在Android开发过程中，经常需要获取Window或某个View的可见性变化时机，以便在View的Visibility变化时进行相应的处理。目前，比较常用的判断View可见性时机的回调有

onWindowVisibilityChanged
onVisibilityChanged
OnAttachStateChangeListener#onViewAttachedToWindow

一、onWindowVisibilityChanged

         /*** Called when the window containing has change its visibility* (between {@link #GONE}, {@link #INVISIBLE}, and {@link #VISIBLE}).  Note* that this tells you whether or not your window is being made visible* to the window manager; this does <em>not</em> tell you whether or not* your window is obscured by other windows on the screen, even if it* is itself visible.** @param visibility The new visibility of the window.*/protected void onWindowVisibilityChanged(@Visibility int visibility) {if (visibility == VISIBLE) {initialAwakenScrollBars();}}

由方法注释可知，它是在窗口可见性改变时调用，而且注意这只是在Window对WindowManager可见时调用，并不是告知你当前可见的Window是否被遮挡。

查看代码，发现其调用位置有3处，添加时在performTraversals方法中(代码有省略)，Activity onStop生命周期会remove掉DecorView和对应的Window，在removeView方法中会调用dispatchDetachedFromWindow方法，该方法内又会调用onWindowVisibilityChanged

private void performTraversals() {// cache mView since it is used so much below...final View host = mView;......final int viewVisibility = getHostVisibility();final boolean viewVisibilityChanged = !mFirst&& (mViewVisibility != viewVisibility || mNewSurfaceNeeded// Also check for possible double visibility update, which will make current// viewVisibility value equal to mViewVisibility and we may miss it.|| mAppVisibilityChanged);mAppVisibilityChanged = false;final boolean viewUserVisibilityChanged = !mFirst &&((mViewVisibility == View.VISIBLE) != (viewVisibility == View.VISIBLE));......if (mFirst) {......// We used to use the following condition to choose 32 bits drawing caches:// PixelFormat.hasAlpha(lp.format) || lp.format == PixelFormat.RGBX_8888// However, windows are now always 32 bits by default, so choose 32 bitsmAttachInfo.mUse32BitDrawingCache = true;mAttachInfo.mWindowVisibility = viewVisibility;mAttachInfo.mRecomputeGlobalAttributes = false;mLastConfigurationFromResources.setTo(config);mLastSystemUiVisibility = mAttachInfo.mSystemUiVisibility;// Set the layout direction if it has not been set before (inherit is the default)if (mViewLayoutDirectionInitial == View.LAYOUT_DIRECTION_INHERIT) {host.setLayoutDirection(config.getLayoutDirection());}/***  ①*/host.dispatchAttachedToWindow(mAttachInfo, 0);mAttachInfo.mTreeObserver.dispatchOnWindowAttachedChange(true);dispatchApplyInsets(host);} else {......}}if (viewVisibilityChanged) {mAttachInfo.mWindowVisibility = viewVisibility;/***  ②*/host.dispatchWindowVisibilityChanged(viewVisibility);if (viewUserVisibilityChanged) {host.dispatchVisibilityAggregated(viewVisibility == View.VISIBLE);}if (viewVisibility != View.VISIBLE || mNewSurfaceNeeded) {endDragResizing();destroyHardwareResources();}if (viewVisibility == View.GONE) {// After making a window gone, we will count it as being// shown for the first time the next time it gets focus.mHasHadWindowFocus = false;}}}

void dispatchDetachedFromWindow() {AttachInfo info = mAttachInfo;if (info != null) {int vis = info.mWindowVisibility;if (vis != GONE) {onWindowVisibilityChanged(GONE);// ③if (isShown()) {// Invoking onVisibilityAggregated directly here since the subtree// will also receive detached from windowonVisibilityAggregated(false);}}}onDetachedFromWindow();onDetachedFromWindowInternal();......ListenerInfo li = mListenerInfo;final CopyOnWriteArrayList<OnAttachStateChangeListener> listeners =li != null ? li.mOnAttachStateChangeListeners : null;if (listeners != null && listeners.size() > 0) {// NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to// perform the dispatching. The iterator is a safe guard against listeners that// could mutate the list by calling the various add/remove methods. This prevents// the array from being modified while we iterate it.for (OnAttachStateChangeListener listener : listeners) {listener.onViewDetachedFromWindow(this);}}......}

