/**
* Set a hint to the system about whether this fragment's UI is currently visible
* to the user. This hint defaults to true and is persistent across fragment instance
* state save and restore.
*
* <p>An app may set this to false to indicate that the fragment's UI is
* scrolled out of visibility or is otherwise not directly visible to the user.
* This may be used by the system to prioritize operations such as fragment lifecycle updates
* or loader ordering behavior.</p>
*
* @param isVisibleToUser true if this fragment's UI is currently visible to the user (default),
* false if it is not.
*/
public void setUserVisibleHint(boolean isVisibleToUser) {
if (!mUserVisibleHint && isVisibleToUser && mState < STARTED
&& mFragmentManager != null && isAdded()) {
mFragmentManager.performPendingDeferredStart(this);
}
mUserVisibleHint = isVisibleToUser;
mDeferStart = mState < STARTED && !isVisibleToUser;
}
其中有一句话是这么说的：This may be used by the system to prioritize operations such as fragment lifecycle updates
这个可能被用来在一组有序的fragmen里 ，例如 fragment生命周期的更新。告诉我们这个方法被调用希望在一个pager里，因此 FragmentPagerAdapter 可以使用这个，然后调用这个 setUserVisibleHint() 在fragment里，因此有些人也建议重写 setUserVisibileHint() 来知道当前一个fragment对用户来说是隐藏还是显示，这个方法仅仅工作在FragmentPagerAdapter中，不能被使用在一个普通的activity中。
因此我这个单个fragment 就是不被调用的 ，可以重写 onHiddenChanged() 方法来得知当前 fragment 的状态。
