手把手教你读懂源码，View的Touch事件传递流程详细剖析

昨天看到有童鞋在聊面试时经常被问到Android的事件传递问题，不知道从何答起。本来近段时间我就一直在分析有关View的源码：

手把手教你读懂源码，View的加载流程详细剖析

手把手教你读懂源码，View的绘制流程详细剖析

手把手教你读懂源码，View事件的注册和接收详细剖析

继上一篇分析，今天我们来接着分析Activity的Touch事件是如何分发传递的。

都知道在Android中的事件主要包括三部分内容：分发事件dispatchTouchEvent、拦截事件onInterceptTouchEvent、消费事件onTouchEvent。这几乎是所有开发者都要面临的问题，无论是解决一些事件冲突问题，还是自定义View，都会或多或少涉及到。由于其独特的重要性，大多数面试的时候也基本会有所涉及，所以很好的掌握View的Touch事件传递显得尤其重要。

1、Activity的dispatchTouchEvent

首先来看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 (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}

如果事件为按下状态，则先调用onUserInteraction方法：

/**
* Called whenever a key, touch, or trackball event is dispatched to the
* activity.  Implement this method if you wish to know that the user has
* interacted with the device in some way while your activity is running.
* This callback and {@link #onUserLeaveHint} are intended to help
* activities manage status bar notifications intelligently; specifically,
* for helping activities determine the proper time to cancel a notfication.
*
* <p>All calls to your activity's {@link #onUserLeaveHint} callback will
* be accompanied by calls to {@link #onUserInteraction}.  This
* ensures that your activity will be told of relevant user activity such
* as pulling down the notification pane and touching an item there.
*
* <p>Note that this callback will be invoked for the touch down action
* that begins a touch gesture, but may not be invoked for the touch-moved
* and touch-up actions that follow.
*
* @see #onUserLeaveHint()
*/
public void onUserInteraction() {
}

该方法为空，从注释可以知道，当此activity在栈顶时，触屏点击按home、back、menu键等都会触发此方法，一般会用于屏保。

接着调用了getWindow().superDispatchTouchEvent(ev)，即PhoneWindow类的superDispatchTouchEvent方法：

@Override
public boolean superDispatchTouchEvent(MotionEvent event) {
return mDecor.superDispatchTouchEvent(event);
}

简单调用了mDecor.superDispatchTouchEvent(event)，即DecorView的superDispatchTouchEvent方法：

public boolean superDispatchTouchEvent(MotionEvent event) {
return super.dispatchTouchEvent(event);
}

我们知道DecorView继承自FrameLayout，FrameLayout又继承了ViewGroup，所以这里就是调用了ViewGroup的dispatchTouchEvent方法。

所以执行getWindow().superDispatchTouchEvent(ev)实际上是执行了ViewGroup.dispatchTouchEvent(event)，这样事件就从 Activity 传递到了 ViewGroup。这里后续会接着分析。

这里需要注意的是：

当getWindow().superDispatchTouchEvent(ev)返回true时，即Activity的子View拦截了TouchEvent事件，那么接下来就不会再传递给Activity的onTouchEvent 方法，同时Activity的dispatchTouchEvent方法返回true；

反之返回false时，这个事件就交给Activity的onTouchEvent 方法来处理。

/**
* Called when a touch screen event was not handled by any of the views
* under it.  This is most useful to process touch events that happen
* outside of your window bounds, where there is no view to receive it.
*
* @param event The touch screen event being processed.
*
* @return Return true if you have consumed the event, false if you haven't.
* The default implementation always returns false.
*/
public boolean onTouchEvent(MotionEvent event) {
if (mWindow.shouldCloseOnTouch(this, event)) {
finish();
return true;
}

return false;
}

可以看到Activity的onTouchEvent 方法返回了false，也就意味着当getWindow().superDispatchTouchEvent(ev)返回false时，Activity的dispatchTouchEvent方法默认返回false。

