事件分发机制 ViewGroup分析
事件分发从手指触摸屏幕开始，即产生了触摸信息，被底层系统捕获后会传递给Android的输入系统服务IMS，通过Binder把消息发送到activity，activity会通过phoneWindow、DecorView最终发送给ViewGroup。这里就直接分析ViewGroup的事件分发
整体流程
配合图在看一段伪代码：
public boolean dispatchTouchEvent(MotionEvent ev) :Boolean{
val result = false //处理结果，默认是没消费过的
if (!onInterceptTouchEvent(ev)){ //是否拦截
result = child.dispatchTouchEvent(ev) // 分发给子view处理
}
if (!result){ //事件没有消费
if (onTouchListener != null) { //先询问是否设置了onTouchListener
result = onTouchListener.onTouch(ev)
}
if (!result) { //还是没有消费就交给onTouchEvent处理
result = onTouchEvent(ev)
}
}
return result
}
这张图和这段伪代码实际上已经概括了ViewGroup和View对事件处理的整个流程，注意只有ViewGroup有拦截机制即onInterceptTouchEvent
源码分析
在分析源码之前先了解个基本概念 同一事件序列：同一个事件序列是指从手指接触屏幕的那一刻起，到手指离开屏幕的那一刻结束，在这个过程中所产生的一系列事件，这个事件序列以down事件开始，中间含有数量不定的move事件，最终以up事件结束
public boolean dispatchTouchEvent(MotionEvent ev) {
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
/**
* step1
* ACTION_DOWN是一个系列事件的起点，终点是ACTION_UP
* 如果是ACTION_DOWN会重置一些flag并且会把mFirstTouchTarget置空
*/
if (actionMasked == MotionEvent.ACTION_DOWN) {
cancelAndClearTouchTargets(ev);
resetTouchState();
}
final boolean intercepted;//变量判断消息是否被拦截
/**
* step2
* 从以下代码可以看出如果事件不是ACTION_DOWN并且mFirstTouchTarget为空的话那么ViewGroup是不能再拦截同一系列的事件了
* mFirstTouchTarget 代表的就是一个单链表，它会把处理当前这一系列事件的view保存下来
* 假如当前事件是ACTION_MOVE，并拦截了该事件那么会在step9中把mFirstTouchTarget置空
*
* 结论1：
* 如果View决定拦截一个事件那么该View的 onInterceptTouchEvent 方法不会再被调用了，
* 同一序列事件后续的所有事件都只能由该View处理(当然前提是事件能分发到该view，有可能在上层被拦截了)
*/
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
/**
* disallowIntercept表示是否禁用拦截功能，子view通过 requestDisallowInterceptTouchEvent 方法
* 可以要求父view不准拦截事件，不过该方法在MotionEvent.ACTION_DOWN事件中不起作用，因为在step1中会把所有标志位重置
*
*/
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 {
/**
* 如果进不到上面的if判断则表示当前系列事件viewGroup已经拦截过某个事件了
* intercepted 直接置为true
*/
intercepted = true;
}
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
final boolean isMouseEvent = ev.getSource() == InputDevice.SOURCE_MOUSE;
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0
&& !isMouseEvent;
TouchTarget newTouchTarget = null;
boolean alreadyDispatchedToNewTouchTarget = false;
/**
* step3
* 看这里如果ViewGroup拦截了该事件则不会进入step3里面了，而是直接走到step9中
*/
if (!canceled && !intercepted) {
/**
* step4
* 这里我们只考虑单指的点击、移动和抬起
* ACTION_POINTER_DOWN和多点触控有关，ACTION_HOVER_MOVE和鼠标有关
* 所以如果当前事件是MOVE也不会走step4也是直接走到step9中找到对应的子view继而分发事件
* 结论2：如果DOWN事件被某个view消耗那么后续的事件都会直接交给这个view(前提是父view没有拦截)
*/
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;
final int childrenCount = mChildrenCount;
if (newTouchTarget == null && childrenCount != 0) {
final float x =
isMouseEvent ? ev.getXCursorPosition() : ev.getX(actionIndex);
final float y =
isMouseEvent ? ev.getYCursorPosition() : 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;
/**
* step5
* 遍历所有的子view
*/
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(
preorderedList, children, childIndex);
//省略部分代码。。。
/**
* step6
* 当找到一个合适的子view时，在 dispatchTransformedTouchEvent 中会调用子view的dispatchTouchEvent
* 如果该子view消耗了事件，会把子view保存到mFirstTouchTarget对应的链表中，并结束for循环
*
* 结论3：
* 如果一个view没有消耗DOWN事件那么后续的事件都不会再分发给该view
* 该结论和结论2呼应上了，因为在这个for循环中只有子view的 dispatchTransformedTouchEvent返回true才会被加入到链表中
* 下一次的事件并不会再到step4中来了
*/
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();
/**
* step7
* 把子view保存到链表中，mFirstTouchTarget指向表头
* alreadyDispatchedToNewTouchTarget置为true
* 结束for循环
*/
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);
}
}
}
}
/**
* step8
* 如果拦截了事件会把 mFirstTouchTarget 置空这个时候就直接调用viewGroup的super.dispatchTouchEvent
* 即view中的dispatchTouchEvent
*/
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;
/**
* step9
* 如果拦截了就把mFirstTouchTarget置空，没有拦截就找到对应的childView把事件分发下去
*/
while (target != null) {
final TouchTarget next = target.next;
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
//注意这里cancelChild如果为true，并且target.child不为空的话，dispatchTransformedTouchEvent会把事件转成CANCEL分发给target.child
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;
}
}
}
return handled;
}
看下cancel事件的由来，这里需要结合上文代码step9看
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) {
/**
* 把事件转换成ACTION_CANCEL
*/
event.setAction(MotionEvent.ACTION_CANCEL);
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
/**
* 如果child不为空就分发给它
*/
handled = child.dispatchTouchEvent(event);
}
event.setAction(oldAction);
return handled;
}
return handled;
}