转载请标明出处!http://blog.csdn.net/sahadev_/article/details/23839039 ,当然一般也没人转载。。。
一直想彻底了解View的事件分发过程,在网上也看了很多大神的博客,但是总有一些东西不是很明白,于是自己就根据源码画了一个流程草图,思路总算是清晰了。
文章分为View和ViewGroup两部分介绍,首先来讲讲View的事件分发:
打开View的源码,顺着onTouch方法向上找到dispatchPointerEvent(MotionEvent event)的这个方法,在View中再没有其它地方调用它,估计事件就是从这里传过来的吧。
来贴一下这个方法的源码:
public final boolean dispatchPointerEvent(MotionEvent event) {if (event.isTouchEvent()) {return dispatchTouchEvent(event);} else {return dispatchGenericMotionEvent(event);}}
看到在这个方法中调用了dispatchTouchEvent的这个方法,dispatchTouchEven()暂时没找到它的相关说明,权当它返回了true,进入dispatchTouchEvent方法:
public boolean dispatchTouchEvent(MotionEvent event) {if (mInputEventConsistencyVerifier != null) {mInputEventConsistencyVerifier.onTouchEvent(event, 0);}if (onFilterTouchEventForSecurity(event)) {//noinspection SimplifiableIfStatementListenerInfo 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;}
可以看到在这个方法中调用了onTouchEvent(Event),所以可知onTouchEvent(event)是由dispatchTouchEvent触发的。如果有人对onFilterTouchEventForSecurity这方法有疑问,API中是这么解释的:True if the event should be dispatched, false if the event should be dropped.这句话就不用翻译了吧,相信大家都懂。
如果onTouchEvent返回了true,那么dispatchTouchEvent的返回值也为true. 对于dispatchTouchEvent的返回值说明是这么解释的:True if the event was handled by the view, false otherwise. 如果返回true,说明这个事件就被当前的这个View消费掉了,这里的返回值大家一定要清楚,一会在ViewGroup的说明中需要对这块很熟悉。
接下来说一说ViewGroup的事件分发方法:
ViewGroup的事件处理相对View相对有些复杂,ViewGroup继承于View,在View中dispatchTouchEvent是由dispatchPointerEvent触发的,所以ViewGroup也是由dispatchPointerEvent触发的,这里不重要,接着看ViewGroup的dispatchTouchEvent的源代码:
@Overridepublic 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 downfinal 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;}
ViewGroup的dispatchTouchEvent的方法有些长,在代码中可以看到,ViewGroup将View的该方法彻底重写了,没有很明显的看到super.dispatchTouchEvent的字样,也就是说还没有着急把事件交给View去处理。
前几行和View都差不错,往下看
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;}
在第5行中调用了onInterceptTouchEvent(ev);该方法表示当按下事件触发的时候是否要拦截本次事件,这个方法的源码是这样的:
public boolean onInterceptTouchEvent(MotionEvent ev) {return false;}
如果不对它进行重写的话,它总是返回false,也就是说intercepted的值恒为false,接着往下看,看到 if (!canceled && !intercepted) {这一行的时候,由于intercepted为false,所以事件可以进入if内执行,接着往下看:
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;}}
看到这段代码中dispatchTransformedTouchEvent的这个方法开始对ViewGroup的子View进行事件传递了:
/*** 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;}
该方法的最终返回值是布尔值,意思为是否被处理了,请看这里:
// 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);}
这里开始对子View进行事件传递,调用子View的dispatchTouchEvent的方法,如果被子View消费掉了本次事件,dispatchTransformedTouchEvent就返回true,再回到那个循环:
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;}
请注意这个方法:
newTouchTarget = addTouchTarget(child, idBitsToAssign);
这个方法中的代码是这样的:
private TouchTarget addTouchTarget(View child, int pointerIdBits) {TouchTarget target = TouchTarget.obtain(child, pointerIdBits);target.next = mFirstTouchTarget;mFirstTouchTarget = target;return target;}
注意将mFirstTouchTarget赋了值,不再是null,先记住这里,待会有用。
好了,子View循环完成之后,代码会执行到这里:
// 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;}}
注意到刚刚mFirstTouchTarget被赋了值,不再是null,不为null,就不会执行if中的语句,if中的语句都做了些什么呢?
handled = dispatchTransformedTouchEvent(ev, canceled, null,TouchTarget.ALL_POINTER_IDS);
注意第3个参数是Null,咱们再返回到dispatchTransformedTouchEvent中(刚才已经贴过方法内的代码):
该方法内有3处super.dispatchTouchEvent(event);因为刚才mFirstTouchTarget的值不为null,所以没有进入if,所以ViewGroup没法去调用super.dispatchTouchEvent(event);所以当子View把事件消费完成之后,ViewGroup就无法接收到onTouchEvent事件,这就是为什么子View返回true,ViewGroup就无法处理的原因。
事件传递到这里也就讲完了,不知道大家有没有听明白,如果有什么疑问可以在下面留言,欢迎转载!