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Android -- onAttachedToWindow()

onAttachedToWindow在Activity生命周期中的位置

放大招：

onAttachedToWindow不能在onCreate中哟~

View view = getWindow().getDecorView();   WindowManager.LayoutParams lp = (WindowManager.LayoutParams)view.getLayoutParams();   lp.gravity = Gravity.CENTER;   lp.width = (dm.widthPixels * 4) / 5;   lp.height = (dm.widthPixels * 4) / 5;   getWindowManager().updateViewLayout(view,lp);

放到onCreate中进行测试，结果在lp.gravity = Gravity.CENTER;这行报了空指针异常，所以view.getLayoutParams()获取的LayoutParams是空。

public final class ActivityThread {   ......   final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward) {    ......    ActivityClientRecord r = performResumeActivity(token, clearHide);    if (r != null) {     final Activity a = r.activity;     ......     // If the window hasn't yet been added to the window manager,     // and this guy didn't finish itself or start another activity,     // then go ahead and add the window.     boolean willBeVisible = !a.mStartedActivity;     if (!willBeVisible) {      try {       willBeVisible = ActivityManagerNative.getDefault().willActivityBeVisible(         a.getActivityToken());      } catch (RemoteException e) {      }     }     if (r.window == null && !a.mFinished && willBeVisible) {      r.window = r.activity.getWindow();      View decor = r.window.getDecorView();      decor.setVisibility(View.INVISIBLE);      ViewManager wm = a.getWindowManager();      WindowManager.LayoutParams l = r.window.getAttributes();      a.mDecor = decor;      l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;      ......      if (a.mVisibleFromClient) {       a.mWindowAdded = true;       wm.addView(decor, l);      }     }      ......    }    ......   }   ......  }

原来在ActivityThread 执行handleResumeActivity时就会为PhoneWindow（r.activity.getWindow）中的DecorView设置LayoutParam ，并且通过源码发现 handleResumeActivity函数首先会执行performResumeActivity，此时会调用Activity的onResume()生命周期函数，这时问题就比较清晰了，看来只要在Activity的onResume生命周期后就能获取DecorView的LayoutParam，进而可以设置高度和宽度了。根据上面贴出的生命周期图，onResume()后面是onAttachedToWindow()，并且onAttachedToWindow只会调用一次，不会受用户操作行为影响。所以在onAttachedToWindow中进行窗口尺寸的修改再合适不过了。