Android6.0源码解读之ViewGroup点击事件分发机制

本篇博文是Android点击事件分发机制系列博文的第三篇，主要是从解读ViewGroup类的源码入手，根据源码理清ViewGroup点击事件分发原理，明白ViewGroup和View点击事件分发的关系，并掌握ViewGroup点击事件分法机制。特别声明的是，本源码解读是基于最新的Android6.0版本。

各位童鞋可以参考下面链接进行系统学习

（一）Android6.0触摸事件分发机制解读

（二）Android6.0源码解读之View点击事件分发机制

（三）Android6.0源码解读之ViewGroup点击事件分发机制

（四）Android6.0源码解读之Activity点击事件分发机制

ViewGroup事件分发中的三个重要方法的源码解析

关于ViewGroup事件分发，我们重点需要解读dispatchTouchEvent、onInterceptTouchEvent和onTouchEvent三个方法。ViewGroup比View多了一个onInterceptTouchEvent拦截事件方法，该方法源码默认返回false，即ViewGroup默认不拦截任何事件。

（一）dispatchTouchEvent源码解析

/**
* 重写了父类View的dispatchTouchEvent方法
*/
@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;// 是否处理
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;

// 手指按下去进行一些初始化的处理
if (actionMasked == MotionEvent.ACTION_DOWN) {
// 当开启了一个新的手势触摸时，我要先去重置之前的状态
// 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);// 取消、清理
resetTouchState();// 重置
}

// Check for interception.
final boolean intercepted;// 是否被拦截
// 如果事件类型ACTION_DOWN或者mFirstTouchTarget不为空
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {// 当disallowIntercept为false时
intercepted = onInterceptTouchEvent(ev);// 这里是重点，它会调用onInterceptTouchEvent方法，当该方法为true时拦截，为false时不拦截
// 所谓的拦截，是指按下去自身以及以后的后续事件move up，拦截下来给自己onTouch使用
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;

// 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) {// 这里的canceled和intercepted都为false时，条件成立，也就是说不拦截

// 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) {// 按下去或多指触摸或者移动划过
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) {// 如果子控件的个数不为0 且 newTouchTarget为空
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.
// 考虑到两个View交叉重合的情况，下面的先放进集合，但是按常理说我们手指先按到上面的，这里做了一个倒序
final ArrayList<View> preorderedList = buildOrderedChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
for (int i = childrenCount - 1; i >= 0; i--) {// 遍历ViewGroup中的子控件
final int childIndex = customOrder
? getChildDrawingOrder(childrenCount, i) : i;// 得到子控件绘画的顺序
final View child = (preorderedList == null)
? children[childIndex] : preorderedList.get(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);
// 判断是否有子控件，如果没有就不会执行里面的操作，如果有子控件则执行内部的操作
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();// 拿到X、Y位置
mLastTouchDownY = ev.getY();
// newTouchTarget用到了单向链表
newTouchTarget = addTouchTarget(child, idBitsToAssign);// 找到最终触摸的对象
alreadyDispatchedToNewTouchTarget = true;// 已经分发给一个新触摸的对象
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) {// 如果newTouchTarge为空 且 mFirstTouchTarget不为空
// 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.分发给touch targets
if (mFirstTouchTarget == null) {
// 没有触摸的对象就把它当做一个普通的View
// No touch targets so treat this as an ordinary view.
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);// 第三个参数本应为child，这里是null意味着需要调用父类View的dispatchTouchEvent方法，然后调用onTouch方法
} else {// 找到最终消费事件的对象
// 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;// 找到了
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
if (cancelChild) {
if (predecessor == null) {// 为空
mFirstTouchTarget = next;// 里面没有子控件
} else {
predecessor.next = next;// 里面有子控件，后面需要消耗此事件
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
}

// 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;
}

ViewGroup拦截情况源码分析

首先我们来看一下第34行~48行的代码，ViewGroup在如下两种情况下会判断是否要拦截当前事件：

事件类型为ACTION_DOWN或者 mFirstTouchTarget != null

即，当事件由ViewGroup的子元素成功处理时，mFirstTouchTarget 会被赋值并指向子元素，换句话说，当ViewGroup不拦截事件并将事件交由子元素处理时mFirstTouchTarget != null。反过来，一旦事件由当前的ViewGroup拦截时，mFirstTouchTarget != null条件就不成立。那么当ACTION_MOVE和UP事件到来时，由于actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null这个条件为false，导致ViewGroup的onInterceptTouchEvent不会再被调用，并且同一序列中的其他事件都会默认交给它处理。

另外，这里有一种特殊情况，我们看36行代码，有个FLAG_DISALLOW_INTERCEPT标记为，这个标记是通过requestDisallowInterceptTouchEvent()方法来设置的，一般用在子View中。如果我们通过reqeustDisallowInterceptTouchEvent()方法设置了FLAG_DISALLOW_INTERCEPT标记位后，ViewGroup将无法拦截除了ACTION_DOWN以外的其他方法（即调用该方法并不影响ACTION_DOWN事件处理）。因为ViewGroup会在ACTION_DWON事件到来时做重置状态操作，这里从代码第22~29行可以看出。

requestDisallowInterceptTouchEvent源码解析

/**
* {@inheritDoc}
*/
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);
}
}

