Android事件分发拦截机制(图解) Android事件分发拦截机制(图解

转载请注明出处：http://blog.csdn.net/kiddTeb/article/details/52562567
前言

祝中秋佳节愉快！本文为了自己复习留下一个脚印，有错误望指正，望轻喷~~

分析

在我们平常开发过程中，有时候有出现View与ViewGroup的嵌套，也就是view放在一个viewGroup里面，而这个viewGroup又放在这个另外一个viewGroup当中,那么这个时候的触摸事件应该分配给谁呢？这就涉及了事件分发拦截机制。
首先分析一下，ViewGroup中关键的几个函数

public boolean dispatchTouchEvent(MotionEvent ev){ ... }

/** * 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. */ public boolean onInterceptTouchEvent(MotionEvent ev) { return false; }

再来看看View中的几个关键函数

/** * Checks a touch event. * @param event The event. * @param nestingLevel The nesting level: 0 if called from the base class, * or 1 from a subclass.If the event was already checked by this consistency verifier * at a higher nesting level, it will not be checked again.Used to handle the situation * where a subclass dispatching method delegates to its superclass's dispatching method * and both dispatching methods call into the consistency verifier. */ public boolean onTouchEvent(MotionEvent event){ 。。 }

/** * 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){ 。。 }

上面的几个函数在后面都会讲到，现在可以看看官方注释留个大概印象。

小案例 【Android事件分发拦截机制(图解)】

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mRelativeLayout1.setOnTouchListener(new OnTouchListener() { @Override public boolean onTouch(View v, MotionEvent event) { Log.d("TAG", "mRelativeLayout1 on touch"); return false; } }); mRelativeLayout2.setOnTouchListener(new OnTouchListener() { @Override public boolean onTouch(View v, MotionEvent event) { Log.d("TAG", "mRelativeLayout2 on touch"); return false; } }); mButton.setOnClickListener(new OnClickListener() { @Override public void onClick(View v) { Log.d("TAG", "clicked button"); } });

假如一个屏幕内有两个Viewgroup和一个view进行嵌套，那么如果我们点击button的时候会出现什么情况呢？可以打印log出来观察一下会发现 “clicked button” 。这应该很熟悉，那么这个事件是如何进行分发的呢？

/** * {@inheritDoc} */ @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(); }// 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; // 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 preorderedList = buildOrderedChildList(); final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled(); final View[] children = mChildren; for (int i = childrenCount - 1; i >= 0; i--) { final int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i; 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); 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; } } }// 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; }

/** * 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; }

当我们点击按钮的时候，会去调用这个按钮所在布局的dispatchTouchEvent方法，在这个方法第92行中会遍历这个布局中的所有子View，在168和182行都有调用dispatchTransformedTouchEvent方法，那么我们进入这个方法看看，在这个方法中的15-19行就有个判断，如果孩子是否为空来决定调用父类的还是调用孩子的dispatchTouchEvent方法

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事件的大概方向就是如此，从Viewgroup往View下进行一个传递，最后再逐级往上面ViewGroup回传结果。其中ViewGroup中的onInterceptTouchEvent方法，返回一个boolean值，默认是返回false。看看官方的注释就知道，如果返回false的时候，是不会对事件进行一个拦截，如果我们去重写这个方法返回true的话，那么这个事件就会在这里被拦截掉，从而不会再往下进行一个分发。
另外还有一些要注意的

/** * 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)) { //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; }

在子View的dispatchTouchEvent方法中，第34-38行中有个判断，第一个值在getListenerInfo()方法中new出一个mListenerInfo，只要有添加监听就不会为空，至于第二个值只要注册setOnTouchListener(OnTouchListener l)就不会为空，第三个值是判断当前点击的控件是否是enable的，按钮默认都是enable的，因此这个条件恒定为true，第四个值呢，比较关键，只要这里返回true那么整个方法就会返回true，这就取决去在注册setOnTouchListener的时候你返回了什么值，如果你返回了true那么事件在这里就会被消费掉，不会再执行按钮的onClick，因此如果你在setOnTouchListener中返回true，你又在按钮注册了setOnClickListener的话，你试着在onClick中打印下日志就可以知道，是不会打印出来的。这就说明事件被消费掉了。这就是在View中的事件分发。
但是如果在setOnTouchListener返回来的是false，那么显然onClick是没有问题的，是可以打印出来的，那就说明了按钮的onClick是在第40行中的onTouchEvent(event)方法中的，这就不列源码出来了，其实在这个方法中就是进行一个switch的判断当前事件是抬起手指还是手指移动还是手指抬起，对应进入case中，MotionEvent.ACTION_UP这个case当中有一个performClick()方法，进入这个方法就很明确能找到mOnClickListener.onClick(this); 这一句关键代码，就是在这里进行按钮的onClick的。值得留意的是，ACTION_DOWN，ACTION_MOVE，ACTION_UP等事件中只有前一个事件返回true，后一个事件才能被触发。

总结