Android Asynctask 在 fragment 选项卡中运行两次

Question

当我的 fragment 被重新附加时，我的 Asynctask 运行了两次。我怎样才能让它只附加一次。当我在运行我的异步任务时在选项卡之间快速切换时，这会导致内容 View 尚未创建。任何人都可以解决这个问题吗？

  public class AndroidFragment extends SherlockListFragment  implements ActionBar.TabListener{

static final String URL = "https://xml.xml";
static final String KEY_SONG = "song";
static final String KEY_ID = "id";
static final String KEY_TITLE = "title";
static final String KEY_CAT_ARTIST = "artistcat";
static final String KEY_DURATION = "duration";
static final String KEY_THUMB_URL = "thumb_url";
static final String KEY_BIG_URL = "big_url";
static final String KEY_CAT_URL = "cat_url";
static final String KEY_DESC = "cat_desc";
ArrayList<HashMap<String, String>> menuItems;
ListAdapter adapter;
Context appContext;
private Fragment mFragment;

@Override
public void onViewCreated(View view, Bundle savedInstanceState) {


        new loadListView().execute();

        if (getActivity() != null) {
            adapter=new MainPageLazyAdapter(getActivity(), menuItems);
            setListAdapter(adapter);}

     super.onViewCreated(view, savedInstanceState);
        }

public class loadListView extends AsyncTask<Integer, String, String> {

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }

    @Override
    protected String doInBackground(Integer... args) {
        // updating UI from Background Thread
        menuItems = new ArrayList<HashMap<String, String>>();


        XMLParser parser = new XMLParser();
        String xml = parser.getXmlFromUrl(URL); // getting XML
        Document doc = parser.getDomElement(xml); // getting DOM element

        NodeList nl = doc.getElementsByTagName(KEY_SONG);
        // looping through all item nodes <item>
        for (int i = 0; i < nl.getLength(); i++) {
            // creating new HashMap
            HashMap<String, String> map = new HashMap<String, String>();
            Element e = (Element) nl.item(i);
            // adding each child node to HashMap key => value
            map.put(KEY_ID, parser.getValue(e, KEY_ID));
            map.put(KEY_TITLE, parser.getValue(e, KEY_TITLE));
            map.put(KEY_CAT_ARTIST, parser.getValue(e, KEY_CAT_ARTIST));
            map.put(KEY_DURATION, parser.getValue(e, KEY_DURATION));
            map.put(KEY_THUMB_URL, parser.getValue(e, KEY_THUMB_URL));
            map.put(KEY_BIG_URL, parser.getValue(e, KEY_BIG_URL));
            map.put(KEY_CAT_URL, parser.getValue(e, KEY_CAT_URL));
            map.put(KEY_DESC, parser.getValue(e, KEY_DESC));
            // adding HashList to ArrayList
            menuItems.add(map);
        }
            return null;
        }   


    @Override
    protected void onPostExecute(String args) {

     }
}

    @Override
    public void onTabSelected(Tab tab, FragmentTransaction ft) {

        if (mFragment == null) {
            mFragment = new AndroidFragment();
            ft.add(android.R.id.content, mFragment);
            //ft.attach(mFragment);

        } else {

            if (mFragment.isDetached()){
                ft.attach(mFragment);
            }


        }
        }


    @Override
    public void onTabUnselected(Tab tab, FragmentTransaction ft) {

        ft.detach(mFragment);

    }

    @Override
    public void onTabReselected(Tab tab, FragmentTransaction ft) {

    }


}

洛卡特:

  10-09 15:21:47.141: E/AndroidRuntime(7582): FATAL EXCEPTION: main
10-09 15:21:47.141: E/AndroidRuntime(7582): java.lang.RuntimeException: Unable to start activity ComponentInfo{in.wptrafficanalyzer.actionbarsherlocknavtab/in.wptrafficanalyzer.actionbarsherlocknavtab.MainActivity}: java.lang.NullPointerException
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1970)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1995)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.app.ActivityThread.access$600(ActivityThread.java:127)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1161)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.os.Handler.dispatchMessage(Handler.java:99)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.os.Looper.loop(Looper.java:137)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.app.ActivityThread.main(ActivityThread.java:4512)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at java.lang.reflect.Method.invokeNative(Native Method)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at java.lang.reflect.Method.invoke(Method.java:511)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:982)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:749)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at dalvik.system.NativeStart.main(Native Method)
10-09 15:21:47.141: E/AndroidRuntime(7582): Caused by: java.lang.NullPointerException
10-09 15:21:47.141: E/AndroidRuntime(7582):     at in.wptrafficanalyzer.actionbarsherlocknavtab.MainPageLazyAdapter.getCount(MainPageLazyAdapter.java:34)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.widget.ListView.setAdapter(ListView.java:460)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.support.v4.app.ListFragment.setListAdapter(ListFragment.java:182)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.support.v4.app.ListFragment.ensureList(ListFragment.java:365)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.support.v4.app.ListFragment.onViewCreated(ListFragment.java:145)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at in.wptrafficanalyzer.actionbarsherlocknavtab.AndroidFragment.onViewCreated(AndroidFragment.java:52)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:884)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1080)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:622)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1416)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:505)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1136)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.app.Activity.performStart(Activity.java:4475)
10-09 15:21:47.141: E/AndroidRuntime(7582):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1943)
10-09 15:21:47.141: E/AndroidRuntime(7582):     ... 11 more

