作为链表的头结点
其中比较重要的几个成员变量
public View child表示被触摸的子viewpublic int pointerIdBits这个int值是用来标记被触摸的id集合,因为是多指触摸，而一个bit可以表示一个id,int是32bit,所以可以表示32个id
对象缓存池
因为整个触摸过程中会不断的创建对象，所以内部实现了对象的缓存池，通过obtain获取对象，recycle缓存对象，
如何事件分发
dispatchTransformedTouchEvent方法作用：将一个motion event（事件序列）转换为特定子view的坐标，过滤掉无关的pointer ids,如果有必要重写其action。如果cilid为空，则假定将此MotionEvent发送给此ViewGroup。
整个事件分发的核心在于ViewGroup的dispatchTouchEvent
dispatchTouchEvent
该方法其实总共做了三件事情
- 1.检查是否要拦截事件分发
- 2.如果不拦截，就遍历子view,询问是否拦截
- 3.根据变量
mFirstTouchTarget重新分发事件
下面，逐步剖析下每步具体做了什么操作
1）检查是否要拦截事件分发
final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
//A
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
intercepted = onInterceptTouchEvent(ev);
ev.setAction(action);
} else {
intercepted = false;
}
} else {
intercepted = true;
}
注释A：该标记对应的requestDisallowInterceptTouchEvent方法，如果为true代表子view请求父view不拦截该事件，如果不是子view请求的不拦截的话，再有onInterceptTouchEvent判断是否拦截
从这段逻辑也可以看出，如果down事件中拦截了事件，那么整个事件序列都会交给其处理，除非子view请求不要拦截
2）如果不拦截，就遍历子view,寻找可以消费触摸事件的子view
整个第二步的核心,就是找到TouchTarget
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
...
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
那如果找不到呢？就会将触摸id添加到最近最少使用的target,才用头插法添加到TouchTarget链表
第二步中还有个关键点，就是会按照层级的高低顺序遍历子view,这也体现了常规的“所见即所得”的思想，即用户看到的最上面view的就是级别最高的
比如
<LinearLayout>
<FrameLayout>
<TextView/>
</FrameLayout>
</LinearLayout>
对应的层级顺序可以用下图表示
3）根据变量mFirstTouchTarget重新分发事件
如果mFirstTouchTarget存在，则遍历TouchTarget,设置事件
如果mFirstTouchTarget不存在的话，就会将child置为null,传入dispatchTransformedTouchEvent,实际是将cancel封装成MotionEvent,传入自己的OnTouchEvent
其中还有需要注意的点，就是
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
//注意这个布尔值
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
...
}
本来已经分发给子view的事件，父view会取消掉，那什么情况会触发这个判断呢？
有个经典的例子就是ScollView内包裹Button,按住Button的时候持有这个事件，但是如果滚动屏幕，这时候ScollView就会抢占事件
事件分发的本质
整个事件分发的过程中，会从上到下查看是否需要拦截事件，不管拦截不拦截，都会逐级上报，把拦截结果告诉上层，实际体现的是递归的思路
参考
附完整释义
ViewGruop#dispatchTouchEvent
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
//分发前的准备工作
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;
// 处理初始down事件
if (actionMasked == MotionEvent.ACTION_DOWN) {
//重置之前的触摸手势
cancelAndClearTouchTargets(ev);
resetTouchState();
}
// 检查是否拦截事件
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); // 保存action
} else {
intercepted = false;
}
} else {
//触摸view不存在或者不是最初的down事件，所以view group会继续拦截事件
intercepted = true;
}
//如果已经拦截或者有view捕获了手势，就开始正常的事件分发。
if (intercepted || mFirstTouchTarget != null) {
ev.setTargetAccessibilityFocus(false);
}
//检查是否是cancel事件
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
// 更新包含多指操作down事件的touch target
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
TouchTarget newTouchTarget = null;
boolean alreadyDispatchedToNewTouchTarget = false;
if (!canceled && !intercepted) {
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;
//清除早期该触摸id的touch target,防止数据不同步
removePointersFromTouchTargets(idBitsToAssign);
final int childrenCount = mChildrenCount;
if (newTouchTarget == null && childrenCount != 0) {
final float x = ev.getX(actionIndex);
final float y = ev.getY(actionIndex);
//从层级高的到层级低的，逐级遍历，查找可以接受事件的子view
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 (childWithAccessibilityFocus != null) {
if (childWithAccessibilityFocus != child) {
continue;
}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
if (!child.canReceivePointerEvents()
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
newTouchTarget = getTouchTarget(child);
if (newTouchTarget != null) {
//子view在范围之内接收到了触摸事件，更新触摸id,除了之前已经持有的触摸id还需要增加触摸id
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
resetCancelNextUpFlag(child);
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
//子view想要接收触摸事件
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;
}
ev.setTargetAccessibilityFocus(false);
}
if (preorderedList != null) preorderedList.clear();
}
if (newTouchTarget == null && mFirstTouchTarget != null) {
//没有找到可以接收触摸事件的子view，则将触摸id添加到最近最少使用的target
newTouchTarget = mFirstTouchTarget;
while (newTouchTarget.next != null) {
newTouchTarget = newTouchTarget.next;
}
newTouchTarget.pointerIdBits |= idBitsToAssign;
}
}
}
// 根据mFirstTouchTarget的值分发事件
if (mFirstTouchTarget == null) {
//如果没有touch targets,则将此ViewGroup作为普通view
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else {
//分发给touch targets,但是要排除掉已经分发过的target touch。必要情况下，取消touch targets的分发
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;
}
}
// 更新up或者cancel事件
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;
}
ViewGroup#requestDisallowInterceptTouchEvent
requestDisallowInterceptTouchEvent方法作用：ture表示子view不想让其父view在执行onInterceptTouchEvent拦截事件。本父view会逐级往上传递。这个parent在响应触摸期间，必须执行chlid的请求（也就是说，只会在parent接收到up或者cancel时，清除标记，对应的标记为FLAG_DISALLOW_INTERCEPT）
ViewGroup#onInterceptTouchEvent
public boolean onInterceptTouchEvent(MotionEvent ev) {
if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
&& ev.getAction() == MotionEvent.ACTION_DOWN
&& ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
&& isOnScrollbarThumb(ev.getX(), ev.getY())) {
return true;
}
return false;
}
作用：用来拦截所有的屏幕触摸事件，可以在分发给chlid时查看事件，并且在任何时刻掌握当前手势的所有权。
ViewGroup#dispatchTransformedTouchEvent
将触摸事件转换为特定子view的坐标空间