调用位置已注释，首先看第一处①，应用启动时，host就是DecorView对象，它是一个ViewGroup，所以在ViewGroup类中查看

     /*** ViewGroup.class*/@Override@UnsupportedAppUsagevoid dispatchAttachedToWindow(AttachInfo info, int visibility) {mGroupFlags |= FLAG_PREVENT_DISPATCH_ATTACHED_TO_WINDOW;// [1]super.dispatchAttachedToWindow(info, visibility);mGroupFlags &= ~FLAG_PREVENT_DISPATCH_ATTACHED_TO_WINDOW;final int count = mChildrenCount;final View[] children = mChildren;for (int i = 0; i < count; i++) {final View child = children[i];// [2]child.dispatchAttachedToWindow(info,combineVisibility(visibility, child.getVisibility()));}final int transientCount = mTransientIndices == null ? 0 : mTransientIndices.size();for (int i = 0; i < transientCount; ++i) {View view = mTransientViews.get(i);view.dispatchAttachedToWindow(info,combineVisibility(visibility, view.getVisibility()));}}

可以看到，ViewGroup#dispatchAttachedToWindow方法主要作用就是

[1] 调用自身的dispatchAttachedToWindow方法，处理自己attach到Window

[2] 向子View分发事件，让每个字View处理attach到Window的事件

由此可知，最终都会调用到View#dispatchAttachedToWindow方法

void dispatchAttachedToWindow(AttachInfo info, int visibility) {mAttachInfo = info;......// Transfer all pending runnables.if (mRunQueue != null) {mRunQueue.executeActions(info.mHandler);mRunQueue = null;}performCollectViewAttributes(mAttachInfo, visibility);onAttachedToWindow();// [1]ListenerInfo li = mListenerInfo;final CopyOnWriteArrayList<OnAttachStateChangeListener> listeners =li != null ? li.mOnAttachStateChangeListeners : null;if (listeners != null && listeners.size() > 0) {// NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to// perform the dispatching. The iterator is a safe guard against listeners that// could mutate the list by calling the various add/remove methods. This prevents// the array from being modified while we iterate it.for (OnAttachStateChangeListener listener : listeners) {listener.onViewAttachedToWindow(this);// [2]}}int vis = info.mWindowVisibility;if (vis != GONE) {onWindowVisibilityChanged(vis);// [3]if (isShown()) {// Calling onVisibilityAggregated directly here since the subtree will also// receive dispatchAttachedToWindow and this same callonVisibilityAggregated(vis == VISIBLE);}}// Send onVisibilityChanged directly instead of dispatchVisibilityChanged.// As all views in the subtree will already receive dispatchAttachedToWindow// traversing the subtree again here is not desired.onVisibilityChanged(this, visibility);......}

View#dispatchAttachedToWindow方法集中处理了onAttachedToWindow、onViewAttachedToWindow、onWindowVisibilityChanged和onVisibilityChanged这4种可见性变化相关的回调函数。

对于onWindowVisibilityChanged方法来说，首先会通过AttachInfo对象获取现在窗口（mWindowVisibility）可见性。mWindowVisibility变量的赋值也在performTraversals方法中。

......
final int viewVisibility = getHostVisibility();
// mViewVisibility在创建ViewRootImpl对象时，初始化值是GONE；在完成测量布局后赋值为viewVisibility
final boolean viewVisibilityChanged = !mFirst&& (mViewVisibility != viewVisibility || mNewSurfaceNeeded// Also check for possible double visibility update, which will make current// viewVisibility value equal to mViewVisibility and we may miss it.|| mAppVisibilityChanged);
......
mAttachInfo.mWindowVisibility = viewVisibility;
......

在Window被添加到屏幕上后(mWindowSession.addToDisplay)，getHostVisibility()就返回Visible。所以只要mWindowVisibility不为GONE就会调用onWindowVisibilityChanged方法。这就是它的第一种调用场景。
查看*getHostVisibility()*方法

// #ViewRootImpl.java
int getHostVisibility() {return (mAppVisible || mForceDecorViewVisibility) ? mView.getVisibility() : View.GONE;}

可见，最终取得的是mView对象的可见性，而mView对象就是DecorView对象(在ViewrootImpl#setView方法中设置的)，所以：

mWindowVisibility方法只会在页面（Activity和Dialog）打开和关闭（具体说是Activity或Dialog的Window可见性改变时）各调用一次

第二处调用位置在②处，主要代码是

 if (viewVisibilityChanged) {mAttachInfo.mWindowVisibility = viewVisibility;/***    ②*/host.dispatchWindowVisibilityChanged(viewVisibility);......}