Answer 1

你确定 AsyncTask 运行了两次吗？因为我从您的代码和 logcat 中得到的是 AsyncTask 完成并调用 onPostExecute 时(考虑到您的 fragment 当前已分离)如果您在未创建列表时调用 setListAdapter，则会出现此异常。因此，不要在 onPostExecute 中调用 setListAdapter，而是将下载的项目保存在 fragment 中，并在 onViewCreated() 中将其设置为适配器。试一试，然后告诉我这是否解决了您的问题。

使用此代码。

public class AndroidFragment extends SherlockListFragment  implements ActionBar.TabListener{

    static final String URL            = "https://xml.xml";
    static final String KEY_SONG       = "song";
    static final String KEY_ID         = "id";
    static final String KEY_TITLE      = "title";
    static final String KEY_CAT_ARTIST = "artistcat";
    static final String KEY_DURATION   = "duration";
    static final String KEY_THUMB_URL  = "thumb_url";
    static final String KEY_BIG_URL    = "big_url";
    static final String KEY_CAT_URL    = "cat_url";
    static final String KEY_DESC       = "cat_desc";
    ArrayList<HashMap<String, String>> menuItems = new ArrayList<HashMap<String, String>>;
    ListAdapter adapter;
    Context     appContext;
    private Fragment mFragment;

    @Override
    public void onViewCreated (View view, Bundle savedInstanceState) {


        new loadListView().execute();

        if (getActivity() != null) {
            adapter = new MainPageLazyAdapter( getActivity(), menuItems );
            setListAdapter( adapter );
        }

        super.onViewCreated( view, savedInstanceState );
    }

    public class loadListView extends AsyncTask<Integer, String, String> {

        @Override
        protected void onPreExecute () {
            super.onPreExecute();
        }

        @Override
        protected String doInBackground (Integer... args) {
            // updating UI from Background Thread
            menuItems = new ArrayList<HashMap<String, String>>();


            XMLParser parser = new XMLParser();
            String xml = parser.getXmlFromUrl( URL ); // getting XML
            Document doc = parser.getDomElement( xml ); // getting DOM element

            NodeList nl = doc.getElementsByTagName( KEY_SONG );
            // looping through all item nodes <item>
            for (int i = 0; i < nl.getLength(); i++) {
                // creating new HashMap
                HashMap<String, String> map = new HashMap<String, String>();
                Element e = (Element) nl.item( i );
                // adding each child node to HashMap key => value
                map.put( KEY_ID, parser.getValue( e, KEY_ID ) );
                map.put( KEY_TITLE, parser.getValue( e, KEY_TITLE ) );
                map.put( KEY_CAT_ARTIST, parser.getValue( e, KEY_CAT_ARTIST ) );
                map.put( KEY_DURATION, parser.getValue( e, KEY_DURATION ) );
                map.put( KEY_THUMB_URL, parser.getValue( e, KEY_THUMB_URL ) );
                map.put( KEY_BIG_URL, parser.getValue( e, KEY_BIG_URL ) );
                map.put( KEY_CAT_URL, parser.getValue( e, KEY_CAT_URL ) );
                map.put( KEY_DESC, parser.getValue( e, KEY_DESC ) );
                // adding HashList to ArrayList
                menuItems.add( map );
            }
            return null;
        }


        @Override
        protected void onPostExecute (String args) {
            if (adapter != null) adapter.notifyDataSetChanged();
        }
    }

    @Override
    public void onTabSelected (Tab tab, FragmentTransaction ft) {

        if (mFragment == null) {
            mFragment = new AndroidFragment();
            ft.add( android.R.id.content, mFragment );
            //ft.attach(mFragment);

        } else {

            if (mFragment.isDetached()) {
                ft.attach( mFragment );
            }


        }
    }


    @Override
    public void onTabUnselected (Tab tab, FragmentTransaction ft) {

        ft.detach( mFragment );

    }

    @Override
    public void onTabReselected (Tab tab, FragmentTransaction ft) {

    }


}