本文详细解析了Android中事件分发机制，包括Activity、ViewGroup和View三个层面的事件分发流程，重点介绍了dispatchTouchEvent、onInterceptTouchEvent及onTouchEvent等关键方法的作用。
Android的事件分发主要涉及以下几个方面
Activity: dispatchTouchEvent, onTouchEvent
ViewGroup: dispatchTouchEvent, onInterceptTouchEvent, onTouchEvent
View: dispatchTouchEvent, onTouchEvent
一、Activity的事件分发：
dispatchTouchEvent的源码如下：
public boolean dispatchTouchEvent(MotionEvent ev) {
if (ev.getAction() == MotionEvent.ACTION_DOWN) {
onUserInteraction();
}
if (getWindow().superDispatchTouchEvent(ev)) {
return true;
}
return onTouchEvent(ev);
}onUserInteraction是一个空方法，我们可以重写该方法来获取key、touch、trackball等的触发
getWindow()返回一个Window，这里是PhoneWindow。PhoneWindow中最终调用DecorView的super.dispatchTouchEvent方法。
DecorView是PhoneWindow的一个内部类，它继承自FrameLayout，是每个Activity的根视图，默认的dispatchTouchEvent是ViewGroup的dispatchTouchEvent。
如果DecorView的dispatchTouchEvent返回false，即子View没有处理，最终会交给Activity的onTouchEvent处理。源码如下：
public boolean onTouchEvent(MotionEvent event) {
if (mWindow.shouldCloseOnTouch(this, event)) {
finish();
return true;
}
return false;
}public boolean shouldCloseOnTouch(Context context, MotionEvent event) {
if (mCloseOnTouchOutside && event.getAction() == MotionEvent.ACTION_DOWN
&& isOutOfBounds(context, event) && peekDecorView() != null) {
return true;
}
return false;
}二、ViewGroup的事件分发：
dispatchTouchEvent首先有一个安全检查 onFilterTouchEventForSecurity，如果当前window是被遮挡的状态，直接跳过事件下发过程。
源码：
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
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();
}
// 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;
}
// 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) {
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 View[] children = mChildren;
final boolean customOrder = isChildrenDrawingOrderEnabled();
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = customOrder ?
getChildDrawingOrder(childrenCount, i) : i;
final View child = children[childIndex];
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
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();
mLastTouchDownIndex = childIndex;
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
}
}
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.
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;
}
}
// 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;
}以下是分发的主要流程：
1.判断是否拦截事件。
如果是ACTION_DOWN事件，或者已经有子View处理事件，如果ViewGroup被允许拦截事件，接下来将调用onInterceptTouchEvent，判断当前的ViewGroup是否需要拦截；否则如果前面的事件没有View处理，默认为拦截事件，这样遍历子View的过程即第2步将不会被执行。
2.查询要处理该事件的TouchTarget。
如果不是取消事件并且没有被截断，当ACTION_DOWN时或者允许多点触摸将遍历子View，找到要处理的子View
1) 首先判断child是否能够接收事件，并且点击在child范围内
2) 如果child已经处理过事件，将当前事件的id设置给他，这样可以同时处理多个事件
3) 然后调用 dispatchTransformedTouchEvent，该方法的核心是转换触摸坐标或变换后，最终调用该child的 dispatchTouchEvent判断是否需要处理该事件。如果需要处理，那么添加到当前的TouchTarget链表中。
4）如果没有找到需要处理的child，那么当前的事件会给最近处理的TouchTarget处理
3.分发事件处理
如果没有处理该事件的TouchTarget，即mFirstTouchTarget==null，那么会调用dispatchTransformedTouchEvent，执行super.dispatchTouchEvent，并最终交给自己的onTouchEvent处理事件；否则，遍历所有的TouchTarget，调用dispatchTransformedTouchEvent处理当前事件。如果当前的child要取消，会从链表中移除，这样后续的事件就不会交给它处理。
4.更新状态。
清空TouchTarget，重置状态等
5.返回结果（true/false）
dispatchTransformedTouchEvent源码如下：
/**
* 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;
}如果是子View，坐标偏移和矩阵转换，然后调用child.dispatchTouchEvent做进一步的事件分发。
如果是ViewGroup，最终调用super.dispatchTouchEvent方法。
需要注意的是，如果触摸的点个数发生变化，会通过event.split获取指定的 desiredPointerIdBits 信息。
三、View的事件分发
dispatchTouchEvent的源码如下
public boolean dispatchTouchEvent(MotionEvent event) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
return true;
}
if (onTouchEvent(event)) {
return true;
}
}
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
return false;
}四、总结
源码基于Android4.4
对于自定义的View，如果想要处理点击事件，必须在ACTION_DOWN时，在onTouchEvent中返回true，这样后续的事件才会继续传递过来。当然也可以重写dispatchTouchEvent方法。
对于自定义的ViewGroup，如果子View没有处理事件，也同样需要在ACTION_DOWN时，在onTouchEvent中返回true；如果已经有子View处理之前的事件，如果需要处理后续的事件，只需要在onInterceptTouchEvent中返回true，并在onTouchEvent中处理事件即可。
上文中的过程是针对于一次完整的触摸事件。
ACTION_DOWN->ACTION_MOVE(&ACTION_POINTER_DOWN->ACTION_POINTER_UP)->ACTION_UP/ACTION_CANCEL
ACTION_POINTER_DOWN和ACTION_POINTER_UP针对于多点触摸的情况
扫一扫
460
被折叠的 条评论
为什么被折叠?
到【灌水乐园】发言
