View事件分发机制全解析
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dispatchTouchEvent方法是用来传递Touch(触摸)事件的,它的返回值就是内部声明的result。如果result为true,则会进行分发,也就是view会继续响应触摸事件,但不会向父视图传递。如果为false,则不会分发,无论你如何触摸你的view,都不会处理你触摸事件的逻辑,但事件会传递给父视图进行处理。
从以上可以得到一个概念:分发表示继续处理触摸事件,不分发表示不继续处理触摸事件。
我们先看看这个方法:
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;}
第3-22行是对组件的属性或者是点击操作进行处理,一般情况下并不会影响返回的结果。只有第8行返回了false,也就是不分发。但是从它的判断语句if (!isAccessibilityFocusedViewOrHost())的名字可以大概知道,它是用来处理view的无障碍属性的,一般情况下该值都是true,也就是不会执行第8行,而是继续处理下面的逻辑。
要注意到第12行声明了布尔变量result,它就是这个函数的返回值。默认值为false,不进行分发。
接下来我们看看24-36行:
第24行有个if (onFilterTouchEventForSecurity(event))的判断,这个判断是此函数最需要注意的地方(在if语句内部,可以清楚地看到返回的result值在满足一定的条件下,被设置为ture了),我们进入这个判断内部,看看它是怎么处理的,以及onFilterTouchEventForSecurity返回值的布尔值是怎么设置的
/** * Filter the touch event to apply security policies. * * @param event The motion event to be filtered. * @return True if the event should be dispatched, false if the event should be dropped. * * @see #getFilterTouchesWhenObscured */public boolean onFilterTouchEventForSecurity(MotionEvent event) { //noinspection RedundantIfStatement if ((mViewFlags & FILTER_TOUCHES_WHEN_OBSCURED) != 0 && (event.getFlags() & MotionEvent.FLAG_WINDOW_IS_OBSCURED) != 0) { // Window is obscured, drop this touch. return false; } return true;}
onFilterTouchEventForSecurity只有一个判断,返回值的布尔值只和判断的结果有关系。如果if满足了,就返回false,否则返回true。我们看看这个if判断,它进行了两个位运算。
第一个位运算:mViewFlags & FILTER_TOUCHES_WHEN_OBSCURED。mViewFlags是该View所包含的标志合成信息(包括是否可点击Clikable、可使用的Enable等等,这些标识,可以在Java代码或者xml文件中进行设置,一般可点击的view,这两个属性都为true),所以只需要找到对应的View标志和mViewFlags进行&运算,就能判断当前View是否支持这个标志。因此,我们值需要查看FILTER_TOUCHES_WHEN_OBSCURED,让我们看看它声明的地方:
/** * Indicates that the view should filter touches when its window is obscured. * Refer to the class comments for more information about this security feature. * {@hide} */
也就是说,这个标志是来表明当窗口被遮挡的时候,当前view是否需要过滤掉touch事件。过滤的意思就是不进行传递,即是不对后续的触摸事件进行处理。为什么不会对其进行处理呢?如果该标志为真,那么与mViewFlags相&的结果就不等于0,也就满足于onFilterTouchEventForSecurity方法的if判断,最后该方法返回false,导致dispatchTouchEvent不进入if (onFilterTouchEventForSecurity(event))语句内,最终导致dispatchTouchEvent返回result的默认值false(第36行以后不会再修改result的值了)
那么,为什么要过滤掉touch事件呢?我们能从第二个&运算中得到问题的答案:event.getFlags() & MotionEvent.FLAG_WINDOW_IS_OBSCURED
首先是得到event事件的标志(如按下、抬起、滑动等等),然后与MotionEvent.FLAG_WINDOW_IS_OBSCURED进行&运算。
查看MotionEvent.FLAG_WINDOW_IS_OBSCURED的声明:
/** * This flag indicates that the window that received this motion event is partly * or wholly obscured by another visible window above it. This flag is set to true * even if the event did not directly pass through the obscured area. * A security sensitive application can check this flag to identify situations in which * a malicious application may have covered up part of its content for the purpose * of misleading the user or hijacking touches. An appropriate response might be * to drop the suspect touches or to take additional precautions to confirm the user's * actual intent. */public static final int FLAG_WINDOW_IS_OBSCURED = 0x1;
