前言
事件传递主要涉及如下重要方法：
- dispatchTouchEvent 负责事件分发
Activity-->PhoneWindow-->DecorView-->ViewGroup-->View
若事件不被拦截，最终会被传递到子View，由子View来进行消费。若子View不消费，则层层回朔之上。 - onInterceptTouchEvent 负责事件拦截
ViewGroup中的方法，是否对事件进行拦截，若拦截，则自己来消费，若不拦截，将事件分发至子View - onTouchEvent 事件消费
下面从代码角度具体来进行分析
ViewGroup中的dispatchTouchEvent
dispatchTouchEvent方法
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
//验证事件是否连续
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
// If the event targets the accessibility focused view and this is it, start
// normal event dispatch. Maybe a descendant is what will handle the click.
if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
ev.setTargetAccessibilityFocus(false);
}
//标记事件是否处理完，若传递给子view，并且子view消费(返回true)，则为true
boolean handled = false;
//过滤掉一些不合法的事件：当前的View的窗口被遮挡了
if (onFilterTouchEventForSecurity(ev)) {
//重置前面为0 ,只留下后八位，用于判断相等时候，可以提高性能。
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
//将DOWN事件定为一连串事件的初始事件，所以要进行一些初始化操作
// 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.
//清空掉之前的一些手势状态
cancelAndClearTouchTargets(ev);
//清空掉mFirstTouchTarget
resetTouchState();
}
//检测是否拦截
// Check for interception.
final boolean intercepted;
//如果当前的事件是DOWN事件，或者是已经有了触摸事件的目标
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
//判断是否允许进行拦截，该变量主要由requestDisallowInterceptTouchEvent方法来确定，该方法由子View发起
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
//允许拦截
if (!disallowIntercept) {
//根据onInterceptTouchEvent方法，判断当前是否拦截
intercepted = onInterceptTouchEvent(ev);
//重置Action，以免被改变
ev.setAction(action); // restore action in case it was changed
} else {
intercepted = false;
}
} else {//如果说，事件已经初始化过了，并且没有子View被分配处理，那么就说明，这个ViewGroup已经拦截了这个事件，那么就继续拦截
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
intercepted = true;
}
// If intercepted, start normal event dispatch. Also if there is already
// a view that is handling the gesture, do normal event dispatch.
if (intercepted || mFirstTouchTarget != null) {
ev.setTargetAccessibilityFocus(false);
}
// Check for cancelation.
//如果viewFlag被设置了PFLAG_CANCEL_NEXT_UP_EVENT ,那么就表示，下一步应该是Cancel事件
//或者如果当前的Action为取消，那么当前事件应该就是取消了。
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
//如果需要（不是取消，也没有被拦截）的话，那么在触摸down事件的时候更新触摸目标列表
//split代表，当前的ViewGroup是不是支持分割MotionEvent到不同的View当中
// Update list of touch targets for pointer down, if needed.
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
//新的触摸对象
TouchTarget newTouchTarget = null;
//标记触摸事件有没有传递给新的触摸对象，并且消费了，即调用子View的dispatchTouchEvent并返回true
boolean alreadyDispatchedToNewTouchTarget = false;
//如果不是取消事件并且也没有拦截
if (!canceled && !intercepted) {
// If the event is targeting accessiiblity focus we give it to the
// view that has accessibility focus and if it does not handle it
// we clear the flag and dispatch the event to all children as usual.
// We are looking up the accessibility focused host to avoid keeping
// state since these events are very rare.
View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
? findChildWithAccessibilityFocus() : null;
//如果是个全新的Down事件
//或者是有新的触摸点
//或者是光标来回移动事件
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
//这个事件的索引，也就是第几个事件，如果是down事件就是0
final int actionIndex = ev.getActionIndex(); // always 0 for down
//获取分配的ID的bit数量
final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
: TouchTarget.ALL_POINTER_IDS;
//清理之前触摸这个指针标识,以防他们的目标变得不同步。
// Clean up earlier touch targets for this pointer id in case they
// have become out of sync.
removePointersFromTouchTargets(idBitsToAssign);
final int childrenCount = mChildrenCount;
//如果新的触摸对象为null（这个不是铁定的吗）并且当前ViewGroup有子元素
if (newTouchTarget == null && childrenCount != 0) {
final float x = ev.getX(actionIndex);
final float y = ev.getY(actionIndex);
//遍历子View，找到一个可以接收事件的子View （就是定位到你当前手指触摸的坐标，找到当前该坐标内的子View）
// Find a child that can receive the event.
