关于Android事件派发流程的理解
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以前看了很多人介绍的Android事件派发流程,但最近使用那些来写代码的时候出现了不少错误。所以回顾一下整个流程,简单介绍从手触摸屏幕开始到事件在View树派发。从源码上分析ViewGroup.dispatchTouchEvent。
事件从触摸到View简述
Android的事件产生是从我们触摸屏幕开始,在经过Input子系统,最后达到我们的应用程序(或者经过WindowManagerService到达应用程序)。
而其中Input子系统在Java层对应着InputManagerService,其主要在native层,由InputReader读取EventHub的元数据,将这些数据加工成InputEvent,最后发到InputDispatcher,而InputDispatcher则负责将时间发到应用程序,Input子系统流程可以参见这篇文章Android Framework——之Input子系统。
对于应用层的时间流程,主要是下面的流程图所示:
其中最后一步就是我们经常说的View事件派发流程。另外上面DecorView是经过了两次,第一次是调用DecorView的dispatchTouchEvent,它的源码是:
@Override public boolean dispatchTouchEvent(MotionEvent ev) { final Callback cb = getCallback(); return cb != null && !isDestroyed() && mFeatureId < 0 ? cb.dispatchTouchEvent(ev) : super.dispatchTouchEvent(ev); }
Callback就是Window.Callback,Activity实现了这个接口。在Activity的attach函数中,会调用window的setCallback,将Activity设置给Window。所以这里getCallback返回的就是Activity,最终会调用Activity的dispatchTouchEvent。下面看一下Activity的dispatchTouchEvent函数:
public boolean dispatchTouchEvent(MotionEvent ev) { if (ev.getAction() == MotionEvent.ACTION_DOWN) { onUserInteraction(); } if (getWindow().superDispatchTouchEvent(ev)) { return true; } return onTouchEvent(ev); }
在ACTION_DOWN的时候会调用onUserInteraction方法,然后调用Window(实际上是PhoneWindow)的superDispatchTouchEvent,如果Window的superDispatchTouchEvent消耗了事件,则直接返回,不会调用Activity的onTouchEvent方法。
@Overridepublic boolean superDispatchTouchEvent(MotionEvent event) { return mDecor.superDispatchTouchEvent(event);}
而DecorView的superDispatchTouchEvent为:
public boolean superDispatchTouchEvent(MotionEvent event) { return super.dispatchTouchEvent(event);}
最终还是调用DecorView的父类的dispatchTouchEvent,DecorView的父类是FrameLayout,它没实现该方法,最终会调用ViewGroup的dispatchTouchEvent方法。从这里开始就进入了view树的时间派发流程了。
View树的事件派发流程
这里从源码上分析事件派发的一些特性。事件派发最开始会进入到ViewGroup的dispatchTouchEvent(DecorVIew父类),下面是ViewGroup的dispatchTouchEvent伪代码的分析,直接在对应的代码部分加了注释:
“`
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
//一开始做一些调试验证,另外如果事件的目标是focused view,并且当前view就是一个focused view,
//有可能view的子View就会处理这次事件,所以将targetAccessibilityFocus设置为false。
…
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) { //检查event是否是安全的
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. //清除之前的事件状态。比如说在app切换,ANR或其他状态改变时,系统框架将会去掉up或cancel事件。在这里会将mFirstTouchTarget清空,mFirstTouchTarget是保存了会接受事件的View处理对象。 cancelAndClearTouchTargets(ev); resetTouchState(); //这里会将mGroupFlags的FLAG_DISALLOW_INTERCEPT标识清除, //1. 每次事件流开始的时候都会先清除FLAG_DISALLOW_INTERCEPT,所以子view的requestDisallowInterceptTouchEvent只有当次事件流有效。 } // 判断是否需要拦截事件,去判断是否拦截事件的条件是此次事件是DOWN事件,或者有子类会处理这次事件(mFirstTouchTarget不为null),并且FLAG_DISALLOW_INTERCEPT没被设置。 final boolean intercepted; if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { //2. 只有有TouchTarget并且没有被disallow,或者是ACTION_DOWN时才会调用onInterceptTouchEvent。 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.判断是否取消 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<View> 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; } //判断当前的child是否可以接收事件,并且事件是否在当前的view范围 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); //3.dispatchTransformedTouchEvent会将Event转化为child坐标空间(getX的变化),然后去除无关的points id,如果有必要更改事件,最后调用child.dispatchTouchEvent。 dispatchTransformedTouchEvent把事件派发给child,如果child成功处理了,则会将child添加到mFirstTouchTarget 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(); //addTouchTarget会创建新的TouchTarget,并将其加入到mFirstTouchTarget 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; } } } //4. 派发事件 // Dispatch to touch targets. if (mFirstTouchTarget == null) { //mFirstTouchTarget为空表示没有子View会处理这次事件,则交给当前的ViewGroup处理。 // 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) {//遍历mFirstTouchTarget链表,一个一个地处理TouchTarget。 final TouchTarget next = target.next; if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {//新添加的不会立刻处理,ACTION_DOWN已经在前面派发了 handled = true; } else { final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted; //dispatchTransformedTouchEvent会将Event转化为child坐标空间(getX的变化),然后去除无关的points id,如果有必要更改事件,最后调用child.dispatchTouchEvent 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); } } ... //用于测试的代码 return handled;}
“`
事件派发流程主要集中在这几个方法的调用:
- dispatchTouchEvent 这是事件派发每个View的时候,第一个被调用的方法,如果是ViewGroup,dispatchTouchEvent会先去调用onInterceptTouchEvent是否应该拦截事件,不拦截的话会先从子View中判断是否有处理该次事件的(在ACTION_DOWN中采用mFirstTouchTarget链接保存会处理事件的TouchTarget),如果没有的话则调用当前View的onTouchEvent。
- onInterceptTouchEvent 判断是否应该拦截事件,ViewGroup默认实现是返回false,子View可以调用getParent.requestDisallowInterceptTouchEvent()来阻止父View拦截。否则只要有子View可能消费事件,该方法都会被调用。
- onTouchEvent这个方法是在没有子View将会消耗事件时才会被调用,onClickListener,onTouchListener,onLongClickListener都是在这个方法中处理的。如果返回true表示消费这次事件。
对于事件派发流程,我觉得有几个地方需要注意的:
1. 应该把ViewGroup以及它所包含的子View都看作是这个ViewGroup的一部分,对于一个ViewGroup是否会处理一次事件,应该是包含了它的子View是否也处理。
2. 如果整个ViewGroup以及它的子类没有一个View处理ACTION_DOWN事件,那么下一次就不会调用这个ViewGroup的dispatchTouchEvent。但是如果ACTION_DOWN返回了true,那么下一次事件还是会继续派发到ViewGroup,即使中间某个ACTION_MOVE返回了false。
3. onInterceptTouchEvent是在子View可能会处理该次事件,并且没有被设置FLAG_DISALLOW_INTERCEPT才会被调用。正常情况下,它在ACTION_DOWN的时候一定会被调用的,因为在ACTION_DOWN的时候会先调用resetTouchState()。
这个是最近略微改了一下很久之前写的一个模仿QQ邮箱滑动退出写的一个东西:https://github.com/xxxzhi/SlideListener,改的过程发现之前看的东西理解地不够好…
源码是解释很多现象的最根本的原因,阅读源码能够更好地理解事件派发流程,理解地更加深刻。
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