简而言之,FLAG_WINDOW_IS_OBSCURED是用来检查组件的安全性的。比如说你的view被其他界面部分遮盖或者完全遮盖(假设有个恶意程序故意做成和你view相同的界面,从而劫持了你的触摸事件,诱导你进入到它程序的界面)。所以,拿到这个标志,就能采取一定的预防措施,确定用户的真实意图,如Toast提示用户界面被拦截。
第24行的作用就显而易见了。如果需要过滤当view被遮盖的touch事件,或者view已经半遮盖或者全遮盖住了,则onFilterTouchEventForSecurity返回false,那么dispatchTouchEvent的result为false,也就是不分发事件。如果onFilterTouchEventForSecurity返回true,则分发事件。从该方法的注释@return True if the event should be dispatched, false if the event should be dropped.也可以看出这个结果。
默认情况下,该方法返回的是true,也就进入了25-35行的方法内部:
//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; }
首先是将类型为ListenerInfo的变量li指向mListenerInfo,mListenerInfo是在哪里声明的呢?进行搜索后,可以找到相应的方法:
ListenerInfo getListenerInfo() { if (mListenerInfo != null) { return mListenerInfo; } mListenerInfo = new ListenerInfo(); return mListenerInfo;}
getListenerInfo是获取mListenerInfo,正是我们要搜索的变量。如果mListenerInfo为空的话,它会new一个新的ListenerInfo()变量。我们看看ListenerInfo类到底是什么:
static class ListenerInfo {
/**
* Listener used to dispatch focus change events.
* This field should be made private, so it is hidden from the SDK.
* {@hide}
*/
protected OnFocusChangeListener mOnFocusChangeListener;
/** * Listeners for layout change events. */ private ArrayList<OnLayoutChangeListener> mOnLayoutChangeListeners; protected OnScrollChangeListener mOnScrollChangeListener; /** * Listeners for attach events. */ private CopyOnWriteArrayList<OnAttachStateChangeListener> mOnAttachStateChangeListeners; /** * Listener used to dispatch click events. * This field should be made private, so it is hidden from the SDK. * {@hide} */ public OnClickListener mOnClickListener; /** * Listener used to dispatch long click events. * This field should be made private, so it is hidden from the SDK. * {@hide} */ protected OnLongClickListener mOnLongClickListener; /** * Listener used to dispatch context click events. This field should be made private, so it * is hidden from the SDK. * {@hide} */ protected OnContextClickListener mOnContextClickListener; /** * Listener used to build the context menu. * This field should be made private, so it is hidden from the SDK. * {@hide} */ protected OnCreateContextMenuListener mOnCreateContextMenuListener; private OnKeyListener mOnKeyListener; private OnTouchListener mOnTouchListener; private OnHoverListener mOnHoverListener; private OnGenericMotionListener mOnGenericMotionListener; private OnDragListener mOnDragListener; private OnSystemUiVisibilityChangeListener mOnSystemUiVisibilityChangeListener; OnApplyWindowInsetsListener mOnApplyWindowInsetsListener;}
定义的全都是Listener!包括OnClickListener,OnTouchListener等等。这么说来,ListenerInfo的作用就是为外界提供接口信息。而如果我们设置了Listener,比如setOnTouchListener:
/** * Register a callback to be invoked when a touch event is sent to this view. * @param l the touch listener to attach to this view */public void setOnTouchListener(OnTouchListener l) { getListenerInfo().mOnTouchListener = l;}
