Low Memory狀態下app狀態的恢復

app結構
     activity帶有ViewPager
     ViewPager上帶有4個fragment

這是非常典型的結構

操作過程
     點icon打開app
     點擊home鍵隱藏app
     再次點擊icon喚回app
     點擊home鍵隱藏app
     開啓多個其他app，要很耗費內存那種
     點擊home鍵隱藏這些app
     再次點擊icon喚回app
     app雖然喚回，但是上面的fragment已經不聽招呼，空白，無反應

仔細debug，發現按鍵能夠操作fragment，並且fragment能否反饋，但屏幕上還是顯示一個空白fragment以及一個初始狀態的變量值
所謂初始狀態的變量值，是指靜態代碼設置的值，比如如下的字符串：

     public class FragmentListview extends Fragment {
         String currentPath = "static_ini_code_null";

在靜態代碼之後的初始化過程中這個值應該被改變，但是卻沒有，可見這是一個僅僅靜態代碼初始化的fragment

困擾了很久。在activity和fragment的所有生命週期函數處添加了打印，終於發現端倪

以下是第一次喚回app的流程，打印了activity，以及0和1兩個fragment

lifecycle: DirPlayerActivity.onRestart()
lifecycle: FragmentListview.onStart() 0:/mnt/sdcard FragmentListview{41c2c1a0 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onStart() 1:/mnt/sdcard FragmentListview{41c3f848 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: DirPlayerActivity.onStart()
lifecycle: MyArrayAdapter.MyArrayAdapter()
lifecycle: MyArrayAdapter.MyArrayAdapter()
lifecycle: FragmentListview.setListviewAdapter() 0:/mnt/sdcard FragmentListview{41c2c1a0 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.setListviewAdapter() 0:/mnt/sdcard FragmentListview{41c2c1a0 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: DirPlayerActivity.updateFileInfor()! FragmentListview{41c2c1a0 #0 id=0x7f0a0001 android:switcher:2131361793:0} [Lcom.maxproj.android.dirplayer.MyArrayAdapter;@41c2c018
lifecycle: MyArrayAdapter.MyArrayAdapter()
lifecycle: FragmentListview.setListviewAdapter() 1:/mnt/sdcard FragmentListview{41c3f848 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: FragmentListview.setListviewAdapter() 1:/mnt/sdcard FragmentListview{41c3f848 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: DirPlayerActivity.updateFileInfor()! FragmentListview{41c3f848 #1 id=0x7f0a0001 android:switcher:2131361793:1} [Lcom.maxproj.android.dirplayer.MyArrayAdapter;@41c2c018
lifecycle: MyArrayAdapter.MyArrayAdapter()
lifecycle: DirPlayerActivity.onResume()
lifecycle: FragmentListview.onResume() 0:/mnt/sdcard FragmentListview{41c2c1a0 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onResume() 1:/mnt/sdcard FragmentListview{41c3f848 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: DirPlayerActivity.onResumeFragments()
lifecycle: FragmentListview.onPause() 0:/mnt/sdcard FragmentListview{41c2c1a0 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onPause() 1:/mnt/sdcard FragmentListview{41c3f848 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: DirPlayerActivity.onPause()
lifecycle: FragmentListview.onStop() 0:/mnt/sdcard FragmentListview{41c2c1a0 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onStop() 1:/mnt/sdcard FragmentListview{41c3f848 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: DirPlayerActivity.onStop()
lifecycle: FragmentListview.onDestroy() 0:/mnt/sdcard FragmentListview{41c2c1a0 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onDetach() 0:/mnt/sdcard FragmentListview{41c2c1a0 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onDestroy() 1:/mnt/sdcard FragmentListview{41c3f848 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: FragmentListview.onDetach() 1:/mnt/sdcard FragmentListview{41c3f848 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: DirPlayerActivity.onDestroy()

可見喚回時是從onRestart()開始，且沒有重新生成fragment的操作
這是第二次喚回app的流程，仍然打印了activity，以及0和1兩個fragment

