NPE：CursorTreeAdapter $ MyCursorHelper.changeCursor（光標，布爾值）」對空對象引用

Question

我想在不同的光標從我的數據庫中的數據，並顯示在按組分開的ExpandableListView。如下所示：

• 歷史任務（移動光標0）
  • 歷史1
  • 歷史2級
• 地理的任務（到光標1）
  • 地理1
  • 地理2
• 外語任務（到光標2）
  • 外語1
  • 外語2

我想它使用CursorTreeAdapter做的，因爲我有我的所有信息數據庫，它可以管理不同的遊標並在ExpandableListView中正確顯示信息。

我得到的問題是，我在我的代碼的某些時候得到一個NPE，但我不能夠檢查它在哪裏調試，因爲它直接切換到另一個文件不知道發生了什麼。

當我調試的CursorTreeAdapter :: setChildrenCursor方法，只是當我踏進

childrenCursorHelper.changeCursor(childrenCursor, false); 

失敗，所以我不知道這個方法裏面發生了什麼。

我的代碼如下：

MtMainActivity.java

public class MtMainActivity extends AppCompatActivity implements LoaderManager.LoaderCallbacks<Cursor> { 

    @InjectView(R.id.elvTaskList) 
    protected ExpandableListView elvTaskList; 
    private TaskCursorAdapter taskListAdapter; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     taskListAdapter = new TaskCursorAdapter(null, this); 
     elvTaskList.setAdapter(taskListAdapter); 
     getSupportLoaderManager().initLoader(MtLoaders.HISTORY_TASK, null, this); 
    } 

    @Override 
    public Loader<Cursor> onCreateLoader(int id, Bundle args) { 
     switch (id) { 
      case MtLoaders.HISTORY_TASK: 
       return mtLoaderFactory.createHistoryTaskLoader(); 
     } 
    } 

    @Override 
    public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) { 
     switch (loader.getId()) { 
      case MtLoaders.HISTORY_TASK: 
       taskListAdapter.setChildrenCursor(0, cursor); 
     } 
    } 

    @Override 
    public void onLoaderReset(Loader<Cursor> loader) { 
     switch (loader.getId()) { 
      case MtLoaders.HISTORY_TASK: 
       taskListAdapter.setChildrenCursor(0, null); 
     } 
    } 

TaskCursorAdapter.java

public class TaskCursorAdapter extends CursorTreeAdapter { 

    private final Cursor cursor; 

    public TaskCursorAdapter(Cursor cursor, Context context) { 
     super(cursor, context); 
     this.cursor = cursor; 
    } 

    @Override 
    protected Cursor getChildrenCursor(Cursor groupCursor) { 
     return cursor; 
    } 

    @Override 
    protected View newGroupView(Context context, Cursor cursor, boolean isExpanded, ViewGroup parent) { 
     final LayoutInflater layoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE); 
     final TextView tvHeader = (TextView) layoutInflater.inflate(R.layout.mt_task_item_subtitle, parent, false); 
     tvHeader.setText(">> Insert name here <<"); 
     return tvHeader; 
    } 

    @Override 
    protected void bindGroupView(View view, Context context, Cursor cursor, boolean isExpanded) { /* nothing */ } 

    @Override 
    protected View newChildView(Context context, Cursor cursor, boolean isLastChild, ViewGroup parent) { 
     final LayoutInflater layoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE); 
     final View view = layoutInflater.inflate(R.layout.mt_task_item, parent, false); 

     return view; 
    } 

    @Override 
    protected void bindChildView(View view, Context context, Cursor cursor, boolean isLastChild) { /* nothing */ } 
} 

