概述
一直想写篇关于Android事件派发机制的文章,却一直没写,这两天刚好是周末,有时间了,想想写一篇吧,不然总是只停留在会用的层次上但是无法了解其内部机制。我用的是4.4源码,打开看看,挺复杂的,尤其是事件是怎么从Activity派发出来的,太费解了。了解Windows消息机制的人会发现,觉得Android的事件派发机制和Windows的消息派发机制挺像的,其实这是一种典型的消息“冒泡”机制,很多平台采用这个机制,消息最先到达最底层View,然后它先进行判断是不是它所需要的,否则就将消息传递给它的子View,这样一来,消息就从水底的气泡一样向上浮了一点距离,以此类推,气泡达到顶部和空气接触,破了(消息被处理了),当然也有气泡浮出到顶层了,还没破(消息无人处理),这个消息将由系统来处理,对于Android来说,会由Activity来处理。
Android点击事件的派发机制
1. 从Activity传递到底层View
点击事件用MotionEvent来表示,当一个点击操作发生时,事件最先传递给当前Activity,由Activity的dispatchTouchEvent来进行事件派发,具体的工作是由Activity内部的Window来完成的,Window会将事件传递给decor view,decor view一般就是当前界面的底层容器(即setContentView所设置的View的父容器),通过Activity.getWindow.getDecorView()可以获得。另外,看下面代码的的时候,主要看我注释的地方,代码很多很复杂,我无法一一说明,但是我注释的地方都是关键点,是博主仔细读代码总结出来的。
源码解读:
事件是由哪里传递给Activity的,这个我还不清楚,但是不要紧,我们从activity开始分析,已经足够我们了解它的内部实现了。
Code:Activity#dispatchTouchEvent
/** * Called to process touch screen events. You can override this to * intercept all touch screen events before they are dispatched to the * window. Be sure to call this implementation for touch screen events * that should be handled normally. * * @param ev The touch screen event. * * @return boolean Return true if this event was consumed. */ public boolean dispatchTouchEvent(MotionEvent ev) { if (ev.getAction() == MotionEvent.ACTION_DOWN) { //这个函数其实是个空函数,啥也没干,如果你没重写的话,不用关心 onUserInteraction(); } //这里事件开始交给Activity所附属的Window进行派发,如果返回true,整个事件循环就结束了 //返回false意味着事件没人处理,所有人的onTouchEvent都返回了false,那么Activity就要来做最后的收场。 if (getWindow().superDispatchTouchEvent(ev)) { return true; } //这里,Activity来收场了,Activity的onTouchEvent被调用 return onTouchEvent(ev); }
Window是如何将事件传递给ViewGroup的
Code:Window#superDispatchTouchEvent
/** * Used by custom windows, such as Dialog, to pass the touch screen event * further down the view hierarchy. Application developers should * not need to implement or call this. * */ public abstract boolean superDispatchTouchEvent(MotionEvent event);
这竟然是一个抽象函数,还注明了应用开发者不要实现它或者调用它,这是什么情况?再看看如下类的说明,大意是说:这个类可以控制顶级View的外观和行为策略,而且还说这个类的唯一一个实现位于android.policy.PhoneWindow,当你要实例化这个Window类的时候,你并不知道它的细节,因为这个类会被重构,只有一个工厂方法可以使用。好吧,还是很模糊啊,不太懂,不过我们可以看一下android.policy.PhoneWindow这个类,尽管实例化的时候此类会被重构,但是重构而已,功能是类似的。
Abstract base class for a top-level window look and behavior policy. An instance of this class should be used as the top-level view added to the window manager. It provides standard UI policies such as a background, title area, default key processing, etc.The only existing implementation of this abstract class is android.policy.PhoneWindow, which you should instantiate when needing a Window. Eventually that class will be refactored and a factory method added for creating Window instances without knowing about a particular implementation.
