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 fragments 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 fragments 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 fragments 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。