Fragment(1)－生命周期方法与Fragment视图是否可见间的细节

前言

根据ViewPager+Frgment的小例子，来研究Fragment生命周期方法与Fragment视图是否可见间的细节。

正文

所用资源：ViewPager+Fragment，ViewPager只默认预加载１页，共三个Fragment。

动作１：执行App，界面显示

//PlaceholderFragment: 1 setUserVisibleHint() called with: isVisibleToUser = [false]//PlaceholderFragment: 2 setUserVisibleHint() called with: isVisibleToUser = [false]//PlaceholderFragment: 1 onAttach() called with: //PlaceholderFragment: 1 onCreate() called with: //PlaceholderFragment: 1 setUserVisibleHint() called with: isVisibleToUser = [true]//PlaceholderFragment: 1 onCreateView() called with://PlaceholderFragment: 1 onViewCreated() called with://PlaceholderFragment: 1 onActivityCreated() called with://PlaceholderFragment: 1 onStart() called with://PlaceholderFragment: 1 onResume() called with://PlaceholderFragment: 2 onAttach() called with: //PlaceholderFragment: 2 onCreate() called with: //PlaceholderFragment: 2 onCreateView() called with://PlaceholderFragment: 2 onViewCreated() called with://PlaceholderFragment: 2 onActivityCreated() called with://PlaceholderFragment: 2 onStart() called with://PlaceholderFragment: 2 onResume() called with:

首先，会调用前两页的setUserVisibleHint()方法并设置为false，表明两个Fragmnet所代表的视图，还尚未显示。

接着执行Fragment1的生命周期流程，并在onCreate()与onCreateView()之间执行setUserVisbleHint()为true，来表示Fragment1已用户可见。

随后执行Fragment2的生命周期流程，直至onResume()被调用。但是此时Fragment2还处于用户不可见状态。

动作２：滑动到第二个Fragment

//PlaceholderFragment: 3 setUserVisibleHint() called with: isVisibleToUser = [false]//PlaceholderFragment: 1 setUserVisibleHint() called with: isVisibleToUser = [false]//PlaceholderFragment: 2 setUserVisibleHint() called with: isVisibleToUser = [true]//PlaceholderFragment: 3 onAttach() called with://PlaceholderFragment: 3 onCreate() called with://PlaceholderFragment: 3 onCreateView() called with://PlaceholderFragment: 3 onViewCreated() called with://PlaceholderFragment: 3 onActivityCreated() called with://PlaceholderFragment: 3 onStart() called with://PlaceholderFragment: 3 onResume() called with:

首先，执行３个Fragment的setUserVisbleHint()方法，去表明Fragment是否为可见状态。此时，Fragment2处于可见状态，Fragment1和预加载的Fragment3处于不可见状态。

紧接着再去执行Fragment3的生命周期方法。

动作３：滑动到第三个Fragment

//PlaceholderFragment: 2 setUserVisibleHint() called with: isVisibleToUser = [false]//PlaceholderFragment: 3 setUserVisibleHint() called with: isVisibleToUser = [true]//PlaceholderFragment: 1 onPause() called with://PlaceholderFragment: 1 onStop() called with://PlaceholderFragment: 1 onDestroyView() called with:

首先，先执行当前Fragment和缓存Fragmnet2的setUserVisibleHint()方法，表明各自的可见状态，此时，Fragment3处于可见状态。

紧接着去释放Fragment1的视图资源，但并不销毁Fragment1

结论

Fragment和Activity的大部分生命周期绑定在一起，但是Fragment的onResume()并不意味着Fragment对于用户可见，真正Fragment对用户可见会执行的方法是setUserVisbleHInt()并且为true时。

在Fragment的声明周期中，setUserVisbleHint()方法会被调用两次，第一次是在Fragment声明周期尚未开始时，用于初始化userVisbleHint变量；第二次实在onCreate()与onCreateView()之间调用，表明Fragment已经处于可见状态。