View机制深入学习(四)View的事件分发机制
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/** \frameworks\base\core\java\android\app\Activity.java **/public boolean dispatchTouchEvent(MotionEvent ev) { if (ev.getAction() == MotionEvent.ACTION_DOWN) { onUserInteraction(); } if (getWindow().superDispatchTouchEvent(ev)) { return true; } return onTouchEvent(ev);}代码很简单,当是ACTION_DOWN事件时,会先调用onUserInteraction方法,该方法其实就是个Activity的空函数,可以在定义Activity继承时进行Override,完成一些ACTION_DOWN事件分发前的处理工作;
/** \frameworks\base\core\java\android\app\Activity.java **/public void onUserInteraction() {}可以看到重写onUserInteraction方法并不会对MotionEvent的派发产生影响,但是每一个对ACTION_DOWN事件的派发都会调用onUserInteraction方法。
/** \frameworks\base\policy\src\com\android\internal\policy\impl\PhoneWindow.java**/private DecorView mDecor;@Overridepublic boolean superDispatchTouchEvent(MotionEvent event) { return mDecor.superDispatchTouchEvent(event);}PhoneWindow继续将事件传递给内部的DecorView;
/** \frameworks\base\policy\src\com\android\internal\policy\impl\PhoneWindow.java**/private final class DecorView extends FrameLayout implements RootViewSurfaceTaker { public boolean superDispatchTouchEvent(MotionEvent event) { return super.dispatchTouchEvent(event); }}DecorView是PhoneWindow的子类,其会进一步调用super.dispatchTouchEvent(event),DecorView继承FrameLayout,FrameLayout中并未实现dispatchTouchEvent方法;FameLayout继承ViewGroup,则继续调用ViewGroup的dispatchTouchEvent方法;即事件从Activity传到Window进而传递到ViewGroup中;DecorView是顶层ViewGroup,可想而知,接下来的dispatch会根据View树进行逐级派发事件。
3、ViewGroup#dispatchTouchEvent:(代码较长,分段阅读如下):
1)处理ACTION_DOWN事件:
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();}ACTION_DOWN操作可以看做一个完整事件流程的起点,故在该系列动作开始时,会清理之前设置过状态,其中cancelAndClearTouchTargets会将下面将会看到的mFirstTouchTarget变量置为null;
2)判断是否对事件进行拦截:
// 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;}可以看到不拦截情况包括有ACTION_DOWN事件,或者mFirstTouchTarget不为null;
// First touch target in the linked list of touch targets.private TouchTarget mFirstTouchTarget;private static final class TouchTarget { ...... // The next target in the target list. public TouchTarget next; private TouchTarget() { } ......}mFirstTouchTarget是TouchTarget对象,可以看到其其实最终形式一个链表。每一个ViewGroup中都包含一个 mFirstTouchTarget对象,其指向处理TouchEvent事件的下一个View,即该ViewGroup的子元素A如果成功处理了该TouchEvent事件,就会将mFirstTouchTarget指向A;
其实容易理解,这些都是针对于一个完整系列的操作事件而设计的;该事件以ACTION_DOWN事件为开端,当当前事件为ACTION_DOWN时,表示又是一个新的操作,所以将所有配置全部重置,尤其是将mFirstTouchTarget置为null;此时不用管mFirstTouchTarget,因为是ACTION_DOWN事件会就直接跳入到判断是否拦截的代码中。
而在接下来会看到,mFirstTouchTarget将会赋值,如果发生了拦截,mFirstTouchTarget将会置为null,否则将指向下一个将要传递事件的子View;当系列操作中ACTION_MOVE或者ACTION_UP事件派发时,如果已经之前事件分发时已经发生了拦截,此时mFirstTouchTarget==null也就是mFirstTouchTarget!=null条件不成立,则会直接建立拦截,无需再次调用onInterceptTouchEvent;
所以有结论:若某个View一旦发生拦截,那么这一个事件序列都只能由它来处理,并且它的onInterceptTouchEvent方法将不再会调用。
同时注意这里有一个标记位FLAG_DISALLOW_INTERCEPT,由字面意思就可以看出,设置它就可以使得拦截无效(从代码中可以看出,因为会直接跳到intercepted为false,即不拦截的情况。但注意,由于ACTION_DOWN的特殊性,其表示一个序列事件的起点,会对所有状态,包括FLAG_DISALLOW_INTERCEPT标志位进行重置无效化,所以设置FLAG_DISALLOW_INTERCEPT并不能拦截ACTION_DOWN事件,ACTION_DOWN事件的拦截只能发生在onInterceptTouchEvent的处理逻辑中。
假设在该层ViewGroup并不发生拦截,即intercepted为false,ViewGroup会将事件分发给它的子View进行处理:
3)当未发生拦截,分发事件给子View:
if (actionMasked == MotionEvent.ACTION_DOWN|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN) || actionMasked == MotionEvent.ACTION_HOVER_MOVE){ ..... final int childrenCount = mChildrenCount; // 获取子对象 if (newTouchTarget == null && childrenCount != 0) { final float x = ev.getX(actionIndex); final float y = ev.getY(actionIndex); ..... final View[] children = mChildren; // 遍历子View 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); ..... /** 这里是重要的处理函数 ***/ if (!canViewReceivePointerEvents(child) || !isTransformedTouchPointInView(x, y, child, null)) { ev.setTargetAccessibilityFocus(false); continue; } newTouchTarget = getTouchTarget(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(); }}