2、ViewGroup的dispatchTouchEvent

如果要很好掌握Touch事件处理，这部分要重点学习，而且不同Android版本的实现不一致，本文仍然使用最新的Android 7.1源码，相比之前的源码加入了更多的复杂逻辑操作，但是最基本的流程保持一致。

接下来直接分析ViewGroup的dispatchTouchEvent方法，这个方法代码比较多，就分开几段来做分析，首先来看下面这段源码：

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
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);
}

boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;

// 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();
}
...
}
...
}

其中第一个if语句主要用于调试可直接忽略，后面的变量handled用于表示是否有view消费了该事件，接着调用了父类View的onFilterTouchEventForSecurity方法来判断是否被其他窗口遮盖，方法具体如下：

/**
* Filter the touch event to apply security policies.
*
* @param event The motion event to be filtered.
* @return True if the event should be dispatched, false if the event should be dropped.
*
* @see #getFilterTouchesWhenObscured
*/
public boolean onFilterTouchEventForSecurity(MotionEvent event) {
//noinspection RedundantIfStatement
if ((mViewFlags & FILTER_TOUCHES_WHEN_OBSCURED) != 0
&& (event.getFlags() & MotionEvent.FLAG_WINDOW_IS_OBSCURED) != 0) {
// Window is obscured, drop this touch.
return false;
}
return true;
}

如果被其他窗口遮挡，该方法返回false，表示需要过滤触摸事件，就会跳过dispatchTouchEvent方法中的if语句代码，直接退出dispatchTouchEvent方法并返回false，表示没有View消费Touch事件；如果没有被其他窗口遮挡，该方法返回true，进而继续执行if语句里面的代码。

每一个事件都是由一个触摸按下事件，一个触摸抬起事件和N个触摸滑动事件组成的，触摸按下事件就是这里的ACTION_DOWN，其为一系列事件的开端。所以在ACTION_DOWN时进行一些初始化操作，分别调用了cancelAndClearTouchTargets方法和resetTouchState方法，用来清楚掉之前消费Touch事件的View信息，并重置触摸状态。

首先来看cancelAndClearTouchTargets方法：

/**
* Cancels and clears all touch targets.
*/
private void cancelAndClearTouchTargets(MotionEvent event) {
if (mFirstTouchTarget != null) {
boolean syntheticEvent = false;
if (event == null) {
final long now = SystemClock.uptimeMillis();
event = MotionEvent.obtain(now, now,
MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0);
event.setSource(InputDevice.SOURCE_TOUCHSCREEN);
syntheticEvent = true;
}

for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
resetCancelNextUpFlag(target.child);
dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits);
}
clearTouchTargets();

if (syntheticEvent) {
event.recycle();
}
}
}

首先判断目标View，如果存在则进行统一清除操作。如果event为空，则将动作设为ACTION_CANCEL，接着用一个for循环不断向下传递触摸事件，然后再清除所有触摸目标，最后在回收拷贝的对象。

接着再来看resetTouchState方法：

/**
* Resets all touch state in preparation for a new cycle.
*/
private void resetTouchState() {
clearTouchTargets();
resetCancelNextUpFlag(this);
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
mNestedScrollAxes = SCROLL_AXIS_NONE;
}

该方法非常简单，就是重置了一些Touch标志位。

然后继续回到dispatchTouchEvent方法，看第二个代码块：

// 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;
}

// 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.
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;

变量intercepted用来标记是否要拦截该Touch事件，true表示拦截，false表示不拦截。

接着一个if判断语句，如果为ACTION_DOWN事件，此时还没有找到消费Touch事件的View，所以mFirstTouchTarget为空；如果为ACTION_MOVE和ACTION_UP事件，当前面的ACTION_DOWN事件找到了消费Touch事件的View则mFirstTouchTarget不为空。这两种情况都可以执行if里面的代码块。

变量disallowIntercept 用来标记是否允许拦截，默认为false，但是可以通过 requestDisallowInterceptTouchEvent方法来重置该变量的值。

