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。

干货

• onAttachedToWindow运行在onResume之后；
• DecorView的LayoutParams是在ActivityThread的handleResumeActivity中设置的，并且该函数会调用Activity的onResume生命周期，所以在onResume之后可以设置窗体尺寸；

我是天王盖地虎的分割线

参考：http://blog.csdn.net/guxiao1201/article/details/41517871