判断当前Fragment是否切换到前台

我们都知道，Activity有onResume()方法，可以很方便程序员在该Activity（重新）展示在用户面前的时候，执行一些数据更新的操作，例如需要重新加载网络数据。那么Fragment呢，查看Fragment源码，可以知道，类似的方法是：

// Hint provided by the app that this fragment is currently visible to the user.
    boolean mUserVisibleHint = true;


/**
     * 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.
     *
     * @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.performPendingDeferredStart(this);
        }
        mUserVisibleHint = isVisibleToUser;
        mDeferStart = !isVisibleToUser;
    }

变量mUserVisibleHint表示当前Fragment是否对用户可见，true为可见，false为不可见。setUserVisibleHint()方法在每次切换Fragment的时候都会被调用，那么，我们只要重写该方法，并添加相应的更新代码即可（注意不要在主线程更新UI）。