前言

根據ViewPager+Frgment的小例子，來研究Fragment生命週期方法與Fragment檢視是否可見間的細節。

正文

所用資源：ViewPager+Fragment，ViewPager只預設預載入１頁，共三個Fragment。

動作１：執行App，介面顯示

//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，會呼叫前兩頁的setUserVisibleHint()方法並設定為false，表明兩個Fragmnet所代表的檢視，還尚未顯示。

接著執行Fragment1的生命週期流程，並在onCreate()與onCreateView()之間執行setUserVisbleHint()為true，來表示Fragment1已使用者可見。

隨後執行Fragment2的生命週期流程，直至onResume()被呼叫。但是此時Fragment2還處於使用者不可見狀態。

動作２：滑動到第二個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，執行３個Fragment的setUserVisbleHint()方法，去表明Fragment是否為可見狀態。此時，Fragment2處於可見狀態，Fragment1和預載入的Fragment3處於不可見狀態。

緊接著再去執行Fragment3的生命週期方法。

動作３：滑動到第三個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，先執行當前Fragment和快取Fragmnet2的setUserVisibleHint()方法，表明各自的可見狀態，此時，Fragment3處於可見狀態。

緊接著去釋放Fragment1的檢視資源，但並不銷燬Fragment1

結論

Fragment和Activity的大部分生命週期繫結在一起，但是Fragment的onResume()並不意味著Fragment對於使用者可見，真正Fragment對使用者可見會執行的方法是setUserVisbleHInt()並且為true時。

在Fragment的宣告週期中，setUserVisbleHint()方法會被呼叫兩次，第一次是在Fragment宣告週期尚未開始時，用於初始化userVisbleHint變數；第二次實在onCreate()與onCreateView()之間呼叫，表明Fragment已經處於可見狀態。