按照我们上面的分析,getListenerInfo()在mListenerInfo不为空的时候会new一个新值,所以当你调用了setOnTouchListener的时候,mListenerInfo就已经是不为空了。
我们看看第27行的判断
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if中有四个条件,只有其中一个不满足,就会进行33行的判断
第一个条件:只要设置了listener,那么li不为空。
第二个条件:因为我们调用了setOnTouchListener,所以li.mOnTouchListener也不为空。
第三个条件:mViewsFlags已经分析过了,是包含该View标识的变量,ENABLED_MASK的注释说明了这个View是否是ENABLE的(按钮默认是enable的,如果不是enable的话,那么当你点击的时候,按钮就不会产生响应)
第四个条件:onTouch的返回值是否为true
在我们的案例中,我们的按钮设置了onTouchListener,所以四个判断可以缩短成一个,也就是
if(li.mOnTouchListener.onTouch(this, event)) result = true;
看到没,dispatchTouchEvent的返回值最终就和我们实现onTouchListener时所实现的onTouch方法的返回值有关系了。
上面已经说过,如果result为true,表示分发,否则表示不分发。当设置onTouch返回为true时,该view也就不会继续处理下一个触摸事件了。我们先试试看,在Button的onTouch设置为true,会出现什么情况。
可以发现,我们的onTouch只处理了DOWN的event,而不再响应其他的event了,甚至就连onClick方法也不再执行。
可是,我们只是改变了onTouch的返回值,为什么会影响到onClick方法呢?
继续查看dispatchTouchEvent的源码,在27行以后的第二个if,也就是33行的if判断:
if (!result && onTouchEvent(event)) { result = true; }
如果onTouch返回的是true,会导致!result为false,不对后面的条件进行判断,从而进不去该if语句内部。而后面的条件为onTouchEvent(event)的返回值。onTouch为true,该条件不判断,而onClick也不执行,是不是可以说其实在onTouchEvent内部可能存在onClick语句?
我们进入到onTouchEvent的内部看看
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) { setPressed(false); } // A disabled view that is clickable still consumes the touch // events, it just doesn't respond to them. return (((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE); } if (mTouchDelegate != null) { 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. 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(); } } } 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(); } 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); checkForLongClick(0); } break; case MotionEvent.ACTION_CANCEL: setPressed(false); removeTapCallback(); removeLongPressCallback(); mInContextButtonPress = false; mHasPerformedLongPress = false; mIgnoreNextUpEvent = false; break; case MotionEvent.ACTION_MOVE: drawableHotspotChanged(x, y); // Be lenient about moving outside of buttons if (!pointInView(x, y, mTouchSlop)) { // Outside button removeTapCallback(); if ((mPrivateFlags & PFLAG_PRESSED) != 0) { // Remove any future long press/tap checks removeLongPressCallback(); setPressed(false); } } break; } return true; } return false;}
第59行有个和click名称类似的方法,我们点进去看看
public boolean performClick() { final boolean result; final ListenerInfo li = mListenerInfo; if (li != null && li.mOnClickListener != null) { playSoundEffect(SoundEffectConstants.CLICK); li.mOnClickListener.onClick(this); result = true; } else { result = false; } sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED); return result;}
果然,这个方法内部,就是调用了ListenerInfo的OnClickListener实例的onClick方法。那么除了if (!result && onTouchEvent(event)),剩余的代码(dispatchTouchEvent的38行以后)会去调用onClick方法吗?
