onAttachedToWindow和onDetachedFromWindow的调用时机分析

缘起

笔者为什么会挑这个话题，是因为长时间以来我自己对这2个方法一直有些疑惑，比如：

为啥叫onAttachedToWindow而不是onAttachedToActivity，Window又是什么，在哪里？毕竟我们平时绝大多数时候接触到的是Activity啊；
Activity有明确的生命周期方法，但View却没有，那么这2个方法可以认为是View的吗？它们又何时会被调用呢？

慢慢地随着在这一行逐渐深入，阅读了些系统源码，开始对这些问题有了自己的答案或者说更加深刻的认识。这篇文章尝试将笔者的这些理解、认识说清楚，希望能帮助更多人加深认识。

onAttachedToWindow的调用过程

我们在前面Activity启动过程的文章中说过，在ActivityThread.handleResumeActivity的过程中，会将Act的DecorView添加到WindowManager中，可能很多人一开始会觉得WindowManager是一个具体的类，但是实际上它却只是个继承了ViewManager的接口，具体代码如下：

/** Interface to let you add and remove child views to an Activity. To get an instance* of this class, call {@link android.content.Context#getSystemService(java.lang.String) Context.getSystemService()}.*/
public interface ViewManager
{/*** Assign the passed LayoutParams to the passed View and add the view to the window.* <p>Throws {@link android.view.WindowManager.BadTokenException} for certain programming* errors, such as adding a second view to a window without removing the first view.* <p>Throws {@link android.view.WindowManager.InvalidDisplayException} if the window is on a* secondary {@link Display} and the specified display can't be found* (see {@link android.app.Presentation}).* @param view The view to be added to this window.* @param params The LayoutParams to assign to view.*/public void addView(View view, ViewGroup.LayoutParams params);public void updateViewLayout(View view, ViewGroup.LayoutParams params);public void removeView(View view);
}

而WindowManager的样子差不多是这样，如下图：

当在ActivityThread.handleResumeActivity()方法中调用WindowManager.addView()方法时，最终是调去了

WindowManagerImpl.addView() -->
WindowManagerGlobal.addView()

这里我们看下最终调用到的代码：

public void addView(View view, ViewGroup.LayoutParams params,Display display, Window parentWindow) {if (view == null) {throw new IllegalArgumentException("view must not be null");}if (display == null) {throw new IllegalArgumentException("display must not be null");}if (!(params instanceof WindowManager.LayoutParams)) {throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");}final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;if (parentWindow != null) {parentWindow.adjustLayoutParamsForSubWindow(wparams);} else {// If there's no parent, then hardware acceleration for this view is// set from the application's hardware acceleration setting.final Context context = view.getContext();if (context != null&& (context.getApplicationInfo().flags& ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;}}ViewRootImpl root;View panelParentView = null;synchronized (mLock) {// Start watching for system property changes.if (mSystemPropertyUpdater == null) {mSystemPropertyUpdater = new Runnable() {@Override public void run() {synchronized (mLock) {for (int i = mRoots.size() - 1; i >= 0; --i) {mRoots.get(i).loadSystemProperties();}}}};SystemProperties.addChangeCallback(mSystemPropertyUpdater);}int index = findViewLocked(view, false);if (index >= 0) {if (mDyingViews.contains(view)) {// Don't wait for MSG_DIE to make it's way through root's queue.mRoots.get(index).doDie();} else {throw new IllegalStateException("View " + view+ " has already been added to the window manager.");}// The previous removeView() had not completed executing. Now it has.}// If this is a panel window, then find the window it is being// attached to for future reference.if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {final int count = mViews.size();for (int i = 0; i < count; i++) {if (mRoots.get(i).mWindow.asBinder() == wparams.token) {panelParentView = mViews.get(i);}}}root = new ViewRootImpl(view.getContext(), display);view.setLayoutParams(wparams);mViews.add(view);mRoots.add(root);mParams.add(wparams);}// do this last because it fires off messages to start doing thingstry {// 这行代码是本文重点关注的！！！root.setView(view, wparams, panelParentView);} catch (RuntimeException e) {// BadTokenException or InvalidDisplayException, clean up.synchronized (mLock) {final int index = findViewLocked(view, false);if (index >= 0) {removeViewLocked(index, true);}}throw e;}}

其中有一句root.setView(view, wparams, panelParentView);，正是这行代码将调用流程转移到了ViewRootImpl.setView()里面，此方法内部最终会触发ViewRootImpl.performTraversals()方法，这个方法就是我们熟悉的View从无到有要经历的3个阶段(measure, layout, draw)，不过这个方法内部和我们这里讨论的内容相关的是其1364行代码：host.dispatchAttachedToWindow(mAttachInfo, 0);，这里的host就是Act的DecorView(FrameLayout的子类)，我们可以看到是通过这样的dispatch方法将这个调用沿着View tree分发了下去，我们分别看下ViewGroup和View中这个方法的实现，如下：