如果允许拦截，则调用onInterceptTouchEvent方法，即我们熟知的拦截事件。该方法代码如下：

/**
* Implement this method to intercept all touch screen motion events.  This
* allows you to watch events as they are dispatched to your children, and
* take ownership of the current gesture at any point.
*
* <p>Using this function takes some care, as it has a fairly complicated
* interaction with {@link View#onTouchEvent(MotionEvent)
* View.onTouchEvent(MotionEvent)}, and using it requires implementing
* that method as well as this one in the correct way.  Events will be
* received in the following order:
*
* <ol>
* <li> You will receive the down event here.
* <li> The down event will be handled either by a child of this view
* group, or given to your own onTouchEvent() method to handle; this means
* you should implement onTouchEvent() to return true, so you will
* continue to see the rest of the gesture (instead of looking for
* a parent view to handle it).  Also, by returning true from
* onTouchEvent(), you will not receive any following
* events in onInterceptTouchEvent() and all touch processing must
* happen in onTouchEvent() like normal.
* <li> For as long as you return false from this function, each following
* event (up to and including the final up) will be delivered first here
* and then to the target's onTouchEvent().
* <li> If you return true from here, you will not receive any
* following events: the target view will receive the same event but
* with the action {@link MotionEvent#ACTION_CANCEL}, and all further
* events will be delivered to your onTouchEvent() method and no longer
* appear here.
* </ol>
*
* @param ev The motion event being dispatched down the hierarchy.
* @return Return true to steal motion events from the children and have
* them dispatched to this ViewGroup through onTouchEvent().
* The current target will receive an ACTION_CANCEL event, and no further
* messages will be delivered here.
*/
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;
}

该方法是ViewGroup中特有的方法，用于表示是否拦截触摸事件。返回为true的话则表示拦截事件，事件不在向子View中分发，若返回false的话，则表示不拦截事件，将继续分发事件。

正常都是返回默认的false，但是一般我们在自定义ViewGroup中会重写该方法，用于拦截事件的分发。当我们在父ViewGroup重写该方法返回为true执行事件拦截的逻辑的时候，可以在子View中通过调用requestDisallowInterceptTouchEvent方法，重新设置父ViewGroup的onInterceptTouchEvent方法为false，不拦截对事件的分发逻辑。

这里也是我们在开发中接触碰到的问题，所以需要好好理解一下，下面为requestDisallowInterceptTouchEvent方法的源码：

@Override
public void requestDisallowInterceptTouchEvent(boolean disallowIntercept) {

if (disallowIntercept == ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0)) {
// We're already in this state, assume our ancestors are too
return;
}

if (disallowIntercept) {
mGroupFlags |= FLAG_DISALLOW_INTERCEPT;
} else {
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
}

// Pass it up to our parent
if (mParent != null) {
mParent.requestDisallowInterceptTouchEvent(disallowIntercept);
}
}

拦截事件判断完成后，会接着调用resetCancelNextUpFlag方法来检查当前事件是否被取消。

/**
* Resets the cancel next up flag.
* Returns true if the flag was previously set.
*/
private static boolean resetCancelNextUpFlag(@NonNull View view) {
if ((view.mPrivateFlags & PFLAG_CANCEL_NEXT_UP_EVENT) != 0) {
view.mPrivateFlags &= ~PFLAG_CANCEL_NEXT_UP_EVENT;
return true;
}
return false;
}

继续回到dispatchTouchEvent方法，看第三个代码块：

// Update list of touch targets for pointer down, if needed.
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
TouchTarget newTouchTarget = null;
boolean alreadyDispatchedToNewTouchTarget = false;
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;

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;

// Clean up earlier touch targets for this pointer id in case they
// have become out of sync.
removePointersFromTouchTargets(idBitsToAssign);

final int childrenCount = mChildrenCount;
if (newTouchTarget == null && childrenCount != 0) {
final float x = ev.getX(actionIndex);
final float y = 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;
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;
}

if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}

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.
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}

