View事件分发机制源码分析
16lz
2021-01-25
View事件分发机制源码分析
1、事件传递规则概述
在解释事件分发机制之前，需要搞清楚几个概念。
1、事件:由于android设备对键盘依赖性的降低，导致触摸事件(MotionEvent)成为android最主要的事件，所以对于事件的分发，其实就是对MotionEvent对象的传递过程。
2、事件序列：从用户手指按下，到手里离开的这一系列事件的集合(可以看成用户在屏幕上的一个手势)。即事件序列以ACTION_DOWN开始，ACTION_UP结束：ACTION_DOWN–>ACTION_MOVE–>ACTION–>…->ACTION_UP事件。
MotionEvent的分发主要涉及到以下几个方法：
1、public boolean dispatchTouchEvent(MotionEvent ev);//当事件传递到当前View时调用。2、public boolean onTouchEvent(MotionEvent event);//在dispatchTouchEvent中被间接调用。3、public boolean onInterceptTouchEvent(MotionEvent ev); //在dispatchTouchEvent中被调用，判断是否对MotionEvent进行拦截。三个方法中最主要的是dispatchTouchEvent方法，其他两个方法只是在该方法中被调用，而该方法细节主要分两种情况：
1、当前View为ViewGroup
将会调用onInterceptTouchEvent()方法判断是否对事件进行拦截，如果拦截将会将事件交给当前控件的onTouchEvent()处理，否则将会分发给子控件处理(即调用子控件的dispatchTouchEvent方法)。
下面是ViewGroup消息处理流程的伪代码
/** *这只是大概的流程，详细的源代码细节会在之后讲解 */boolean dispatchTouchEvent(MotionEvent e){ boolean consumed = false; if(onInterceptTouchEvent(e)){ consumed = onTouchEvent(e); }else{ consume = childView.dispatchTouchEvent(e); } return consumed;}2、当前View为非ViewGroup的普通View
如果当前View设置了onTouchListener将会调用Listener对象中的onTouch方法，如果onTouch返回true直接返回，为false则调用当前View的onTouchEvent方法，默认View的onTouchEvent方法中会根据clickable属性或longClickable等相关属性判断是否调用onClickListener中的onClick方法。
下面是非ViewGroup的普通View分发事件流程的伪代码
/** *这只是大概的流程，详细的源代码细节会在之后讲解 */boolean dispatchTouchEvent(MotionEvent e){ boolean result = false; if(mOnTouchListener != null){ result = mOnTouchListener.onTouch(this,e)； } if(result == false){ result = onTouchEvent(e); }}boolean onTouchEvent(MotionEvent e){ boolean result = false; switch(e.getAction()){ case MotionEvent.ACTION_UP: if(mOnClickListener != null){ mOnClickListener.onClick(this); result = true; } break; ... } ... return false;}一个点击事件产生后，它的传递过程主要按以下顺序：
分发过程：Activity->Window->ViewGroup->View；
按照这种过程一层一层分发下去，如果下层View的onTouchEvent方法没有处理则返回false，交由上一层处理，如果上一层也没有处理，再交由上一层，就是这样将未处理的事件返回给上一层处理。
源代码分析
源代码分析前，先将一些结论整理出来，带着结论看源码也许会更轻松，更容易理解，我们在源代码中对以下结论进行验证。
1、通常，一个事件序列只能被一个View拦截并消耗，因为一旦事件被拦截了，那么同一个事件序列内的所有事件都将直接交给该View处理，所以同一个事件序列中的事件不能同时由两个View处理，但是可以使用其他特殊方法做到，比如View将本该自己处理的事件通过调用其他View的onTouchEvent强行交给其他View处理。
2、某个View一旦决定拦截(onInterceptTouchEvent方法被调用并返回true)，那该事件序列中的所有事件都只能由它来处理，而且在该事件序列传递过程中这个View的onInterceptTouchEvent不会再被调用。换句话说，onInterceptTouchEvent一旦决定拦截会拦截一个事件序列。
3、某个View一旦开始处理事件，如果它不消耗ACTION_DOWN事件(即onTouchEvent返回了false)，那么同一个事件序列中的其他事件都不会在交给它来处理。
4、如果View不消耗除ACTION_DOWN以外的其他事件，那么这个点击事件会消失，此时父控件的onTouchEvent方法并不会被调用，并且当前View可以持续收到后续的事件，最终这些消失的点击事件会传递给Activity来处理。
5、ViewGroup的onInterceptTouchEvent方法默认返回false(默认不拦截任何事件)；View没有onInterceptTouchEvent方法，一旦事件传递给它，它按照onTouchEventListener.onTouch->onTouchEvent->onClickEventListener.onClick的顺序执行
6、一般都是先传递给父控件，再有父控件分发给子控件，通过requestDisallowInterceptTouchEvent方法可以在子控件干预父控件的事件分发过程，但是ACTION_DOWN事件除外。
1、我们从ViewGroup.dispatchTouchEvent开始摘取重点进行分析
public boolean dispatchTouchEvent(MotionEvent ev){ ... boolean handled = false;//设置标志位，判断是否已经处理了该事件 //过滤掉本不应该发送到该控件的事件(比如该控件所在的窗口已经被遮挡了) if (onFilterTouchEventForSecurity(ev)) { final int action = ev.getAction(); final int actionMasked = action & MotionEvent.ACTION_MASK; // 对事件序列的第一个事件ACTION_DOWN进行初始化处理 if (actionMasked == MotionEvent.ACTION_DOWN) { //清除前一个事件序列的相关信息,这些信息保存在名为mFirstTouchTarget的TouchTarget对象中， //TouchTarget是一个链表，里面保存了应当处理该事件的View链 cancelAndClearTouchTargets(ev); resetTouchState(); } // 检查本控件是否需要拦截该事件 final boolean intercepted; if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { //该事件为ACTION_DOWN，或者已经有子控件需要处理该事件序列 //判断是否允许拦截，FLAG_DISALLOW_INTERCEPT标志位可通过requestDisallowInterceptTouchEvent方法设置， //子控件就可以通过该方法来干预父控件的事件分发过程，验证了结论6 final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { //允许拦截该事件，则调用onInterceptTouchEvent方法查看是否需要拦截该事件， //intercepted记录下返回值，true为拦截，false为不拦截 intercepted = onInterceptTouchEvent(ev); ev.setAction(action); //还原Action，防止onInterceptTouchEvent修改了Action } else { //不允许拦截，则直接设置为未拦截。 