// Scan children from front to back.
final ArrayList 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 there is a view that has accessibility focus we want it
// to get the event first and if not handled we will perform a
// normal dispatch. We may do a double iteration but this is
// safer given the timeframe.
if (childWithAccessibilityFocus != null) {
if (childWithAccessibilityFocus != child) {
continue;
}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
//如果child不可以接收这个触摸的事件，或者触摸事件发生的位置不在这个View的范围内
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
//获取新的触摸对象，如果当前的子View在之前的触摸目标的列表当中就返回touchTarget
//子View不在之前的触摸目标列表那么就返回null
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.
//如果新的触摸目标对象不为空，那么就把这个触摸的ID赋予它，这样子，
//这个触摸的目标对象的id就含有了好几个pointer的ID了
//已经获得触摸对象，直接跳出循环
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
//表示之前没有触摸对象缓存，newTouchTarget=null
//如果子View不在之前的触摸目标列表中，先重置childView的标志，去除掉CACEL的标志
resetCancelNextUpFlag(child);
//调用子View的dispatchTouchEvent,并且把pointer的id 赋予进去
//如果说，子View接收并且处理了这个事件，那么就更新上一次触摸事件的信息，
//并且为创建一个新的触摸目标对象，并且绑定这个子View和Pointer的ID
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
//子View在范围内接收并消费了这个事件
// 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();
//将新的触摸目标添加至单链表缓存
newTouchTarget = addTouchTarget(child, idBitsToAssign);
//此时事件已经成功分发至子View，更改状态，防止后续再次分发
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();
}
//如果newTouchTarget为null，就代表，这个事件没有找到子View去处理它，
//那么，如果之前已经有了触摸对象（比如，我点了一张图，另一个手指在外面图的外面点下去）
//那么就把这个之前那个触摸目标定为第一个触摸对象，并且把这个触摸（pointer）分配给最近添加的触摸目标
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.
//如果没有触摸目标
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
//那么就表示我们要自己在这个ViewGroup处理这个触摸事件了
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else {
//遍历TouchTargt树.分发事件，如果我们已经分发给了新的TouchTarget那么我们就不再分发给newTouchTarget
// 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;
//根据上面的标记，防止再次分发
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
//是否让child取消处理事件，如果为true，就会分发给child一个ACTION_CANCEL事件
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
//分发事件
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
//cancelChild也就是说，派发给了当前child一个ACTION_CANCEL事件，
//那么就移除这个child
if (cancelChild) {
//没有父节点，也就是当前是第一个TouchTarget
//那么就把头去掉
if (predecessor == null) {
mFirstTouchTarget = next;
} else {
//把下一个赋予父节点的上一个，这样当前节点就被丢弃了
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
}
//遇到了取消事件、或者是单点触摸下情况下手指离开，我们就要更新触摸的状态
// 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) {
//如果是多点触摸下的手指抬起事件，就要根据idBit从TouchTarget中移除掉对应的Pointer(触摸点)
final int actionIndex = ev.getActionIndex();
final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
removePointersFromTouchTargets(idBitsToRemove);
}
}
if (!handled && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
}
return handled;
}
方法中涉及到涉及到的重要的传递事件方法：
//该方法涉及到是否像子View传递还是调用super传递给自身来处理
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;
}
// Calculate the number of pointers to deliver.
final int oldPointerIdBits = event.getPointerIdBits();
final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;
// If for some reason we ended up in an inconsistent state where it looks like we
// might produce a motion event with no pointers in it, then drop the event.
if (newPointerIdBits == 0) {
return false;
}
// If the number of pointers is the same and we don't need to perform any fancy
// irreversible transformations, then we can reuse the motion event for this
// dispatch as long as we are careful to revert any changes we make.