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;
}

// 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;
}
}
}

该段代码首先是一个if判断语句，如果事件没有被取消，也没有被拦截，就分发该事件。只有ACTION_DOWN事件才会执行第二个if语句里面的代码，对于ACTION_MOVE和ACTION_UP事件则直接传给消费了ACTION_DOWN事件的目标View。

接着获取该ViewGroup中子View的个数，得到该事件发生的位置，获取子View的list集合preorderedList，再通过for循环倒序遍历当前ViewGroup的所有子视图。

有一点值得注意的是，这里采用了倒序遍历，这是由于preorderedList中的顺序是按照addView或XML布局文件中的顺序来的。如点击的地方有两个子View都包含点击事件的坐标，那么后被添加到布局中的那个子view会先响应事件，即点击的时候最上层的那个组件先去响应该事件。

在for循环中第一个if语句调用了canViewReceivePointerEvents(child)和isTransformedTouchPointInView(x, y, child, null)方法。

/**
* Returns true if a child view can receive pointer events.
* @hide
*/
private static boolean canViewReceivePointerEvents(@NonNull View child) {
return (child.mViewFlags & VISIBILITY_MASK) == VISIBLE
|| child.getAnimation() != null;
}

该方法用于判断当前视图的状态，只有其正在显示或正在执行动画，才可以接受触摸事件。

/**
* Returns true if a child view contains the specified point when transformed
* into its coordinate space.
* Child must not be null.
* @hide
*/
protected boolean isTransformedTouchPointInView(float x, float y, View child,
PointF outLocalPoint) {
final float[] point = getTempPoint();
point[0] = x;
point[1] = y;
transformPointToViewLocal(point, child);
final boolean isInView = child.pointInView(point[0], point[1]);
if (isInView && outLocalPoint != null) {
outLocalPoint.set(point[0], point[1]);
}
return isInView;
}

判断视图有scrollTo或scrollBy造成的滚动偏移也需要计算在内，并判断触摸点是否在当前子视图内。

从这两个方法可知，如果当前子View可以消费该ACTION_DOWN事件，并且该ACTION_DOWN事件发生的位置在当前子View的范围内，则继续执行将ACTION_DOWN事件分发给它；否则continue判断下一个子View可否接受该ACTION_DOWN事件。

然后代码通过调用getTouchTarget方法去查找当前子View是否在mFirstTouchTarget.next这条target链中的某一个targe中，如果在则返回这个target，否则返回null。紧接着用if判断找到接收Touch事件的子View，即newTouchTarget，既然已经找到则执行break跳出for循环。

如果该子View还没有消费掉该ACTION_DOWN事件，就直接调用dispatchTransformedTouchEvent方法将该ACTION_DOWN事件传递给该子View。

/**
* 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;

// 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;
}

该方法是一个非常重要的方法，其主要包括三块内容，结构雷同。而且会发现该方法中代码为一个递归调用，若其子View是ViewGroup则重复执行ViewGroup的dispatchTouchEvent方法，若其子View是View则执行View的dispatchTouchEvent方法。

从最开始到这里，我们大概分析了一下事件分发流程，通过调用Activity的dispatchTouchEvent方法，事件会首先被派发到最顶级的DecorView也就是ViewGroup，再由ViewGroup递归传递到View的dispatchTouchEvent方法。对于View的dispatchTouchEvent方法，我们后面再做分析。

如果dispatchTransformedTouchEvent方法返回true，则表示子View消费掉该事件。那么就回到dispatchTouchEvent方法继续执行if语句里的代码块，将子View加入到mFirstTouchTarget链表的表头，并且将该表头赋值给newTouchTarget，同时 alreadyDispatchedToNewTouchTarget置为true，说明有子View消费掉了该down事件。

for循环执行完毕后，如果newTouchTarget为null，且 mFirstTouchTarget不为null，即没找到子View来消耗该事件，但为了保存Touch事件的链表不为空，则把newTouchTarget赋值为最早加进mFirstTouchTarget链表的target。