intercepted = false; } } else { //不是事件序列的第一个事件，而且没有子控件“愿意”处理该事件序列。 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) { //如果事件序列没有被取消而且该控件也没有拦截该事件 // 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. //根据子控件在Z方向的大小，对子控件进行排序，上面的子控件应该先处理触摸事件 final ArrayList preorderedList = buildOrderedChildList(); final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled(); final View[] children = mChildren; for (int i = childrenCount - 1; i >= 0; i--) { //遍历所有的子控件 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) { //当前子控件没有获取焦点，则跳过该子控件 //先获取焦点，然后才能触发点击事件 continue; } //如果子控件已获取焦点，重新迭代，这部分我也搞不懂，注释说为了更安全 childWithAccessibilityFocus = null; i = childrenCount - 1; } //canViewReceivePointerEvents检测子控件是否能接收触摸事件 //不可见的，正在执行动画的View不能接收触摸事件 //isTransformedTouchPointInView检测触摸点是否在子控件中 if (!canViewReceivePointerEvents(child) || !isTransformedTouchPointInView(x, y, child, null)) { //不满足条件的，跳过该子控件 ev.setTargetAccessibilityFocus(false); continue; } newTouchTarget = getTouchTarget(child); if (newTouchTarget != null) { //查看child是否加入到mFirstTouchTarget链表中 // 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); //dispatchTransformedTouchEvent，对事件进行一些转换，比如坐标转换成相对子控件边界的坐标 //并将转换后的事件分发给该子控件，该函数后面还会重点解析 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 //如果childIndex是指向排序后的列表，则需要找到在mChildren中的原始索引 for (int j = 0; j < childrenCount; j++) { if (children[childIndex] == mChildren[j]) { mLastTouchDownIndex = j; break; } } } else { mLastTouchDownIndex = childIndex; } mLastTouchDownX = ev.getX(); mLastTouchDownY = ev.getY(); //将该子控件添加到mFirstTouchTarget链表的头部 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. //没有子控件接收该事件 newTouchTarget = mFirstTouchTarget; while (newTouchTarget.next != null) { newTouchTarget = newTouchTarget.next; } newTouchTarget.pointerIdBits |= idBitsToAssign; } } } // Dispatch to touch targets. // 将事件分发给mFirstTouchTarget保存的事件处理View if (mFirstTouchTarget == null) { // No touch targets so treat this as an ordinary view. // 没有子控件接收该事件，则分发给自己，即调用自己的onTouchEvent 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) { //遍历TouchTarget对象，将事件分发给所有需要处理该事件的所有控件 final TouchTarget next = target.next; if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) { //如果在之前的处理过程中已经发给了target中保存的View，则直接设置handle为true，表示已经处理过了该事件 handled = true; } else { final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted; //转换坐标后将事件分发给子控件 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; } } ... } ... return handled;} 2、接下来分析ViewGroup.dispatchTransformedTouchEvent方法,我们摘取里面最重要的一部分进行解析
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, View child, int desiredPointerIdBits) { ... // Perform any necessary transformations and dispatch. if (child == null) { //调用父类的dispatchTouchEvent方法，也就是调用View的dispatchEvent //结论5说过，View接收到事件，按照onTouchListener.onTouch->onTouchEvent->onClickListener.onClick顺序执行， //onTouchListener默认为空，所以这句话相当于调用自己的onTouchEvent方法 //关于View.diapatchTouchEvent方法，后面还会解析 handled = super.dispatchTouchEvent(transformedEvent); } else { //计算相对于子控件边界的坐标值 final float offsetX = mScrollX - child.mLeft; final float offsetY = mScrollY - child.mTop; transformedEvent.offsetLocation(offsetX, offsetY); if (! child.hasIdentityMatrix()) { transformedEvent.transform(child.getInverseMatrix()); } //将转换后的事件对象分发给子控件 handled = child.dispatchTouchEvent(transformedEvent); } ... return handled; }3、现在我们对View.dispatchTouchEvent进行解析
public boolean dispatchTouchEvent(MotionEvent event) { ... if (onFilterTouchEventForSecurity(event)) { //noinspection SimplifiableIfStatement ListenerInfo li = mListenerInfo; if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED && li.mOnTouchListener.onTouch(this, event)) { //如果该控件设置了OnTouchEventListener监听器，则交由监听器处理 result = true; } if (!result && onTouchEvent(event)) { //如果未设置OnTouchEventListener监听器或者监听器返回false，则调用自己的onTouchEvent方法 result = true; } } ... return result; }4、最后我们看一下控件默认的View.onTouchEvent方法