// Otherwise we need to make a copy.
final MotionEvent transformedEvent;
if (newPointerIdBits == oldPointerIdBits) {
if (child == null || child.hasIdentityMatrix()) {
if (child == null) {
handled = super.dispatchTouchEvent(event);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
event.offsetLocation(offsetX, offsetY);
handled = child.dispatchTouchEvent(event);
event.offsetLocation(-offsetX, -offsetY);
}
return handled;
}
transformedEvent = MotionEvent.obtain(event);
} else {
transformedEvent = event.split(newPointerIdBits);
}
// 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;
}
ViewGroup中的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;
}
自定义View时，重写此方法，根据自己的条件来决定是否拦截
View中的dispatchTouchEvent
上面说到，在ViewGroup的dispatchTouchEvent中，若是事件没被拦截，最终会调用View的dispatchTouchEvent，而且若返回true，则表明子View接收了事件并消费了，那么现在来看看View到底是如何消费的？
public boolean dispatchTouchEvent(MotionEvent event) {
// If the event should be handled by accessibility focus first.
if (event.isTargetAccessibilityFocus()) {
// We don't have focus or no virtual descendant has it, do not handle the event.
if (!isAccessibilityFocusedViewOrHost()) {
return false;
}
// We have focus and got the event, then use normal event dispatch.
event.setTargetAccessibilityFocus(false);
}
//标记是否消费
boolean result = false;
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
final int actionMasked = event.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Defensive cleanup for new gesture
stopNestedScroll();
}
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
//可以看出，如果设置了ouTouch监听，并且返回true，则代表消费了，此时将不会传递到下面的onTouchEvent
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
//如果上述不成立，就交由onTouchEvent来消费。
if (!result && onTouchEvent(event)) {
result = true;
}
}
if (!result && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
// Clean up after nested scrolls if this is the end of a gesture;
// also cancel it if we tried an ACTION_DOWN but we didn't want the rest
// of the gesture.
if (actionMasked == MotionEvent.ACTION_UP ||
actionMasked == MotionEvent.ACTION_CANCEL ||
(actionMasked == MotionEvent.ACTION_DOWN && !result)) {
stopNestedScroll();
}
return result;
}
由上面我们可以看出，如果用户设置了onTouch的监听，并且返回true，那么事件就有onTouch消费，即不会有后续。如果没有，那么将交由onTouchEvent方法来消费。
来看下onTouchEvent：
//前后省略了很多，其实主要就是根据事件的类型，来进行各个事件的消费，然后返回true。这边主要来看下onTouchEvent里面的performClick方法
...
public boolean onTouchEvent(MotionEvent event) {
switch (action) {
case MotionEvent.ACTION_UP:
if (mPerformClick == null) {
mPerformClick = new PerformClick();
}
if (!post(mPerformClick)) {
performClick();
}
break;
}
}
...
具体来看下performClick方法：
public boolean performClick() {
final boolean result;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnClickListener != null) {
playSoundEffect(SoundEffectConstants.CLICK);
li.mOnClickListener.onClick(this);
result = true;
} else {
result = false;
}
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
notifyEnterOrExitForAutoFillIfNeeded(true);
return result;
}
很熟悉，这就是我们的点击事件，看来它的回调是发生在onTouchEvent里面的UP事件里。
所以简单的得出了onTouch—>onTouchEvent—>onClick三个执行顺序。
总结
最后用一段伪代码来总结下：
TouchTarget mFirstTouchTarget=null;//子控件是否消费
public boolean dispatchTouchEvent(MotionEvent event) {
boolean isIntercept =false;//是否拦截
if(onInterceptTouchEvent(event)){//是否拦截
isIntercept=true；
}
if(!isIntercept){//如果没有拦截
//执行子控件的dispatchTouchEvent
boolean consume= child.dispatchTouchEvent(event);
if(!consume){//子控件没有消费事件
mFirstTouchTarget = null;
}else{
//子控件消费了事件，给mFirstTouchTarget赋值
mFirstTouchTarget = addTouchTarget(child, idBitsToAssign);
}
}else{//如果拦截,子控件没有消费事件
mFirstTouchTarget = null;
}
if(mFirstTouchTarget==null){
//子控件没有消费事件
return onTouchEvent(event);
}else{//子控件消费了事件
return true;
}
}