当 Activity 以全屏模式运行时，如何允许 Android 系统状态栏在顶层出现，而不迫使 Activity 重新布局让出空间？

参考资料：

http://www.zhihu.com/question/19760889

private void hideStatusBar() {
WindowManager.LayoutParams attrs = getWindow().getAttributes();
attrs.flags |= WindowManager.LayoutParams.FLAG_FULLSCREEN;
getWindow().setAttributes(attrs);
}

private void showStatusBar() {
WindowManager.LayoutParams attrs = getWindow().getAttributes();
attrs.flags &= ~WindowManager.LayoutParams.FLAG_FULLSCREEN;
getWindow().setAttributes(attrs);
}
关键是，在做了该Activity的全屏设置的前提下，还要在onCreate()方法中加入如下语句：
getWindow().addFlags(WindowManager.LayoutParams.FLAG_LAYOUT_IN_SCREEN);
getWindow().addFlags(WindowManager.LayoutParams.FLAG_LAYOUT_NO_LIMITS);