再看dispatchTouchEvent方法的第四个代码块：

// 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;
}
}

如果没有找到消费Touch事件的子View，则直接把当前的ViewGroup当作普通的View看待，把事件传递给自己，即前面分析的dispatchTransformedTouchEvent方法中child为null的情况；如果之前的ACTION_DOWN事件被子View消费掉了，就会直接找到该子View对应的Target，将ACTION_MOVE和ACTION_UP事件传递给它们。

这里需要注意的是，如果intercepted为true，也就是ACTION_MOVE和ACTION_UP事件被拦截了，则cancelChild为true，则会分发一次ACTION_CANCLE事件。

再看dispatchTouchEvent方法的第五个代码块：

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
...
if (onFilterTouchEventForSecurity(ev)) {
...
// 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) {
final int actionIndex = ev.getActionIndex();
final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
removePointersFromTouchTargets(idBitsToRemove);
}
}

if (!handled && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
}
return handled;
}

如果当前事件是ACTION_CANCLE或ACTION_UP，会调用resetTouchState方法清空Touch状态。

至此，ViewGroup的dispatchTouchEvent方法分析完毕。

3、View的dispatchTouchEvent

在分析ViewGroup的dispatchTouchEvent方法时，里面多处调用了dispatchTransformedTouchEvent方法，最终将事件从ViewGroup传递到 View，那么事件在后续如何传递的，接下来继续分析。

/**
* Pass the touch screen motion event down to the target view, or this
* view if it is the target.
*
* @param event The motion event to be dispatched.
* @return True if the event was handled by the view, false otherwise.
*/
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);
}

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;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}

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;
}

相比较ViewGroup的dispatchTouchEvent方法，View的dispatchTouchEvent方法要简便得多。当View没有被其他窗口遮挡时，判断mOnTouchListener是否为空，即判断该View有没有绑定OnTouchListener监听器。

从源码里面可以找到，mOnTouchListener是通过setOnTouchListener方法来进行绑定的：

/**
* Register a callback to be invoked when a touch event is sent to this view.
* @param l the touch listener to attach to this view
*/
public void setOnTouchListener(OnTouchListener l) {
getListenerInfo().mOnTouchListener = l;
}

OnTouchListener监听器如下：

/**
* Interface definition for a callback to be invoked when a touch event is
* dispatched to this view. The callback will be invoked before the touch
* event is given to the view.
*/
public interface OnTouchListener {
/**
* Called when a touch event is dispatched to a view. This allows listeners to
* get a chance to respond before the target view.
*
* @param v The view the touch event has been dispatched to.
* @param event The MotionEvent object containing full information about
*        the event.
* @return True if the listener has consumed the event, false otherwise.
*/
boolean onTouch(View v, MotionEvent event);
}

当前View一旦执行了setOnTouchListener方法，该View的mOnTouchListener就不为空，就会调用OnTouchListener监听器的OnTouch方法。从返回值可以看到，如果重写的OnTouch方法返回true的话，那么result的值就为true，意味着该事件被消费掉了，就不会继续执行后面的onTouchEvent方法了；否则继续执行onTouchEvent方法。

4、View的onTouchEvent

onTouchEvent方法源码如下：

/**
* Implement this method to handle touch screen motion events.
* <p>
* If this method is used to detect click actions, it is recommended that
* the actions be performed by implementing and calling
* {@link #performClick()}. This will ensure consistent system behavior,
* including:
* <ul>
* <li>obeying click sound preferences
* <li>dispatching OnClickListener calls
* <li>handling {@link AccessibilityNodeInfo#ACTION_CLICK ACTION_CLICK} when
* accessibility features are enabled
* </ul>
*
* @param event The motion event.
* @return True if the event was handled, false otherwise.
*/
public boolean onTouchEvent(MotionEvent event) {
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();

if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return (((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}

if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) ||
(viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
switch (action) {
case MotionEvent.ACTION_UP:
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)) {
performClick();
}
}
}