依次点进各个方法的内部,并没有发现和onClick有关的代码。所以可以肯定,onClick方法必然是在onTouchEvent里面调用,且在dispatchTouchEvent方法内部,只有它一个方法调用了onClick方法。我们也能通过反证法进行验证:假设后续的方法也调用了onClick的话,那么Log打印的onClick应该不止一次,但是Log打印的onClick方法有且仅有一次,所以假设有误,也就是只执行了一次onClick方法,而该方法正是在onTouchEvent内部。
不过在此之前,我们先对dispatchTouchEvent进行简化,根据上面的分析,简化后的代码为:
public boolean dispatchTouchEvent(MotionEvent event) {
if (!isAccessibilityFocusedViewOrHost()) return false;boolean result = false;ListenerInfo li = mListenerInfo;if(li.mOnTouchListener.onTouch(this, event)) result = true;else if(onTouchEvent(event)) result = true;return result;
}
是不是感觉清晰许多了?实际上,当我们onTouch返回false的时候,就会去执行onTouchEvent。
我们继续分析onTouchEvent方法。
在分析前,我们再明确一下onTouch返回值对dispatchTouchEvent造成的影响:
onTouch返回为false,所以result仍然为false。所以,result的值取决于onTouchEvent的返回值,也正好与onTouchEvent返回值相同。如果onTouchEvent返回true,则分发事件,否则不分发事件。
那么,分析开始。
第2-5行,初始化变量:点击的坐标,view的标志,event的动作
第7行的判断:
if ((viewFlags & ENABLED_MASK) == DISABLED) { if (action == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) { setPressed(false); } // A disabled view that is clickable still consumes the touch // events, it just doesn't respond to them. return (((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE);}
如果view是disabled的,且是clickable的,则返回true,如果是disclickable的,则返回false。
如果view是enable的,则进入第17行的判断:
if (mTouchDelegate != null) { if (mTouchDelegate.onTouchEvent(event)) { return true; }}
mTouchDelegate是的Touch事件进行代理处理(比如说,点击A按钮,但是A按钮不做处理,反而是让B按钮来处理触摸事件)。一般情况下是false,所以不会进入该if语句。
这个if判断以后的大段代码里,只有两行代码是和return有关的,分别为133行return true和136行return false,而133行在第23行的if条件判断内部最后一行语句。也就是说,如果进了if,则返回true,分发事件,否则返回false,不分发事件。
进入23行代码:
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. 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(); } } } 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(); } 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); checkForLongClick(0, x, y); } break; case MotionEvent.ACTION_CANCEL: setPressed(false); removeTapCallback(); removeLongPressCallback(); mInContextButtonPress = false; mHasPerformedLongPress = false; mIgnoreNextUpEvent = false; break; case MotionEvent.ACTION_MOVE: drawableHotspotChanged(x, y); // Be lenient about moving outside of buttons if (!pointInView(x, y, mTouchSlop)) { // Outside button removeTapCallback(); if ((mPrivateFlags & PFLAG_PRESSED) != 0) { // Remove any future long press/tap checks removeLongPressCallback(); setPressed(false); } } break; } return true; }
看上去似乎很长,不过大部分的代码不是我们分析重点,我们只需要重点关注该方法的返回值和onClick事件的触发就可以了。因为①返回值是dispatchTouchEvent事件分发的关键,②onClick事件则是为了验证该事件是在onTouch事件后调用,③并且当onTouch返回true的时候不会再调用onClick。
②③已经分析过了,所以只需要验证①,基本上就能理解view的事件分发机制,看看①,也就是23行代码的开始:
if (((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) {
第一行,在view已经是enable的状态下,判断是不是clickable,如果是clickable的话,进入if语句,最后会返回true,否则直接返回false。①②③都验证完成了。
对于onTouchEvent方法,我们进行简化,简化后的代码如下:
public boolean onTouchEvent(MotionEvent event) { if ((viewFlags & ENABLED_MASK) == DISABLED) { return (((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE); } if (((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE) { switch(event){ case MotionEvent.ACTION_UP: li.mOnClickListener.onClick(this); } return true; } return false; }
这是第一步简化,但通常情况下,我们不需要设置长按和上下文点击(上下文点击用于压感式屏幕或者鼠标右键等等),所以我们能进行第二步的简化:
public boolean onTouchEvent(MotionEvent event) { if ((viewFlags & ENABLED_MASK) == DISABLED) { return ((viewFlags & CLICKABLE) == CLICKABLE); } if ((viewFlags & CLICKABLE) == CLICKABLE) { switch(event){ case MotionEvent.ACTION_UP: li.mOnClickListener.onClick(this); } return true; } return false; }
是不是简单许多了?其实onTouchEvent只有在enable并且clickable的时候才返回true,其他情况下才返回false。而在onTouchEvent里,又会去调用onClick方法。
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