android - 使用 ListFragment 和 CursorLoader 时得到 "ListFragment cannot cast to"消息

Question

谁能告诉我一个使用 CursorLoader 查询 SQLite 数据库并填充 ListFragment 的简单示例？下面的代码可以编译，但是当我运行它时，LogCat 告诉我“ListFrag”不能转换为 LoaderManager.LoaderCallbacks。如果我更改 ListFrag，使其不是 ListFrag，只是 ListFrag，我会被告知“ListFrag 无法转换为 android.v4.support.Fragment”。请注意，由于 this forum post 中的信息，我的 Activity 扩展了 FragmentActivity，我的 fragment 扩展了 ListFragment .我一直在努力让这个工作一段时间，但我就是不明白。这是我的包含 fragment 的 Activity 代码，谢谢:

import android.os.Bundle;
import android.support.v4.app.FragmentActivity

public class MyList extends FragmentActivity {
public void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setContentView(R.layout.myfragment);
}
}

这是我的 fragment 代码:

import android.app.ListFragment;
import android.app.LoaderManager.LoaderCallbacks;
import android.content.Intent;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.support.v4.content.CursorLoader;
import android.support.v4.content.Loader;
import android.support.v4.widget.CursorAdapter;
import android.support.v4.widget.SimpleCursorAdapter;
import android.support.v4.app.LoaderManager;
import android.view.View;
import android.widget.ListView;

@SuppressWarnings("hiding")
public class ListFrag<Cursor> extends ListFragment implements LoaderManager.LoaderCallbacks<Cursor> {   
    private static final String TABLE_BASEPATH = "MyTable_tbl";
    private static final String AUTHORITY = "SQLData";
    public static final Uri MY_URI = Uri.parse("content://" + AUTHORITY + "/" + TABLE_BASEPATH);
    private static final String[] PROJECTION = new String[] { "_id", "fieldname" };
    private SimpleCursorAdapter mAdapter;
    private static final int LOADER_ID = 0;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @SuppressWarnings("unchecked")
    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        Intent myData = getActivity().getIntent();
        Bundle info = myData.getExtras();


            SimpleCursorAdapter adapter;
            String[] dataColumns = { "fieldname" };
            int[] viewIDs = { R.id.mydetails };
            adapter = new SimpleCursorAdapter(getActivity(), R.layout.mylist, null, dataColumns, viewIDs, CursorAdapter.FLAG_REGISTER_CONTENT_OBSERVER);
            setListAdapter(adapter);
            getLoaderManager().initLoader(0, info, (LoaderCallbacks<Cursor>) this); 

    }

          @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        String item = (String) getListAdapter().getItem(position);
        DetailFrag frag = (DetailFrag) getFragmentManager().findFragmentById(R.id.frag_detail);
        if (frag != null && frag.isInLayout()) {
            frag.setText(item);
        }
    }

    @SuppressWarnings("unchecked")
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        String selection = "level='" + args.getString("Level") + "'";
        return (Loader<Cursor>) new CursorLoader(getActivity(), MY_URI,
                PROJECTION, selection, null, null); 
    }
    public void onLoadFinished(Loader<Cursor> loader, Cursor cursor) {
        switch (loader.getId()) {
          case LOADER_ID:
            mAdapter.swapCursor((android.database.Cursor) cursor);
            break;
        }


    }
    public void onLoaderReset(Loader<Cursor> loader) {
        mAdapter.swapCursor(null);

    }

}

刚刚看到您对 LogCat 的评论。哎呀。这是我的整个 LogCat:

