viewpager+fragment获取fragment是否可见的状态

前言：近日根据项目需求，需要获取到viewpager+fragment中某个fragment的可见状态。但是fragment的OnResume()和OnPause()方法在切换tab时并不回调，如何才能判断该fragment是否可见呢？

解决办法：

重写setUserVisibleHint方法。

    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        if(isVisibleToUser) {
            //相当于OnResume(),可以做相关逻辑
        }else {
            //相当于OnPause()
        }
    }

Fragment的生命周期

viewpager+fragment中调用setUserVisibleHint的fragment的生命周期：

1   E/myfragment: setUserVisibleHint=false
2   E/myfragment: onAttach
3   E/myfragment: onCreate
4   E/myfragment: onCreateView
5   E/myfragment: onActivityCreated
6   E/myfragment: onStart
7   E/myfragment: onresume
8   E/myfragment: setUserVisibleHint=true   //该fragment可见时
9   E/myfragment: setUserVisibleHint=false  //该fragment不可见时
10  E/myfragment: onPause                   
11  E/myfragment: onStop
12  E/myfragment: onDestroyView
13  E/myfragment: onDestroy
14  E/myfragment: onDetach

说明：1-7生命周期是fragment初始化并加载到viewpager中时执行的。10-14是viewpager所在的activity被销毁过程中，fragment的生命周期执行过程。8是切换到该fragment可见时，9是该fragment不可见时。
使用FragmentPagerAdapter+ViewPager时，切换回上一个Fragment页面时（已经初始化完毕），不会回调任何生命周期方法以及onHiddenChanged()，只有setUserVisibleHint(boolean isVisibleToUser)会被回调。

分析setUserVisibleHint方法

/**
* 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.
*
* 
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.
*
* 
Note: This method may be called outside of the fragment lifecycle.
* and thus has no ordering guarantees with regard to fragment lifecycle method calls.
*
* @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;
}

并参考以下资料：
Fragment 中setUserVisibleHint方法不执行
stackoverflow
我们可以知道，在单个fragment中是不会被系统调用setUserVisibleHint方法的。

另附：onHiddenChanged的回调时机

当使用add()+show()，hide()跳转新的Fragment时，旧的Fragment回调onHiddenChanged()，不会回调onStop()等生命周期方法，而新的Fragment在创建时是不会回调onHiddenChanged()。

参考资料：Fragment全解析系列（二）：正确的使用姿势。