Code:PhoneWindow#superDispatchTouchEvent
@Override public boolean superDispatchTouchEvent(MotionEvent event) { return mDecor.superDispatchTouchEvent(event); }这个逻辑很清晰了,PhoneWindow将事件传递给DecorView了,这个DecorView是啥呢,请看下面 private final class DecorView extends FrameLayout implements RootViewSurfaceTaker // This is the top-level view of the window, containing the window decor. private DecorView mDecor; @Override public final View getDecorView() { if (mDecor == null) { installDecor(); } return mDecor; }
顺便说一下,平时Window用的最多的就是((ViewGroup)getWindow().getDecorView().findViewById(android.R.id.content)).getChildAt(0)即通过Activity来得到内部的View。这个mDecor显然就是getWindow().getDecorView()返回的View,而我们通过setContentView设置的View是它的一个子View。目前事件传递到了DecorView 这里,由于DecorView 继承自FrameLayout且是我们的父View,所以最终事件会传递给我们的View,原因先不管了,换句话来说,事件肯定会传递到我们的View,不然我们的应用如何响应点击事件呢。不过这不是我们的重点,重点是事件到了我们的View以后应该如何传递,这是对我们更有用的。从这里开始,事件已经传递到我们的顶级View了,注意:顶级View实际上是最底层View,也叫根View。
2.底层View对事件的分发过程
点击事件到底层View(一般是一个ViewGroup)以后,会调用ViewGroup的dispatchTouchEvent方法,然后的逻辑是这样的:如果底层ViewGroup拦截事件即onInterceptTouchEvent返回true,则事件由ViewGroup处理,这个时候,如果ViewGroup的mOnTouchListener被设置,则会onTouch会被调用,否则,onTouchEvent会被调用,也就是说,如果都提供的话,onTouch会屏蔽掉onTouchEvent。在onTouchEvent中,如果设置了mOnClickListener,则onClick会被调用。如果顶层ViewGroup不拦截事件,则事件会传递给它的在点击事件链上的子View,这个时候,子View的dispatchTouchEvent会被调用,到此为止,事件已经从最底层View传递给了上一层View,接下来的行为和其底层View一致,如此循环,完成整个事件派发。另外要说明的是,ViewGroup默认是不拦截点击事件的,其onInterceptTouchEvent返回false。
源码解读:
Code:ViewGroup#dispatchTouchEvent
@Override public 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) { //这里判断是否拦截点击事件,如果拦截,则intercepted=true 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; //这里面一大堆是派发事件到子View,如果intercepted是true,则直接跳过 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 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 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(); //注意下面两句,如果有子View处理了点击事件,则newTouchTarget会被赋值, //同时alreadyDispatchedToNewTouchTarget也会为true,这两个变量是直接影响下面的代码逻辑的。 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. //这里如果当前ViewGroup拦截了事件,或者其子View的onTouchEvent都返回了false,则事件会由ViewGroup处理 if (mFirstTouchTarget == null) { // No touch targets so treat this as an ordinary view. //这里就是ViewGroup对点击事件的处理 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; }
下面再看ViewGroup对点击事件的处理
Code:ViewGroup#dispatchTransformedTouchEvent
/** * 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) { //这里就是ViewGroup对点击事件的处理,其调用了View的dispatchTouchEvent方法 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; }
再看
Code:View#dispatchTouchEvent
/** * 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 (mInputEventConsistencyVerifier != null) { mInputEventConsistencyVerifier.onTouchEvent(event, 0); } if (onFilterTouchEventForSecurity(event)) { //noinspection SimplifiableIfStatement ListenerInfo 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; }
这段代码比较简单,View对事件的处理是这样的:如果设置了OnTouchListener就调用onTouch,否则就直接调用onTouchEvent,而onClick是在onTouchEvent内部通过performClick触发的。简单来说,事件如果被ViewGroup拦截或者子View的onTouchEvent都返回了false,则事件最终由ViewGroup处理。
3.无人处理的点击事件
如果一个点击事件,子View的onTouchEvent返回了false,则父View的onTouchEvent会被直接调用,以此类推。如果所有的View都不处理,则最终会由Activity来处理,这个时候,Activity的onTouchEvent会被调用。这个问题已经在1和2中做了说明。
以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持查字典教程网。
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