// ViewGroup中的实现：
void dispatchAttachedToWindow(AttachInfo info, int visibility) {mGroupFlags |= FLAG_PREVENT_DISPATCH_ATTACHED_TO_WINDOW;// 先调用自己的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];// 递归调用每个child的dispatchAttachedToWindow方法// 典型的深度优先遍历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()));}}// View中的实现：
void dispatchAttachedToWindow(AttachInfo info, int visibility) {//System.out.println("Attached! " + this);mAttachInfo = info;if (mOverlay != null) {mOverlay.getOverlayView().dispatchAttachedToWindow(info, visibility);}mWindowAttachCount++;// We will need to evaluate the drawable state at least once.mPrivateFlags |= PFLAG_DRAWABLE_STATE_DIRTY;if (mFloatingTreeObserver != null) {info.mTreeObserver.merge(mFloatingTreeObserver);mFloatingTreeObserver = null;}if ((mPrivateFlags&PFLAG_SCROLL_CONTAINER) != 0) {mAttachInfo.mScrollContainers.add(this);mPrivateFlags |= PFLAG_SCROLL_CONTAINER_ADDED;}performCollectViewAttributes(mAttachInfo, 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);}}int vis = info.mWindowVisibility;if (vis != GONE) {onWindowVisibilityChanged(vis);}// 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);if ((mPrivateFlags&PFLAG_DRAWABLE_STATE_DIRTY) != 0) {// If nobody has evaluated the drawable state yet, then do it now.refreshDrawableState();}needGlobalAttributesUpdate(false);}

从源码我们可以清晰地看到ViewGroup先是调用自己的onAttachedToWindow()方法，再调用其每个child的onAttachedToWindow()方法，这样此方法就在整个view树中遍布开了，注意到visibility并不会对这个方法产生影响。

onDetachedFromWindow的调用过程

和attched对应的，detached的发生是从act的销毁开始的，具体的代码调用流程如下：

ActivityThread.handleDestroyActivity() -->
WindowManager.removeViewImmediate() -->
WindowManagerGlobal.removeViewLocked()方法 —>
ViewRootImpl.die() --> doDie() -->
ViewRootImpl.dispatchDetachedFromWindow()

最终会调用到View层次结构的dispatchDetachedFromWindow方法去，对应的代码如下：

// ViewGroup的：
@Overridevoid dispatchDetachedFromWindow() {// If we still have a touch target, we are still in the process of// dispatching motion events to a child; we need to get rid of that// child to avoid dispatching events to it after the window is torn// down. To make sure we keep the child in a consistent state, we// first send it an ACTION_CANCEL motion event.cancelAndClearTouchTargets(null);// Similarly, set ACTION_EXIT to all hover targets and clear them.exitHoverTargets();// In case view is detached while transition is runningmLayoutCalledWhileSuppressed = false;// Tear down our drag trackingmDragNotifiedChildren = null;if (mCurrentDrag != null) {mCurrentDrag.recycle();mCurrentDrag = null;}final int count = mChildrenCount;final View[] children = mChildren;for (int i = 0; i < count; i++) {// 先调用child的方法children[i].dispatchDetachedFromWindow();}clearDisappearingChildren();final int transientCount = mTransientViews == null ? 0 : mTransientIndices.size();for (int i = 0; i < transientCount; ++i) {View view = mTransientViews.get(i);view.dispatchDetachedFromWindow();}// 最后才是自己的super.dispatchDetachedFromWindow();}// View的：
void dispatchDetachedFromWindow() {AttachInfo info = mAttachInfo;if (info != null) {int vis = info.mWindowVisibility;if (vis != GONE) {onWindowVisibilityChanged(GONE);}}// 调用回调onDetachedFromWindow();onDetachedFromWindowInternal();InputMethodManager imm = InputMethodManager.peekInstance();if (imm != null) {imm.onViewDetachedFromWindow(this);}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);}}if ((mPrivateFlags & PFLAG_SCROLL_CONTAINER_ADDED) != 0) {mAttachInfo.mScrollContainers.remove(this);mPrivateFlags &= ~PFLAG_SCROLL_CONTAINER_ADDED;}mAttachInfo = null;if (mOverlay != null) {mOverlay.getOverlayView().dispatchDetachedFromWindow();}}

至此，onDetachedFromWindow()就在整个view树上传播开了。

总结

从上面的分析中我们可以得出下面的结论：

onAttachedToWindow方法是在Act resume的时候被调用的，也就是act对应的window被添加的时候，且每个view只会被调用一次，父view的调用在前，不论view的visibility状态都会被调用，适合做些view特定的初始化操作；

onDetachedFromWindow方法是在Act destroy的时候被调用的，也就是act对应的window被删除的时候，且每个view只会被调用一次，父view的调用在后，也不论view的visibility状态都会被调用，适合做最后的清理操作；
这些结论也正好解释了方法名里带有window的原因，有些人可能会想，那为啥不叫onAttachedToActivity/onDetachedFromActivity，因为在Android里不止是Activity，这里说的内容同样适用于Dialog/Toast，Window只是个虚的概念，是Android抽象出来的，最终操作的实体还是View，这也说明了前面的WindowManager接口为啥是从ViewManager接口派生的，因为所有一切的基石归根结底还是对View的操作。

转载自：https://www.jianshu.com/p/e7b6fa788ae6