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;

case MotionEvent.ACTION_DOWN:
mHasPerformedLongPress = false;

if (performButtonActionOnTouchDown(event)) {
break;
}

// 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) {
mPrivateFlags |= PFLAG_PREPRESSED;
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
mPendingCheckForTap.x = event.getX();
mPendingCheckForTap.y = event.getY();
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true, x, y);
checkForLongClick(0, x, y);
}
break;

case MotionEvent.ACTION_CANCEL:
setPressed(false);
removeTapCallback();
removeLongPressCallback();
mInContextButtonPress = false;
mHasPerformedLongPress = false;
mIgnoreNextUpEvent = false;
break;

case MotionEvent.ACTION_MOVE:
drawableHotspotChanged(x, y);

// Be lenient about moving outside of buttons
if (!pointInView(x, y, mTouchSlop)) {
// Outside button
removeTapCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
// Remove any future long press/tap checks
removeLongPressCallback();

setPressed(false);
}
}
break;
}

return true;
}

return false;
}

该方法代码比较多，但是思路非常清晰。可以从第一个if语句看到，即使View为 disable 状态，其依然可以消耗事件。从后面的if语句可以看到，当 View 的 LongClick 或 Clickable 属性，只要有一个为 true则能消耗事件，执行onClick和onLongClick方法。

其中onClick是在ACTION_UP事件中执行的，onLongClick是在ACTION_DOWN事件中执行的，分别对应performClick和checkForLongClick方法。

/**
* Call this view's OnClickListener, if it is defined.  Performs all normal
* actions associated with clicking: reporting accessibility event, playing
* a sound, etc.
*
* @return True there was an assigned OnClickListener that was called, false
*         otherwise is returned.
*/
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);
return result;
}

上面代码判断mOnClickListener是否为空，即判断该View有没有绑定OnClickListener监听器。如果通过调用setOnClickListener方法绑定了OnClickListener监听器，则调用onClick方法。

/**
* Register a callback to be invoked when this view is clicked. If this view is not
* clickable, it becomes clickable.
*
* @param l The callback that will run
*
* @see #setClickable(boolean)
*/
public void setOnClickListener(@Nullable OnClickListener l) {
if (!isClickable()) {
setClickable(true);
}
getListenerInfo().mOnClickListener = l;
}

接着来看checkForLongClick方法的源码：

private void checkForLongClick(int delayOffset, float x, float y) {
if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) {
mHasPerformedLongPress = false;

if (mPendingCheckForLongPress == null) {
mPendingCheckForLongPress = new CheckForLongPress();
}
mPendingCheckForLongPress.setAnchor(x, y);
mPendingCheckForLongPress.rememberWindowAttachCount();
postDelayed(mPendingCheckForLongPress,
ViewConfiguration.getLongPressTimeout() - delayOffset);
}
}

由于长按事件比较复杂，需要根据ACTION_DOWN事件开始计时，所以这里新建了一个CheckForLongPress对象，其实际为一个Runnable对象：

private final class CheckForLongPress implements Runnable {
private int mOriginalWindowAttachCount;
private float mX;
private float mY;

@Override
public void run() {
if (isPressed() && (mParent != null)
&& mOriginalWindowAttachCount == mWindowAttachCount) {
if (performLongClick(mX, mY)) {
mHasPerformedLongPress = true;
}
}
}

public void setAnchor(float x, float y) {
mX = x;
mY = y;
}

public void rememberWindowAttachCount() {
mOriginalWindowAttachCount = mWindowAttachCount;
}
}

run方法中调用了performLongClick 方法，继续追踪：

/**
* Calls this view's OnLongClickListener, if it is defined. Invokes the
* context menu if the OnLongClickListener did not consume the event,
* anchoring it to an (x,y) coordinate.
*
* @param x x coordinate of the anchoring touch event, or {@link Float#NaN}
*          to disable anchoring
* @param y y coordinate of the anchoring touch event, or {@link Float#NaN}
*          to disable anchoring
* @return {@code true} if one of the above receivers consumed the event,
*         {@code false} otherwise
*/
public boolean performLongClick(float x, float y) {
mLongClickX = x;
mLongClickY = y;
final boolean handled = performLongClick();
mLongClickX = Float.NaN;
mLongClickY = Float.NaN;
return handled;
}

