判断ViewPager2的哪个fragment可见visible

1.创建fragment时,将fragment作为成员变量持有

2.监听tablayout或viewPager2的pageChange方法知道哪个fragment被selected

3.调用getSupportFragmentManager().beginTransaction().setMaxLifecycle(fragment, Lifecycle.State.RESUMED),fragment的onResume方法就会被回调

4.ViewPager2注意事项