取消、清理、重置之前的触摸状态

/**
* Cancels and clears all touch targets.
*/
private void cancelAndClearTouchTargets(MotionEvent event) {
if (mFirstTouchTarget != null) {// 如果保存的第一个触摸View对象不为空
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();
}
}
}
/**
* Resets all touch state in preparation for a new cycle.
*/
private void resetTouchState() {
clearTouchTargets();
resetCancelNextUpFlag(this);
mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
mNestedScrollAxes = SCROLL_AXIS_NONE;
}

因此我们可以得出如下结论：

1.当ViewGroup决定拦截事件后，那么点击事件将会默认交给它处理并且不再调用它的onInterceptTouchEvent方法，FLAG_DISALLOW_INTERCEPT这个标记的作用是ViewGroup不再拦截事件，前提是ViewGroup不拦截ACTION_DOWN事件处理。

2.如果事件能够传递到当前的ViewGroup，且我们要提前处理所有点击事件，应该选择dispatchTouchEvent方法，因为只有这个方法能确保每次都会被调用；而onInterceptTouchEvent()却无法保证每次事件都会被调用。

3.FLAG_DISALLOW_INTERCEPT标记位可以用于解决滑动冲突问题。

ViewGroup不拦截情况源码分析

ViewGroup不拦截事件的时候，事件会向下分发交由它的子View进行处理。先来看下代码64行（!canceled && !intercepted）这里的canceled和intercepted都为false时，条件成立，也就是说不拦截。接下来74行的条件判断：

actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE

在该if条件内，看到第86行，如果newTouchTarget == null && childrenCount != 0，即子控件的个数不为0 且 newTouchTarget为空，在95行中遍历整个ViewGroup中的子控件，这里的集合做了个倒序排列，如果两个View交叉覆盖在一起，下面的子控件先放进集合，因为后被添加的子控件会浮在上面，通常我们会希望点击的时候最上层的那个组件先去响应事件。接着105行代码开始判断子控件是否能够接收到点击事件，主要依赖于两个条件：第一子控件是否在播动画；第二点击事件是否落在子控件的区域内。如果某个子控件满足这两个条件，那么事件就会传递给它来处理。

buildOrderedChildList方法解析

/**
* 实现倒序排序
* Populates (and returns) mPreSortedChildren with a pre-ordered list of the View's children,
* sorted first by Z, then by child drawing order (if applicable). This list must be cleared
* after use to avoid leaking child Views.
* Uses a stable, insertion sort which is commonly O(n) for ViewGroups with very few elevated
* children.
*/
ArrayList<View> buildOrderedChildList() {
final int count = mChildrenCount;
if (count <= 1 || !hasChildWithZ()) return null;

if (mPreSortedChildren == null) {
mPreSortedChildren = new ArrayList<View>(count);
} else {
mPreSortedChildren.ensureCapacity(count);
}

final boolean useCustomOrder = isChildrenDrawingOrderEnabled();
for (int i = 0; i < mChildrenCount; i++) {
// add next child (in child order) to end of list
int childIndex = useCustomOrder ? getChildDrawingOrder(mChildrenCount, i) : i;
View nextChild = mChildren[childIndex];
float currentZ = nextChild.getZ();

// insert ahead of any Views with greater Z
int insertIndex = i;
while (insertIndex > 0 && mPreSortedChildren.get(insertIndex - 1).getZ() > currentZ) {
insertIndex--;
}
mPreSortedChildren.add(insertIndex, nextChild);
}
return mPreSortedChildren;
}

接着看代码，129行通过dispatchTransformedTouchEvent()这一重要方法（后面有详细分析），判断是否有子控件，如果有子控件则执行内部的操作，并找到最终触摸的对象，通过addTouchTarget方法赋值给newTouchTarget。在dispatchTransformedTouchEvent()方法中，如果子控件的dispatchTouchEvent()方法返回true，那么mFirstTouchTarget就会被赋值，同时跳出for循环，详见148行代码。同样如果dispatchTouchEvent()方法返回false，ViewGroup就会把事件分发给下一个子控件（如果还有下一个子控件）。

mFirstTouchEvent的真正赋值其实是在addTouchTarget方法中完成的，mFirstTouchEvent其实是一个单链表结构，如果mFirstTouchEvent为null，那么ViewGroup就会默认拦截下来同一序列中所有的点击事件。

addTouchTarget方法解析

/**
* Adds a touch target for specified child to the beginning of the list.
* Assumes the target child is not already present.
*/
private TouchTarget addTouchTarget(View child, int pointerIdBits) {
TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
// 往链表中插入一个元素
target.next = mFirstTouchTarget;// 将mFirstTouchTarget赋值给target.next
mFirstTouchTarget = target;// 将target赋值给mFirstTouchTarget
return target;
}

