我的listview只在使用调试器时更新

Question

每当我正常运行我的应用程序时，listfragment总是为bnf片段空白。但是，如果我在fetch stories方法之前设置一个断点并逐行执行代码，listfragment就会得到更新!我粘贴的最后一个类是一个asyncntask，它获取用于更新listview的所有数据。我想我正在做一些错误的线程，但我不知道发生了什么。为了它的价值，得到添加的第一个片段也是一个异步任务，它工作得很好。

public class NasaAppActivity extends Activity implements ActionBar.TabListener{
private Bundle savedInstanceState;
private static View mainView;
private FragmentTransaction ft;
BreakingNewsFragment bnf; //a list fragment
NasaDailyImage ndi; //another fragment
@Override
public void onStart(){
    super.onStart(); 
    if(savedInstanceState==null){
        ndi=new NasaDailyImage(this);
        bnf=new BreakingNewsFragment(this);     
        ft=getFragmentManager().beginTransaction();
        ft.add(R.id.focused_view_container,ndi).commit();
        ft=getFragmentManager().beginTransaction();
        ft.add(R.id.focused_view_container,bnf).commit();
    }
public void onTabSelected(Tab tab, FragmentTransaction f) {
    switch(tab.getPosition()){ //switches which fragment is visible
    case 0:
        ft=getFragmentManager().beginTransaction();

            ft.hide(bnf);
            ft.show(ndi);
            ft.commit();
           getFragmentManager().executePendingTransactions();

        break;
    case 1:
            ft=getFragmentManager().beginTransaction();
            ft.hide(ndi);
            ft.show(bnf);
            ft.commit();
            bnf.fetchStories();
            bnf.updateList(); //updates the list fragment
          getFragmentManager().executePendingTransactions();
         break;
    }
}

}

---

@SuppressLint("ValidFragment")
public class BreakingNewsFragment extends ListFragment {
    private static Activity mainActivity;
    private static ArrayList<Story> stories=new ArrayList<Story>();
    private static ArrayList<String> storyTitles=new ArrayList<String>();
    public BreakingNewsFragment(){
    }
    public BreakingNewsFragment(Activity mainActivity){
        this.mainActivity=mainActivity;
    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        return super.onCreateView(inflater, container, savedInstanceState);
    }
    @Override
    public void onHiddenChanged(boolean hidden) {
        // TODO Auto-generated method stub
        super.onHiddenChanged(hidden);
    }
    @Override
    public void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);

    }
    public void fetchStories(){
        RssNewsParser parser=new RssNewsParser(mainActivity);
        parser.execute("");
        stories=parser.getStories();
        for(Story story:stories){
            storyTitles.add(story.getTitle());
        }
        String result;
        result = storyTitles.toString();
    }
public void updateList(){
    String result;
    result = storyTitles.toString();
    setListAdapter(new ArrayAdapter<String>(mainActivity,android.R.layout.simple_list_item_1,storyTitles));
}
    @Override
    public void onStart() {
        // TODO Auto-generated method stub
        super.onStart();


    }
}

---

public class RssNewsParser extends AsyncTask<ArrayList<Story>,String,ArrayList<Story>>{
    private ProgressDialog dialog;
    URL newsURL;
    ArrayList<Story> stories=new ArrayList<Story>();
    int eventType;
    int storyCount= -1;
    private Activity mainActivity;
    RssNewsParser(Activity mainActivity){
        this.mainActivity=mainActivity;
    }
    public ArrayList<Story> getStories(){
        return stories;
    }


        @Override
        protected void onPostExecute(ArrayList<Story> result) {
            // TODO Auto-generated method stub
            super.onPostExecute(result);
            dialog.hide();
        //this.cancel(true);
    }
    @Override
    protected void onPreExecute() {
        // TODO Auto-generated method stub
        super.onPreExecute();
        dialog=ProgressDialog.show(mainActivity, "Loading", "loading news");
    }
    @Override
    protected ArrayList<Story> doInBackground(ArrayList<Story>... params) {
        try{
        newsURL = new URL("http://www.nasa.gov/rss/breaking_news.rss");//set URl                         
        BufferedReader in = new BufferedReader(new InputStreamReader(newsURL.openStream()));//get rss
        XmlPullParserFactory factory;
        factory = XmlPullParserFactory.newInstance();//new factory
        factory.setNamespaceAware(true);
        XmlPullParser xpp;
        xpp = factory.newPullParser();
        xpp.setInput(in);
        eventType = xpp.getEventType();//returns an int which mean different things (START_DOCUMENT,START_TAG,etc)

    while(eventType!=XmlPullParser.END_DOCUMENT){//while the document has words
     switch(eventType){
        case XmlPullParser.START_DOCUMENT://beginning of xml
            break;
        case XmlPullParser.START_TAG://case : at beginning of new tag
            String tagName=xpp.getName();
            if(tagName.equals("item")){
                Story story=new Story();
                stories.add(story);
                storyCount++;
                xpp.getDepth();
            }
            if(tagName.equals("title")&& !stories.isEmpty()){
                xpp.getDepth();
                stories.get(storyCount).setTitle(xpp.nextText());
            }
            if(tagName.equals("link") && !stories.isEmpty()){
                stories.get(storyCount).setURL(xpp.nextText());
                xpp.getDepth();
            }
            if(tagName.equals("description")&& !stories.isEmpty()){
                stories.get(storyCount).setDescription(xpp.nextText());
                xpp.getDepth();
            }
            break;
        }
        eventType=xpp.next();
    }//switch   
        in.close();//close BufferedReader
    } catch (MalformedURLException e){
        e.printStackTrace();
    }catch(XmlPullParserException e1){
        e1.printStackTrace();
    }catch(IOException e2){
        e2.printStackTrace();
    }     
        return stories;
    }
}

Answer 1

我的listview只在使用调试器时更新

这里

    parser.execute("");
    stories=parser.getStories(); //<<<<<<< here

您试图在执行AsyncTask后返回AsyncTask结果。要么使用AsyncTask.get()方法获得结果返回，并等待doInBackground执行未完成，要么使用onPostExecute更新ListView与从doInBackground返回的新数据。将onPostExecute更改为:

@Override
 protected void onPostExecute(ArrayList<Story> result) {
   // TODO Auto-generated method stub
    super.onPostExecute(result);
     // get data here
     stories=getStories();
    BreakingNewsFragment.this.setListAdapter(new ArrayAdapter<String>(mainActivity,
                         android.R.layout.simple_list_item_1,stories));
     dialog.hide();
    }

Answer 2

您将fetchStories方法视为阻塞调用。这意味着，你期望你的代码等待，直到该方法完成，直到调用下一行调用updateStories方法。然而，由于fetchStories是异步工作的，所以在调用updateStories时它还没有完成。

解决这个问题的方法是从AsyncTask的onPostExecute方法中调用updateStories。这个方法的目的是允许你在启动AsyncTask的线程(通常是UI线程，所以你可以做更新)上执行代码，在任务完成了它的主要工作(你放入doInBackground的东西)

它在调试时工作的原因是因为你放慢了代码的执行速度，以便在调用updateStories之前完成fetchStories。在设备上，情况就不一样了。

简短的回答，从onPostExecute更新你的列表