探究onAttachedToWindow和onDetachedFromWindow
探究onAttachedToWindow和onDetachedFromWindow
ProgressBar，是很多开发者一开始接触安卓开发就使用过的控件，用来显示加载进度。查看ProgressBar源码，在onAttachedToWindow()方法中开启进度条加载动画：
@Override
protected void onAttachedToWindow() {
super.onAttachedToWindow();
if (mIndeterminate) {
startAnimation();
}
if (mRefreshData != null) {
synchronized (this) {
final int count = mRefreshData.size();
for (int i = 0; i < count; i++) {
final RefreshData rd = mRefreshData.get(i);
doRefreshProgress(rd.id, rd.progress, rd.fromUser, true, rd.animate);
rd.recycle();
}
mRefreshData.clear();
}
}
mAttached = true;
}在与之对应的onDetachedFromWindow()方法中停止动画：
@Override
protected void onDetachedFromWindow() {
if (mIndeterminate) {
stopAnimation();
}
if (mRefreshProgressRunnable != null) {
removeCallbacks(mRefreshProgressRunnable);
mRefreshIsPosted = false;
}
// This should come after stopAnimation(), otherwise an invalidate message remains in the
// queue, which can prevent the entire view hierarchy from being GC'ed during a rotation
super.onDetachedFromWindow();
mAttached = false;
}onAttachedToWindow()和onDetachedFromWindow()类似Activity的onCreate()和onDestroy()。作为成对匹配的方法：可以在attach的时候做一些初始化、启动操作；在detach的时候释放资源、暂停操作。
非常有必要了解onAttachedToWindow()和onDetachedFromWindow()这两个方法的来龙去脉。
1、View的onAttachedToWindow、onDetachedFromWindow
在View类中就定义了onAttachedToWindow()和onDetachedFromWindow()这两个方法，它们不是继承来的。子类重写的时候注意需要调用super父类方法。
/**
* This is called when the view is attached to a window. At this point it
* has a Surface and will start drawing. Note that this function is
* guaranteed to be called before {@link #onDraw(android.graphics.Canvas)},
* however it may be called any time before the first onDraw -- including
* before or after {@link #onMeasure(int, int)}.
*
* @see #onDetachedFromWindow()
*/
@CallSuper
protected void onAttachedToWindow() {
if ((mPrivateFlags & PFLAG_REQUEST_TRANSPARENT_REGIONS) != 0) {
mParent.requestTransparentRegion(this);
}
mPrivateFlags3 &= ~PFLAG3_IS_LAID_OUT;
jumpDrawablesToCurrentState();
AccessibilityNodeIdManager.getInstance().registerViewWithId(this, getAccessibilityViewId());
resetSubtreeAccessibilityStateChanged();
// rebuild, since Outline not maintained while View is detached
rebuildOutline();
if (isFocused()) {
notifyFocusChangeToImeFocusController(true /* hasFocus */);
}
}
/**
* This is called when the view is detached from a window. At this point it
* no longer has a surface for drawing.
*
* @see #onAttachedToWindow()
*/
@CallSuper
protected void onDetachedFromWindow() {
}1.1、onAttachedToWindow流程
找到源码的切入点，从Activity的resume开始：当View被添加到ViewRootImpl的时候，会通过dispatchAttachedToWindow(AttachInfo info, int visibility)方法逐级分发View被添加到Window的事件。
源码就不贴上来了，用PlantUML“手写”View的onAttachedToWindow()回调时序图如下：
1.2、onDetachedFromWindow流程
对应的，是在Activity销毁destroy时会将View从ViewRootImpl移除，通过dispatchDetachedFromWindow()完成事件分发给每个View。
源码较多太占篇幅，对应的时序图简化如下：
2、Activity的onAttachedToWindow、onDetachedFromWindow
在Activity中也有一对onAttachedToWindow()和onDetachedFromWindow()方法，是Activity实现Window.Callback接口而定义的空方法：
/**
* Called when the main window associated with the activity has been
* attached to the window manager.
* See {@link View#onAttachedToWindow() View.onAttachedToWindow()}
* for more information.
* @see View#onAttachedToWindow
*/
public void onAttachedToWindow() {
}
/**
* Called when the main window associated with the activity has been
* detached from the window manager.
* See {@link View#onDetachedFromWindow() View.onDetachedFromWindow()}
* for more information.
* @see View#onDetachedFromWindow
*/
public void onDetachedFromWindow() {
}Activity的onAttachedToWindow()比View的onAttachedToWindow()早；Activity的onDetachedFromWindow()比View的onDetachedFromWindow()晚；通过Log可以验证：
2.1、onAttachedToWindow流程
Activity的onAttachedToWindow()从哪里回调的呢？在DecorView的onAttachedToWindow()中回调：
@Override
protected void onAttachedToWindow() {
//先回调View的onAttachedToWindow()
super.onAttachedToWindow();
//Window中的Callback正是Activity,Activity实现了Window.Callback接口
final Window.Callback cb = mWindow.getCallback();
//再这里回调Activity的onAttachedToWindow()
if (cb != null && !mWindow.isDestroyed() && mFeatureId < 0) {
cb.onAttachedToWindow();
}
...
}在DecorView添加到Window的流程一文中提到相关成员的初始化：
@Override
public void handleResumeActivity(IBinder token, boolean finalStateRequest, ...) {
...
//获取Activity中的Window
r.window = r.activity.getWindow();
//获取DecorView
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
//获取WindowManager
ViewManager wm = a.getWindowManager();
...
//将DecorView添加到Window中
wm.addView(decor, l);
....
}其中Activity中的Window在attach()创建的时候就初始化为PhoneWindow，并且给PhoneWindow设置的Window.Callback就是当前Activity：
@UnsupportedAppUsage
final void attach(Context context, ActivityThread aThread...) {
attachBaseContext(context);
mFragments.attachHost(null /*parent*/);
//创建一个PhoneWindow
mWindow = new PhoneWindow(this, window, activityConfigCallback);
mWindow.setWindowControllerCallback(mWindowControllerCallback);
//给Window设置Callback，就是当前Activity
mWindow.setCallback(this);
...
}2.2、onDetachedFromWindow流程
同样的，Activity的onDetachedFromWindow()方法也是借助DecorView回调：
@Override
protected void onDetachedFromWindow() {
//ViewGroup分发onDetachedFromWindow()
super.onDetachedFromWindow();
//拿到当前Window的Activity
final Window.Callback cb = mWindow.getCallback();
//回调Activity的onDetachedFromWindow()方法
if (cb != null && mFeatureId < 0) {
cb.onDetachedFromWindow();
}
...
} 再看一下Log，各周期执行情况和前面所看源码顺序一致：