錯誤

E/AndroidRuntime﹕ FATAL EXCEPTION: main 
Process: project.android, PID: 11592 
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.CursorTreeAdapter$MyCursorHelper.changeCursor(android.database.Cursor, boolean)' on a null object reference 
     at android.widget.CursorTreeAdapter.setChildrenCursor(CursorTreeAdapter.java:164) 
     at project.android.ui.MtMainActivity.onLoadFinished(MtMainActivity.java:215) 
     at project.android.ui.MtMainActivity.onLoadFinished(MtMainActivity.java:29) 
     at android.support.v4.app.LoaderManagerImpl$LoaderInfo.callOnLoadFinished(LoaderManager.java:427) 
     at android.support.v4.app.LoaderManagerImpl$LoaderInfo.onLoadComplete(LoaderManager.java:395) 
     at android.support.v4.content.Loader.deliverResult(Loader.java:104) 
     at android.support.v4.content.AsyncTaskLoader.dispatchOnLoadComplete(AsyncTaskLoader.java:223) 
     at android.support.v4.content.AsyncTaskLoader$LoadTask.onPostExecute(AsyncTaskLoader.java:61) 
     at android.support.v4.content.ModernAsyncTask.finish(ModernAsyncTask.java:461) 
     at android.support.v4.content.ModernAsyncTask.access$500(ModernAsyncTask.java:47) 
     at android.support.v4.content.ModernAsyncTask$InternalHandler.handleMessage(ModernAsyncTask.java:474) 
     at android.os.Handler.dispatchMessage(Handler.java:102) 
     at android.os.Looper.loop(Looper.java:135) 
     at android.app.ActivityThread.main(ActivityThread.java:5221) 
     at java.lang.reflect.Method.invoke(Native Method) 
     at java.lang.reflect.Method.invoke(Method.java:372) 
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) 
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 

PS：光標正常工作，我查採用了獨特的ListView和沒有問題與此有關。

Answer 1

我已經在我的項目相同的問題。看來，這是在CursorTreeAdapter一個問題：

at android.widget.CursorTreeAdapter.setChildrenCursor(CursorTreeAdapter.java:164) 

看那個方法的Android源裏面：

public void setChildrenCursor(int groupPosition, Cursor childrenCursor) { 

    /* 
    * Don't request a cursor from the subclass, instead we will be setting 
    * the cursor ourselves. 
    */ 
    MyCursorHelper childrenCursorHelper = getChildrenCursorHelper(groupPosition, false); 

    /* 
    * Don't release any cursor since we know exactly what data is changing 
    * (this cursor, which is still valid). 
    */ 
    childrenCursorHelper.changeCursor(childrenCursor, false); 
} 

在某些情況下getChildrenCursorHelper返回空到childrenCursorHelper。我通常在配置更改後（例如屏幕旋轉）得到它。

爲了解決這個問題，我想補充CursorFilter + CursorTreeAdapter源到我的項目，未來加空檢查childrenCursorHelper

public void setChildrenCursor(int groupPosition, Cursor childrenCursor) { 

    /* 
    * Don't request a cursor from the subclass, instead we will be setting 
    * the cursor ourselves. 
    */ 
    MyCursorHelper childrenCursorHelper = getChildrenCursorHelper(groupPosition, false); 

    /* 
    * Don't release any cursor since we know exactly what data is changing 
    * (this cursor, which is still valid). 
    */ 
    if (childrenCursorHelper != null) 
     childrenCursorHelper.changeCursor(childrenCursor, false); 
} 

最後延長從固定CursorTreeAdapter我的適配器。
詳情請參閱this commit。

Answer 2

您不需要將CursorTreeAdapter源添加到項目中。

只重寫setChildrenCursor：

public class MyAdapter extends CursorTreeAdapter { 

    @Override 
    public void setChildrenCursor(int groupPosition, Cursor childrenCursor) { 
     try { 
      Method getChildrenCursorHelper = CursorTreeAdapter.class.getDeclaredMethod("getChildrenCursorHelper", int.class, boolean.class); 
      getChildrenCursorHelper.setAccessible(true); 
      Object childrenCursorHelper = getChildrenCursorHelper.invoke(this, groupPosition, false); 
      if (childrenCursorHelper != null) { 
       Method changeCursor = childrenCursorHelper.getClass().getDeclaredMethod("changeCursor", Cursor.class, boolean.class); 
       changeCursor.setAccessible(true); 
       changeCursor.invoke(childrenCursorHelper, childrenCursor, false); 
      } 
     } catch (IllegalAccessException e) { 
     } catch (InvocationTargetException e) { 
     } catch (NoSuchMethodException e) { 
     } 
    } 
}