关于onhiddenchanged和setUserVisibleHint函数的知识

/**
* Called when the hidden state (as returned by {@link #isHidden()} of
* the fragment has changed.  Fragments start out not hidden; this will
* be called whenever the fragment changes state from that.
* @param hidden True if the fragment is now hidden, false if it is not
* visible.
*/
onhiddenchanged(boolean hidden)

v4包下的Fragment里面的文档，大概意思就是：当你用FragmentTransaction来控制fragment的hide和show时，那么这个方法就会被调用。每当你对某个Fragment使用hide或者是show的时候，那么这个Fragment就会自动调用这个方法。

（使用情况：你自己去管理Fragment，而不是用viewpager管理的时候）

/**
* 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.
*/
setUserVisibleHint(boolean isVisibleToUser)

比如说当我们在使用viewpager的时候，viewpager内部有个提前缓存的机制(默认是提前缓存一页)，比如你在看第一个Fragment的时候，隔壁的Fragment已经创建好了，但此时的状态却是不可见的。

但是这时候Fragment不会去调用上面说的onhiddenchanged方法，只会调用setUserVisibleHint这个方法。