android SDK-25事件分发机制--源码正确解析

android SDK-25事件分发机制–源码正确解析

Android 事件分发分为View和ViewGroup的事件分发，ViewGroup比View过一个拦截判断，viewgroup可以拦截事件，从而决定要不要把事件传递给子view，因为view没有子view所以不存在拦截事件的情况。

事件分发主要从事件的分发，拦截，和处理三个函数的调用逻辑关系来分析。

public boolean dispatchTouchEvent(MotionEvent event) {}public boolean onInterceptTouchEvent(MotionEvent ev) {}public boolean onTouchEvent(MotionEvent event) {}

首先，屏幕上面一个点击事件，通过传感器捕获到点击，然后知道把点击事件传递到activity 到PhoneWindow，再到，DecorView 最后就到我们自己在布局文件中的view或者viewgroup。

下面分析dispatchTouchEvent源码：（sdk-25）（==>这个标记为重点，不用全懂，只要把==>这个标记处的逻辑理清楚就行）

 @Override
public boolean dispatchTouchEvent(MotionEvent ev) {//检验是不是利用辅助功能点击事件，根本不用管 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);}//辅助功能点击事件的判断和处理根本不用管，直接不要管，跳转到下一个重点// ==> 一个标志，初始值为不处理，意识就是不处理MotionEventboolean handled = false;if (onFilterTouchEventForSecurity(ev)) {final int action = ev.getAction();final int actionMasked = action & MotionEvent.ACTION_MASK;// Handle an initial down.// ==> 处理第一个点击事件（事件分为，点击事件，move事件，up事件，所以点击事件是第一个事件）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.// ==> 如果是点击事件，说明是一个新的事件序列开始了（按下，移动，手指抬起离开屏幕）// 由于是新的事件序列开始，所以要重置一些参数，和清空一下数据等等操作。cancelAndClearTouchTargets(ev);resetTouchState();}// Check for interception.final boolean intercepted;// ==> 判断要不要打断，如果不是点击事件，并且mFirstTouchTarget为null，则该viewgroup则打断拦截事件。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;// 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;// ==>如果没有返回并且没有被拦截，则进行分发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down事件，进行遍历子view，并且把事件分发给子view，前提是down事件的坐标必须在子view中等等条件==>这句话，请把下面的源码分析完再一定回来看一看，你要思考一下，这里如果是down事件才会进去，如果是move事件那么不会进去分发了，其实往后看源码会发现，在down事件分发给一个child，如果这个child消费了这个down事件，那么这个child就会被保存起来，以后的move（可能0到多次move），up都会直接分发给这个child，就不用每次再去遍历所有的Child这些效率就提高和很多，但是你也会想到，如果一个down事件被某个child消费只有，其它child就无法被分发事件了，除非我们手动调用child的分发方法，或者打断事件序列，从发分发一个down事件，这段话请看完后面的源码再来推敲一下，就完全理解事件分发机制了。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 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;}// ==>判断这个子view能否接受点击事件和子view是否包含这个点击的坐标，具体的判断点击这两个方法进去看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);// ==>  关键之处，这里将事件传递给child进行处理了，马上进入这个方法看看吧if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign))

// ==>dispatchTransformedTouchEvent方法

 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.//根本都不是什么返回事件，所以跳过，直接到重点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) {// ==>如果child为空，直接调用该viewgroup自己父类的dispatchTouchEvent，也就是viewdispatchTouchEvent方法，点击进去会发现它会调用onTouchEvent，也就是说如果viewGroup如果没有child那么他就会调用自己的onTouchEvent方法来消费这个事件，这个handle就表明了这个事件viewgroup自己是否消费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());}// ==>我们的child不为空的时候，就调用child事件分发方法，于是到这里可以看到事件的传递了，先来分析简单的情况，如果child是view不是viewgroup，那么dispatchTouchEvent流程就简单了，会调用onTouchEvent来告诉父view，child它是否消费父亲分发给他的事件，这个handle就表明了这个事件clild是否消费handled = child.dispatchTouchEvent(transformedEvent);}// Done.transformedEvent.recycle();return handled;
}

//这个重要的方法分析完之后，马上返回刚才源码的地方，继续。。。

//如果刚才，我们的child不为空并且child消费down事件，那么就很棒了，说明有child处理事件了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 indexfor (int j = 0; j < childrenCount; j++) {if (children[childIndex] == mChildren[j]) {mLastTouchDownIndex = j;break;}}} else {mLastTouchDownIndex = childIndex;}mLastTouchDownX = ev.getX();mLastTouchDownY = ev.getY();// ==>重点，从上面的if条件可以看出，有child消费down事件才会执行到这里，这个方法点进去发现mFirstTouchTarget = target;     这句代码，很重要哦。很明显就是把消费down事件的child赋值给mFirstTouchTarget，从而保存起来newTouchTarget = addTouchTarget(child, idBitsToAssign);// ==> alreadyDispatchedToNewTouchTarget = true; 已经把事件分发给新的触摸目标下面在分发的时候会用到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();}//这里，如果是down事件，并且mFirstTouchTarget != null则加入链表newTouchTarget，这里不要管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;}}}// ==> 由于刚才mFirstTouchTarget被赋值为消息了down事件的child，所以不为空了// Dispatch to touch targets.if (mFirstTouchTarget == null) {// 这里非常重要，如果child为空，表明viewgroup在分发事件给child的dispatchTouchEvent被返回了false，说明子view都不消费down事件，那么这里会调用dispatchTransformedTouchEvent这个重要的方法，点进去就会发现，当child为null的时候，会调用viewgroup自己的父类View的dispatchTouchEvent，从而调用onTouchEvent，就是说儿子不消费，给老子再看看要不要消费。// 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;// ==>看到这里，你是不是发现怎么又调用了这个重要的，吧事件分发给child的方法，刚才不是已经分发了，这不是第二次又来分发吗？，当然不会，你看前面的，第一次分发事件的时候已经讲alreadyDispatchedToNewTouchTarget=true；了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;
}

总结：事件分发，viewgroup自己先判断要不要拦截事件，和有没有调用过requestDisallowInterceptTouchEvent方法来不拦截事件，如果不拦截，当down事件的时候，遍历child，看child是否消费，child如果消息，则被保存下来，后面的事件就不遍历直接分发给他，如果child不消费，那么viewgruop继续执行，调用自己onTouchEvent方法来判断自己是不是要消费事件。

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