public boolean onTouchEvent(MotionEvent event) { final float x = event.getX(); final float y = event.getY(); final int viewFlags = mViewFlags; final int action = event.getAction(); if ((viewFlags & ENABLED_MASK) == DISABLED) { if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) { //ACTION_UP触发修改PFLAG_PRESSED标志位， //同时更换drawableStateList设置的不同状态下的drawable资源 setPressed(false); } // A disabled view that is clickable still consumes the touch // events, it just doesn't respond to them. // 即使DISABLED状态下的控件，只要满足： // CLICKABLE(可点击)， // LONG_CLICKABLE(可长按)， // CONTEXT_CLICKABLE(手写笔、鼠标相关，基本不用考虑) // 仍然消耗MotionEvent事件，只是不做响应 return (((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE); } if (mTouchDelegate != null) { //委托的触摸代理，将本控件一个区域映射到另一个控件的某个区域 //如果点击了委托的区域则将MotionEvent交给委托的View控件来处理 //详细的内容可以自行阅读TouchDelegate源码 if (mTouchDelegate.onTouchEvent(event)) { return true; } } if (((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) { switch (action) { case MotionEvent.ACTION_UP: boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0; if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) { // take focus if we don't have it already and we should in // touch mode. // 如果没有获得焦点，那么点击后让其获取焦点 boolean focusTaken = false; if (isFocusable() && isFocusableInTouchMode() && !isFocused()) { focusTaken = requestFocus(); } if (prepressed) { // The button is being released before we actually // showed it as pressed. Make it show the pressed // state now (before scheduling the click) to ensure // the user sees it. setPressed(true, x, y); } if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) { // This is a tap, so remove the longpress check removeLongPressCallback(); // Only perform take click actions if we were in the pressed state if (!focusTaken) { // Use a Runnable and post this rather than calling // performClick directly. This lets other visual state // of the view update before click actions start. if (mPerformClick == null) { mPerformClick = new PerformClick(); } if (!post(mPerformClick)) { //performClick中将会执行已经设置了onClickListener.onClick方法 performClick(); } } } if (mUnsetPressedState == null) { mUnsetPressedState = new UnsetPressedState(); } if (prepressed) { postDelayed(mUnsetPressedState, ViewConfiguration.getPressedStateDuration()); } else if (!post(mUnsetPressedState)) { // If the post failed, unpress right now mUnsetPressedState.run(); } removeTapCallback(); } mIgnoreNextUpEvent = false; break; case MotionEvent.ACTION_DOWN: mHasPerformedLongPress = false; if (performButtonActionOnTouchDown(event)) { break; } // Walk up the hierarchy to determine if we're inside a scrolling container. boolean isInScrollingContainer = isInScrollingContainer(); // For views inside a scrolling container, delay the pressed feedback for // a short period in case this is a scroll. if (isInScrollingContainer) { mPrivateFlags |= PFLAG_PREPRESSED; if (mPendingCheckForTap == null) { mPendingCheckForTap = new CheckForTap(); //CheckForTap的run方法中会调用checkForLongClick } mPendingCheckForTap.x = event.getX(); mPendingCheckForTap.y = event.getY(); postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout()); } else { // Not inside a scrolling container, so show the feedback right away setPressed(true, x, y); //该方法中会执行一个延时的LongClick checkForLongClick(0); } break; case MotionEvent.ACTION_CANCEL: ... break; case MotionEvent.ACTION_MOVE: ... break; } return true; } return false; }到此已经大致的将View的事件分发机制讲解完了。
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