接着我们看到171行代码中，如果mFirstTouchEvent为null，也就是说要么ViewGroup中没有子控件，要么是子控件处理了点击事件，但是在dispatchTouchEvent中返回了false（子控件在onTouchEvent中返回了false），那么ViewGroup就会自己处理点击事件，需要说明的是175行代码中，第三个参数本应为child，这里是null意味着需要调用父类View的dispatchTouchEvent方法，然后调用onTouch方法。

TouchTarget 内部类源码解析

/* Describes a touched view and the ids of the pointers that it has captured.
* 链表实现的内部类，解决多指触控问题用来指定当前触摸的对象，多个手指触控(0~31个手指)
* This code assumes that pointer ids are always in the range 0..31 such that
* it can use a bitfield to track which pointer ids are present.
* As it happens, the lower layers of the input dispatch pipeline also use the
* same trick so the assumption should be safe here...
*/
private static final class TouchTarget {
private static final int MAX_RECYCLED = 32;
private static final Object sRecycleLock = new Object[0];
private static TouchTarget sRecycleBin;
private static int sRecycledCount;

public static final int ALL_POINTER_IDS = -1; // all ones

// The touched child view.
public View child;

// The combined bit mask of pointer ids for all pointers captured by the target.
public int pointerIdBits;

// The next target in the target list.
public TouchTarget next;

private TouchTarget() {
}

public static TouchTarget obtain(View child, int pointerIdBits) {
final TouchTarget target;
synchronized (sRecycleLock) {
if (sRecycleBin == null) {
target = new TouchTarget();
} else {
target = sRecycleBin;
sRecycleBin = target.next;
sRecycledCount--;
target.next = null;
}
}
target.child = child;
target.pointerIdBits = pointerIdBits;// 手指的ID
return target;
}

public void recycle() {
synchronized (sRecycleLock) {
if (sRecycledCount < MAX_RECYCLED) {
next = sRecycleBin;
sRecycleBin = this;
sRecycledCount += 1;
} else {
next = null;
}
child = null;
}
}
}

至关重要的dispatchTransformedTouchEvent方法解析

/**
* 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) {// 如果child为空，则调用自己的分发方法
handled = super.dispatchTouchEvent(event);
} else {// 否则调用child的分发方法
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) {// 如果ViewGroup中没有子控件，调用父类View的dispatchTouchEvent
handled = super.dispatchTouchEvent(transformedEvent);
} else {// 如果有子控件
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
transformedEvent.offsetLocation(offsetX, offsetY);
if (! child.hasIdentityMatrix()) {
transformedEvent.transform(child.getInverseMatrix());
}
// 调用子控件的dispatchTouchEvent，有两种情况，如果是子控件是View，又分成两种情况（Button返回true，TextView返回false）；如果是ViewGroup则进入递归了，又回到了这段代码，最终要么没有任何消耗事件的View，要么找到消费事件的View
handled = child.dispatchTouchEvent(transformedEvent);
}

// Done.
transformedEvent.recycle();
return handled;
}

该方法在dispatchTouchEvent()中被调用，用于将事件分发给子View处理。我们重点看一下60~71行代码。在dispatchTransformedTouchEvent()方法一共有三个参数，其中第三个参数View child有时为null，有时不为null。61行代码中，child==null意味着事件没有被消费，ViewGroup中没有子控件需要调用父类View的dispatchTouchEvent方法，即super.dispatchTouchEvent(event)。

接着我们关注下handled这个变量，可以发现dispatchTransformedTouchEvent()方法return handled，而handled的值其实是取决于dispatchTransformedTouchEvent()方法递归调用dispatchTouchEvent()方法的结果，也就是说在子控件中dispatchTouchEvent()方法的onTouchEvent()是否消费了Touch事件的返回值决定了dispatchTransformedTouchEvent()的返回值，从而决定mFirstTouchTarget是否为null，更进一步决定了ViewGroup是否处理Touch事件。

（二）onInterceptTouchEvent源码解析

public boolean onInterceptTouchEvent(MotionEvent ev) {
return false;// 默认不拦截
}

你没有看错，这方法就是简简单单的一个布尔值返回，当返回true时，对事件进行拦截，返回false则不拦截。

（三）ViewGroup点击事件分发小结

Android点击事件分发是到达顶级View后（一般是ViewGroup），会调用ViewGroup的dispatchTouchEvent方法，其中它的onInterceptTouchEvent方法如果返回true，则会对事件传递进行拦截，事件由ViewGroup处理；如果onInterceptTouchEvent方法返回false，则代表不对事件进行拦截，默认返回false，即所有的ViewGroup都是默认不拦截的。则此时子View中的dispatchTouchEvent方法将被调用，到此，事件已经由顶级View传递给了下一层的View，接下来的过程是一个递归循环的过程，和顶级View事件分发过程是一致的，直到完成整个事件分发。