上面主要是对事件向子View转发的处理;系统如果采用将事件全部转发,也就是广播给所有子View,然后在子View中去判断自己是否应当处理该event,这样效率显然是相当低的。因此系统采取的做法是先进行判断,遍历所有子View,判断适合接收该event的子View,然后将该事件传递给它。上面的代码中完成就是这样的问题。
主要涉及到的函数为canViewReceivePointerEvents和isTransformedTouchPointInView,可以看到如果两个判断均通过的话,该child即是所要寻找的结果,会调用dispatchTransformedTouchEvent方法将事件传递给它;
I) canViewReceivePointerEvents方法是判断该View是否能够该收该event,源码如下:
private static boolean canViewReceivePointerEvents(View child) { return (child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null;}可以看到当child的状态为VISIBLE状态,或者child当前在播放动画时,此时该View是可以接受该event的;
II)isTransformedTouchPointInView方法是判断点击事件的坐标是否在child的区域范围内:
protected boolean isTransformedTouchPointInView(float x, float y, View child, PointF outLocalPoint) { final float[] point = getTempPoint(); point[0] = x; point[1] = y; transformPointToViewLocal(point, child); final boolean isInView = child.pointInView(point[0], point[1]); if (isInView && outLocalPoint != null) { outLocalPoint.set(point[0], point[1]); } return isInView;}III)当两个条件都满足,即调用dispatchTransformedTouchEvent方法将事件传递给child:
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, View child, int desiredPointerIdBits) { final boolean handled; .... // Perform any necessary transformations and dispatch. if (child == null) { handled = super.dispatchTouchEvent(transformedEvent); } else { ...... handled = child.dispatchTouchEvent(transformedEvent); } return handled;}如果child为null,表示无子View或者改事件点击范围内没有满足条件的子View,则调用ViewGroup的父类View中的dispatchTouchEvent函数来处理event;否则,调用child的方法继续下一轮事件派发;
IV)在dispatchTransformedTouchEvent函数结果值为true的情况下,会继续往下执行,执行的主要代码如下:
newTouchTarget = addTouchTarget(child, idBitsToAssign); alreadyDispatchedToNewTouchTarget = true;
即如果child子元素的dispatchTouchEvent返回结果为true时,会执行addTouchTarget,
private TouchTarget addTouchTarget(View child, int pointerIdBits) { TouchTarget target = TouchTarget.obtain(child, pointerIdBits); target.next = mFirstTouchTarget; mFirstTouchTarget = target; return target;}这里完成对mFirstTouchTarget的初始化。
4、View对事件的处理过程:
由前面可以看到,当发生拦截或者事件分发到最底层的View时,事件会交给View的dispatchTouchEvent方法进行处理;
View#dispatchTouchEvent:
public boolean dispatchTouchEvent(MotionEvent event) { .... 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; } } ......}这里是对Touch事件处理的主要逻辑,由View机制深入学习(三) View中的消息传递及InputManagerService 知ListenerInfo是对View注册的一系列Listener的管理者,比如OnClickListener;这里和上文中的逻辑相同,若是View注册了onTouchListener,则先调用onTouchListener的onTouch方法处理事件,若onTouch方法返回true,则拦截事件,不再继续向下处理;若onTouch返回false,则会继续调用View中的onTouchEvent方法处理事件。
View#onTouchEvent:
I、状态为DISABLED的View相应点击事件
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);}可以看到当View为DISABLED状态时,依然会消耗点击事件,只不过是不做处理而已。View的enable属性是不影响View中onTouchEvent默认的返回值的,即使是DIABLED状态,只要其clickable或者long_clickable为true,就会返回true,消耗该点击事件。
II、主要的事务处理逻辑:
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;View会针对TouchEvent事件类型的不同进行分别处理,这里以ACTION_UP为例,可以看到最核心的响应就是触发performClick()方法:
III、View#performClick:
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;}可以看到OnClickListener的处理逻辑就在这里,如果View注册了 OnClickListener,在ACTION_UP事件到达该View时,会最终触发performClick方法,该方法中进而调用OnClickListener的onClick函数,故而在日常使用中则是重写onClick方法来实现点击的响应,并且onClick(View view)这里将this作参数传递进来,因此onClick中的view即代表被click的组件View。
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