lifecycle: FragmentListview.newInstance() 0 FragmentListview{41c40198}
lifecycle: DirPlayerActivity.StaticCodeIniFragment{} 0 FragmentListview{41c40198}
lifecycle: FragmentListview.newInstance() 1 FragmentListview{41c4e3e8}
lifecycle: DirPlayerActivity.StaticCodeIniFragment{} 1 FragmentListview{41c4e3e8}
lifecycle: FragmentListview.onAttach() 0:static_ini_code_null FragmentListview{41c5ee80 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onCreate() 0:static_ini_code_null FragmentListview{41c5ee80 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onAttach() 0:static_ini_code_null FragmentListview{41c5f0c8 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: FragmentListview.onCreate() 0:static_ini_code_null FragmentListview{41c5f0c8 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: DirPlayerActivity.onCreate()
lifecycle: FragmentListview.onCreateView() 0:static_ini_code_null FragmentListview{41c5ee80 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onActivityCreated() 0:static_ini_code_null FragmentListview{41c5ee80 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onCreateView() 0:static_ini_code_null FragmentListview{41c5f0c8 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: FragmentListview.onActivityCreated() 0:static_ini_code_null FragmentListview{41c5f0c8 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: FragmentListview.onStart() 0:static_ini_code_null FragmentListview{41c5ee80 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onStart() 0:static_ini_code_null FragmentListview{41c5f0c8 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: DirPlayerActivity.onStart()
lifecycle: MyArrayAdapter.MyArrayAdapter()
lifecycle: MyArrayAdapter.MyArrayAdapter()
lifecycle: FragmentListview.setListviewAdapter() 0:static_ini_code_null FragmentListview{41c40198}
lifecycle: FragmentListview.setListviewAdapter() 0:/mnt/sdcard FragmentListview{41c40198}
lifecycle: DirPlayerActivity.updateFileInfor()! FragmentListview{41c40198} [Lcom.maxproj.android.dirplayer.MyArrayAdapter;@41c3f260
lifecycle: MyArrayAdapter.MyArrayAdapter()
lifecycle: FragmentListview.setListviewAdapter() 1:static_ini_code_null FragmentListview{41c4e3e8}
lifecycle: FragmentListview.setListviewAdapter() 1:/mnt/sdcard FragmentListview{41c4e3e8}
lifecycle: DirPlayerActivity.updateFileInfor()! FragmentListview{41c4e3e8} [Lcom.maxproj.android.dirplayer.MyArrayAdapter;@41c3f260
lifecycle: MyArrayAdapter.MyArrayAdapter()
lifecycle: DirPlayerActivity.onResume()
lifecycle: FragmentListview.onResume() 0:static_ini_code_null FragmentListview{41c5ee80 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onResume() 0:static_ini_code_null FragmentListview{41c5f0c8 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: DirPlayerActivity.onResumeFragments()
lifecycle: FragmentListview.onPause() 0:static_ini_code_null FragmentListview{41c5ee80 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onPause() 0:static_ini_code_null FragmentListview{41c5f0c8 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: DirPlayerActivity.onPause()
lifecycle: FragmentListview.onSaveInstanceState() 0:static_ini_code_null FragmentListview{41c5ee80 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onSaveInstanceState() 0:static_ini_code_null FragmentListview{41c5f0c8 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: DirPlayerActivity.onSaveInstanceState()
lifecycle: FragmentListview.onStop() 0:static_ini_code_null FragmentListview{41c5ee80 #0 id=0x7f0a0001 android:switcher:2131361793:0}
lifecycle: FragmentListview.onStop() 0:static_ini_code_null FragmentListview{41c5f0c8 #1 id=0x7f0a0001 android:switcher:2131361793:1}
lifecycle: DirPlayerActivity.onStop()

可見，app不是從onRestart()開始，而是完全從activity靜態代碼開始初始化activity並且到達onCreate()
並且看到來歷不明的兩個fragment被onAttach()過來，這並不是activity靜態代碼生成的fragment

一個訣竅：可以將一個fragment或其他object當成字符串打印，會打印出該fragment的內存地址(估計是)
通過這個我們可以看出每次操作的是哪個fragment或其他object，儘管它們可能是同一類對象

如果高亮上面被打印出的各個fragment地址，就會發現activity靜態代碼生成的兩個fragment被後續的activity代碼初始化了
比如被setListviewAdapter()設置了adapter，但是這兩個fragment卻走不到onCreateView()和onResume()

被attach過來的fragment未經歷activity代碼的初始化，但是卻走到了onCreateView()和onResume()
也即，app顯示了被attach過來的fragment，僅僅被該fragment的靜態代碼初始化過
這正可以解釋操作時看到的現象，被attach過來的應該是系統幫忙恢復的fragment

在官網上找到了low memory下系統可能的恢復操作，其中包括幫忙恢復fragment
但是，這個恢復的fragment怎樣接收？怎樣關聯回activity？官網上卻找不到具體的處理方法

在stackoverflow上找到若干類似的問題，其中之一和我預想的方法一致
也即，在fragment的onAttach()中將自己關聯給activity，具體關鍵代碼如下

//fragment代碼
public class FragmentListview extends Fragment {
    int tab = -1; // attach時如果爲-1，說明是系統幫忙恢復的。正常初始化後會被設置爲0和1

    @Override
    public void onAttach(Activity activity){
        super.onAttach(activity);
        
        if (tab == -1){ // 系統幫忙創建的，更新到activity
         tab = ((DirPlayerActivity) getActivity()).sysAttachFragment; // 這裏計數被系統attach了多少個fragment
         fragmentListviewInterface.sysAttachFragmentListviewLowMem(
         tab, 
         this);
         ((DirPlayerActivity) getActivity()).sysAttachFragment++;
        }
    }

}
//activity代碼
public class DirPlayerActivity extends FragmentActivity implements
    FragmentListview.FragmentListviewInterface
    {
        int sysAttachFragment = 0;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_dir_player);
            for (int i = 0; i < LocalConst.tabCount; i++) {
                if(sysAttachFragment == 0){ // 正常啓動，而不是系統幫忙恢復
                    fragmentListview[i] = FragmentListview.newInstance(i);
                }
        }
    }

    public void sysAttachFragmentListviewLowMem(int tab, FragmentListview fragment){
               fragmentListview[tab] = fragment;
    }

}

簡單測試了一下，似乎可行

但是，感覺很不好
因爲fragment也許和activity有很多關聯，有很複雜的初始化過程
系統幫忙恢復的操作卻非常的粗魯
直接生成一個未正常初始化的fragment就強塞過來，並佔領了對應的位置
這時候千頭萬緒，關聯和恢復處理相當麻煩不可靠

在low memory的情況下恢復app的狀態有必要嗎？值得嗎？
至少有一些app不會有這樣的需求
所以，系統應該提供一個選項，讓app自己選擇要不要恢復原來的狀態
有這樣的選項嗎？我沒找到。或有，只是我不知道而已？

在我的這個項目中我就不希望系統幫忙恢復fragment
讓app代碼自己處理，結果會乾淨清爽得多，安心得多