帶AsyncTaskLoader的片段影響其他片段附加到活動

Question

我使用AsyncTaskLoader從數據庫查詢加載遊標。 我跟着Android開發者樣本： http://developer.android.com/reference/android/content/AsyncTaskLoader.html

但不知何故，正在添加到頁面適配器該片段後（使用加載器）的碎片不會得到重視的活動，它會嘗試當使用需要方法一個活動（如getString（））拋出一個異常，並表示這個片段沒有附加到任何活動。

下面是一些代碼：

1. 添加片段到頁面適配器。

   mAdapter = new PlaceFragmentPagerAdapter(getSupportFragmentManager()); 
   
   
   NewFragment newFrag = new NewFragment(); 
   mAdapter.addFragment(newShiftFrag); 
   
   ListFragment listFrag = new ListFragment(); 
   mAdapter.addFragment(listFrag); 
   
   SettingsFragment settingsFrag = new SettingsFragment(); 
   mAdapter.addFragment(settingsFrag); 
   
   mPager = (ViewPager)findViewById(R.id.pager); 
   mPager.setAdapter(mAdapter); 
   

2. AsyncTaskLoader實現：

   抽象公共類AbstractCursorLoader擴展AsyncTaskLoader {

   abstract protected Cursor buildCursor(); 
       Cursor lastCursor=null; 
   
       public AbstractCursorLoader(Context context) { 
        super(context); 
       } 
   
       /** 
       * Runs on a worker thread, loading in our data. Delegates 
       * the real work to concrete subclass' buildCursor() method. 
       */ 
       @Override 
       public Cursor loadInBackground() { 
        Cursor cursor=buildCursor(); 
   
        if (cursor!=null) { 
         // Ensure the cursor window is filled 
         cursor.getCount(); 
        } 
   
        return(cursor); 
       } 
   
       /** 
       * Runs on the UI thread, routing the results from the 
       * background thread to whatever is using the Cursor 
       * (e.g., a CursorAdapter). 
       */ 
       @Override 
       public void deliverResult(Cursor cursor) { 
        if (isReset()) { 
         // An async query came in while the loader is stopped 
         if (cursor!=null) { 
          cursor.close(); 
         } 
   
         return; 
        } 
   
        Cursor oldCursor=lastCursor; 
        lastCursor=cursor; 
   
        if (isStarted()) { 
         super.deliverResult(cursor); 
        } 
   
        if (oldCursor!=null && oldCursor!=cursor && !oldCursor.isClosed()) { 
         oldCursor.close(); 
        } 
       } 
   
       /** 
       * Starts an asynchronous load of the list data. 
       * When the result is ready the callbacks will be called 
       * on the UI thread. If a previous load has been completed 
       * and is still valid the result may be passed to the 
       * callbacks immediately. 
       * 
       * Must be called from the UI thread. 
       */ 
       @Override 
       protected void onStartLoading() { 
        if (lastCursor!=null) { 
         deliverResult(lastCursor); 
        } 
        if (takeContentChanged() || lastCursor==null) { 
         forceLoad(); 
        } 
       } 
   
       /** 
       * Must be called from the UI thread, triggered by a 
       * call to stopLoading(). 
       */ 
       @Override 
       protected void onStopLoading() { 
        // Attempt to cancel the current load task if possible. 
        cancelLoad(); 
       } 
   
       /** 
       * Must be called from the UI thread, triggered by a 
       * call to cancel(). Here, we make sure our Cursor 
       * is closed, if it still exists and is not already closed. 
       */ 
       @Override 
       public void onCanceled(Cursor cursor) { 
        if (cursor!=null && !cursor.isClosed()) { 
         cursor.close(); 
        } 
       } 
   
       /** 
       * Must be called from the UI thread, triggered by a 
       * call to reset(). Here, we make sure our Cursor 
       * is closed, if it still exists and is not already closed. 
       */ 
       @Override 
       protected void onReset() { 
        super.onReset(); 
   
        // Ensure the loader is stopped 
        onStopLoading(); 
   
        if (lastCursor!=null && !lastCursor.isClosed()) { 
         lastCursor.close(); 
        } 
   
        lastCursor=null; 
       } 
   } 
   
       private static class ListLoader extends AbstractCursorLoader { 
        private String mName; 
   
        public ShiftsListLoader(Context context, String name) { 
         super(context); 
         mName = name; 
        } 
   
        @Override 
        protected Cursor buildCursor() { 
         PlacesHandler wph = new PlacesHandler(this.getContext()); 
         return wph.GetShifts(mName); 
        } 
       } 
   

3. Initializng裝載機：

   @Override 
       public void onActivityCreated(Bundle savedInstanceState) { 
       super.onActivityCreated(savedInstanceState); 
   
       // Give some text to display if there is no data. In a real 
       // application this would come from a resource. 
       // TODO change to resource and back 
       setEmptyText("Nothing here.."); 
   
       // Start out with a progress indicator. 
       setListShown(false); 
   
   
   
       // Prepare the loader. Either re-connect with an existing one, 
       // or start a new one. 
       getLoaderManager().initLoader(0, null, this); 
   

   }

   @Override 
       public Loader<Cursor> onCreateLoader(int id, Bundle args) { 
        return new ListLoader(getActivity(), mWorkPlaceName); 
       } 
       @Override 
       public void onLoadFinished(Loader<Cursor> loader, Cursor data) { 
        // Create an empty adapter we will use to display the loaded data. 
       mAdapter = new ListCursorAdapter(getActivity(), data, CursorAdapter.FLAG_REGISTER_CONTENT_OBSERVER); 
       setListAdapter(mAdapter); 
   
       } 
   @Override 
   public void onLoaderReset(Loader<Cursor> loader) { 
       // TODO Auto-generated method stub 
   
   } 
   

我真的不知道爲什麼會發生這種情況。

P.S.我在評論中遇到了一些問題，我覺得它有一個bug，所以很抱歉。

在此先感謝，Elad。

Answer 1

簡短的回答是，你永遠不應該假設一個片段處於任何特定的狀態，直到它收到相應的生命週期回調信號爲止。

你所看到的是在ViewPager利用ICS期間添加的優化。 FragmentPagerAdapter通過調用setUserVisibleHint專門將屏幕外片段標記爲不是用戶可見的。 FragmentManager使用它來優先執行加載程序的執行方式，以便用戶首先看到完全加載的可見頁面，加載側頁面不會減慢加載可見頁面的過程。具體來說，它會延遲將碎片移動到「啓動」狀態，這也是裝載機開始運行時的情況。

如果用戶在這個過程中滾動到另一個頁面，FragmentManager會將片段移動到啓動狀態，並立即開始運行它的加載器作爲FragmentPagerAdapter＃setPrimaryItem（）的一部分，因爲此方法將當前頁面標記爲現在正在用戶可見。