继续调用了重载的performLongClick 方法：

/**
* Calls this view's OnLongClickListener, if it is defined. Invokes the
* context menu if the OnLongClickListener did not consume the event.
*
* @return {@code true} if one of the above receivers consumed the event,
*         {@code false} otherwise
*/
public boolean performLongClick() {
return performLongClickInternal(mLongClickX, mLongClickY);
}

直接调用了performLongClickInternal方法：

/**
* Calls this view's OnLongClickListener, if it is defined. Invokes the
* context menu if the OnLongClickListener did not consume the event,
* optionally anchoring it to an (x,y) coordinate.
*
* @param x x coordinate of the anchoring touch event, or {@link Float#NaN}
*          to disable anchoring
* @param y y coordinate of the anchoring touch event, or {@link Float#NaN}
*          to disable anchoring
* @return {@code true} if one of the above receivers consumed the event,
*         {@code false} otherwise
*/
private boolean performLongClickInternal(float x, float y) {
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_LONG_CLICKED);

boolean handled = false;
final ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLongClickListener != null) {
handled = li.mOnLongClickListener.onLongClick(View.this);
}
if (!handled) {
final boolean isAnchored = !Float.isNaN(x) && !Float.isNaN(y);
handled = isAnchored ? showContextMenu(x, y) : showContextMenu();
}
if (handled) {
performHapticFeedback(HapticFeedbackConstants.LONG_PRESS);
}
return handled;
}

上面代码判断mOnLongClickListener是否为空，即判断该View有没有绑定OnLongClickListener监听器。如果通过调用setOnLongClickListener方法绑定了OnLongClickListener监听器，则调用onLongClick方法。

/**
* Register a callback to be invoked when this view is clicked and held. If this view is not
* long clickable, it becomes long clickable.
*
* @param l The callback that will run
*
* @see #setLongClickable(boolean)
*/
public void setOnLongClickListener(@Nullable OnLongClickListener l) {
if (!isLongClickable()) {
setLongClickable(true);
}
getListenerInfo().mOnLongClickListener = l;
}

从以上的代码分析知道，如果在ACTION_DOWN事件中已经执行了onLongClick的话，则mHasPerformedLongPress变量会被置为true，这样在ACTION_UP事件中，就会把onClick的回调remove掉，就不会再执行onClick了。

至此，Touch事件的传递流程分析完毕。

总结

按照上面一步一步分析，流程确实比较复杂，只是便于理解具体如何传递的，最后再把其中的关键流程总结一下。主要有以下几点：



事件从Activity.dispatchTouchEveent()开始传递，只要没有拦截，就会从最上层(ViewGroup)开始一直往下传递，子View通过onTouchEvent()消费事件。如果事件从上往下一直传递到最底层的子View，但是该View并没有消费该事件，那么该事件就会反序往上传递，即从该View传递给自己的ViewGroup，然后再传给更上层的ViewGroup直至传递给Activity.onTouchEvent()。

事件从ViewGroup传递给子View时，其中ViewGroup可以通过onInterceptTouchEvent()方法对事件进行拦截，停止其往下传递，如果拦截（即返回true）后该事件会直接走到该ViewGroup中的onTouchEvent()方法，不会再往下传递给子View。如果从DOWN开始，之后的MOVE、UP都会直接在该ViewGroup.onTouchEvent()中进行处理。



如果子View之前在处理某个事件，但是后续被ViewGroup拦截，那么子View会接收到ACTION_CANCEL。如果View没有消费ACTION_DOWN事件，之后其他的ACTION_MOVE和ACTION_UP等事件都不会传递过来。

OnTouchListener优先于onTouchEvent()对事件进行消费，onLongClick优先于oClick对事件进行消费。


这一块的内容详细分析确实比较麻烦，但是整体疏通以后看起来大体还算比较简单的。如果有疑问，欢迎留言一起相互探讨共同进步。

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