11-05 15:47:27.953: D/dalvikvm(553): Not late-enabling CheckJNI (already on)
11-05 15:47:28.643: I/dalvikvm(553): threadid=3: reacting to signal 3
11-05 15:47:28.683: I/dalvikvm(553): Wrote stack traces to '/data/anr/traces.txt'
11-05 15:47:29.143: I/dalvikvm(553): threadid=3: reacting to signal 3
11-05 15:47:29.153: I/dalvikvm(553): Wrote stack traces to '/data/anr/traces.txt'
11-05 15:47:29.643: I/dalvikvm(553): threadid=3: reacting to signal 3
11-05 15:47:29.653: I/dalvikvm(553): Wrote stack traces to '/data/anr/traces.txt'
11-05 15:47:30.143: I/dalvikvm(553): threadid=3: reacting to signal 3
11-05 15:47:30.153: I/dalvikvm(553): Wrote stack traces to '/data/anr/traces.txt'
11-05 15:47:30.643: I/dalvikvm(553): threadid=3: reacting to signal 3
11-05 15:47:30.693: D/gralloc_goldfish(553): Emulator without GPU emulation detected.
11-05 15:47:30.693: I/dalvikvm(553): Wrote stack traces to '/data/anr/traces.txt'
11-05 15:47:32.843: E/ActivityThread(553): Failed to find provider info for SQLData
11-05 15:47:32.883: D/AndroidRuntime(553): Shutting down VM
11-05 15:47:32.883: W/dalvikvm(553): threadid=1: thread exiting with uncaught exception    (group=0x409c01f8)
11-05 15:47:32.903: E/AndroidRuntime(553): FATAL EXCEPTION: main
11-05 15:47:32.903: E/AndroidRuntime(553): java.lang.NullPointerException
11-05 15:47:32.903: E/AndroidRuntime(553):  at com.MyKnitCards.project.ListFrag.onLoadFinished(ListFrag.java:71)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.support.v4.app.LoaderManagerImpl$LoaderInfo.callOnLoadFinished(LoaderManager.java:425)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.support.v4.app.LoaderManagerImpl$LoaderInfo.onLoadComplete(LoaderManager.java:393)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.support.v4.content.Loader.deliverResult(Loader.java:103)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.support.v4.content.CursorLoader.deliverResult(CursorLoader.java:81)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.support.v4.content.CursorLoader.deliverResult(CursorLoader.java:35)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.support.v4.content.AsyncTaskLoader.dispatchOnLoadComplete(AsyncTaskLoader.java:221)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.support.v4.content.AsyncTaskLoader$LoadTask.onPostExecute(AsyncTaskLoader.java:61)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.support.v4.content.ModernAsyncTask.finish(ModernAsyncTask.java:461)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.support.v4.content.ModernAsyncTask.access$500(ModernAsyncTask.java:47)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.support.v4.content.ModernAsyncTask$InternalHandler.handleMessage(ModernAsyncTask.java:474)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.os.Handler.dispatchMessage(Handler.java:99)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.os.Looper.loop(Looper.java:137)
11-05 15:47:32.903: E/AndroidRuntime(553):  at android.app.ActivityThread.main(ActivityThread.java:4424)
11-05 15:47:32.903: E/AndroidRuntime(553):  at java.lang.reflect.Method.invokeNative(Native Method)
11-05 15:47:32.903: E/AndroidRuntime(553):  at java.lang.reflect.Method.invoke(Method.java:511)
11-05 15:47:32.903: E/AndroidRuntime(553):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
11-05 15:47:32.903: E/AndroidRuntime(553):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
11-05 15:47:32.903: E/AndroidRuntime(553):  at dalvik.system.NativeStart.main(Native Method)
11-05 15:47:33.213: I/dalvikvm(553): threadid=3: reacting to signal 3
11-05 15:47:33.233: I/dalvikvm(553): Wrote stack traces to '/data/anr/traces.txt'
11-05 15:47:33.513: I/dalvikvm(553): threadid=3: reacting to signal 3
11-05 15:47:33.533: I/dalvikvm(553): Wrote stack traces to '/data/anr/traces.txt'

Answer 1

首先尝试将这些导入切换到支持库版本。变化:

import android.app.ListFragment;
import android.app.LoaderManager.LoaderCallbacks;

到:

import android.support.v4.app.ListFragment;
import android.support.v4.app.LoaderManager.LoaderCallbacks;

接下来确保您要求支持 fragment 管理器:

DetailFrag frag = (DetailFrag) getActivity().getSupportFragmentManager().findFragmentById(R.id.frag_detail);
//                                              ^^^^^^^

(这可能无法解决所有问题，如果您仍然需要帮助发布这些更改的 LogCat 错误，以便我们可以准确了解发生了什么。)

---

添加mAdapter 为null，在此处将adapter 更改为mAdapter:

mAdapter = new SimpleCursorAdapter(getActivity(), R.layout.mylist, null, dataColumns, viewIDs, CursorAdapter.FLAG_REGISTER_CONTENT_OBSERVER);