在DecorView加载时，如果mFirst==false（非首次加载），很可能进行该条件体进行调用。

第三种情况③，例如打开一个新页面，老页面走到onStop声明周期方法，如③处，只要不是GONE就会调用。

综上，onWindowVisibilityChanged的调用：

每当一个页面打开或移除时，如果关联的Window可见（不等于GONE），则会调用
打开时，是在View#dispatchAttachedToWindow中进行调用，分离时在View#dispatchDetachFromWindow时调用，并传入默认参数GONE

二、onVisibilityChanged

onVisibilityChanged调用时机和onWindowVisibilityChanged非常类似，对于APP启动打开页面时，会处理重写该方法的View attach到Window的事件，此时默认传入的参数是Visible(值为0)。

 // ViewRootImpl.classprivate void performTraversals() {......host.dispatchAttachedToWindow(mAttachInfo, 0);......}// View.classvoid dispatchAttachedToWindow(AttachInfo info, int visibility) {......// Send onVisibilityChanged directly instead of dispatchVisibilityChanged.// As all views in the subtree will already receive dispatchAttachedToWindow// traversing the subtree again here is not desired.onVisibilityChanged(this, visibility);......}

然后，每次调用setVisibility方法来控制视图的可见性时都会回调该方法。

 // View.classpublic void setVisibility(@Visibility int visibility) {setFlags(visibility, VISIBILITY_MASK);}void setFlags(int flags, int mask) {......if ((changed & VISIBILITY_MASK) != 0) {......if (mAttachInfo != null) {dispatchVisibilityChanged(this, newVisibility);......}}        .......}protected void dispatchVisibilityChanged(@NonNull View changedView,@Visibility int visibility) {onVisibilityChanged(changedView, visibility);}

同样，在页面关闭时（或打开新页面覆盖旧页面），会执行onStop生命周期方法，其实是调用ActivityThread#handleStopActivity方法，然后会调用到updateVisibility方法

// ActivityThread.class
private void updateVisibility(ActivityClientRecord r, boolean show) {View v = r.activity.mDecor;if (v != null) {if (show) {if (!r.activity.mVisibleFromServer) {r.activity.mVisibleFromServer = true;mNumVisibleActivities++;if (r.activity.mVisibleFromClient) {r.activity.makeVisible();}}......} else {if (r.activity.mVisibleFromServer) {r.activity.mVisibleFromServer = false;mNumVisibleActivities--;v.setVisibility(View.INVISIBLE);}}}}

mVisibleFromServer默认是false，在onResume后赋值为true，此时mVisibleFromServer为true，进入条件体，首先将mVisibleFromServer设置为false，然后通过DecorView调用setVisibility方法来控制视图显示，并默认传输View.INVISIBLE。我们知道调用setVisibility就可能触发onVisibilityChanged的执行。

总结：

页面加载时，会在View attach到Window时（dispatchAttachedToWindow方法）调用onVisibilityChanged
通过setVisibility来改变View的可见性时会调用onVisibilityChanged
关闭页面时，在handleStopActivity方法中会调用updateVisibility方法，内部也会调用setVisibility

，并传入默认参数INVISIBLE。

三、OnAttachStateChangeListener#onViewAttachedToWindow

OnAttachStateChangeListener定义了2个接口方法，分别在View Attach/Detach to Window时候调用。要使用该接口首先需要注册这个监听器。

public void addOnAttachStateChangeListener(OnAttachStateChangeListener listener) {ListenerInfo li = getListenerInfo();if (li.mOnAttachStateChangeListeners == null) {li.mOnAttachStateChangeListeners= new CopyOnWriteArrayList<OnAttachStateChangeListener>();}li.mOnAttachStateChangeListeners.add(listener);}

调用位置很单纯，就在View#dispatchAttachedToWindow方法里，如果有注册过该监听器，就会调用

void dispatchAttachedToWindow(AttachInfo info, int visibility) {......onAttachedToWindow();ListenerInfo li = mListenerInfo;final CopyOnWriteArrayList<OnAttachStateChangeListener> listeners =li != null ? li.mOnAttachStateChangeListeners : null;if (listeners != null && listeners.size() > 0) {// NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to// perform the dispatching. The iterator is a safe guard against listeners that// could mutate the list by calling the various add/remove methods. This prevents// the array from being modified while we iterate it.for (OnAttachStateChangeListener listener : listeners) {listener.onViewAttachedToWindow(this);}}......}

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Android View可见性判断方法

一、onWindowVisibilityChanged

二、onVisibilityChanged

三、OnAttachStateChangeListener#onViewAttachedToWindow

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