Как загрузить ListView с использованием AsyncTask в android

Question

У меня есть метод, называемый fetchData(), чтобы извлечь некоторые данные из базы данных и загрузить их в ListView. Но когда начинается активность, из-за этого происходит небольшое отставание. Поэтому мне нужно загрузить данные в фоновом режиме. Мне было интересно, может ли кто-нибудь сказать мне, как это сделать, используя AsyncTask.

Это мой метод fetchData().

public void fetchData() { 
    database = helper.getReadableDatabase(); 
    Cursor c; 
    Date cDate = new Date(); 
    SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd"); 
    final String fDate = sdf.format(cDate); 

    int thisMonth=Integer.parseInt(fDate.split("-")[1]); 
    Month mn=new Month(); 
    String month=mn.getMonth(thisMonth); 

    Calendar cal=Calendar.getInstance(); 

    int today=Integer.parseInt(fDate.split("-")[2]); 

    int curTab=position; 
    String whereClause=""; 
    String sort=""; 
    if(curTab==0){ 
     whereClause=null; 
     sort=Database.NAME; 
    } 
    else if(curTab==1){ 
     whereClause=Database.MONTH+" = '"+month+"' and "+Database.DAY+" ="+today; 
     sort=Database.NAME; 
    } 
    else if(curTab==2){ 
     cal.add(Calendar.DAY_OF_MONTH, 1); 
     int monthn=cal.get(Calendar.MONTH)+1; 
     Month mnN=new Month(); 
     String monthTomorrow=mnN.getMonth(monthn); 

     int tomorrow=cal.get(Calendar.DAY_OF_MONTH); 
     whereClause=Database.MONTH+" = '"+monthTomorrow+"' and "+Database.DAY+" ="+tomorrow; 
     sort=Database.DAY; 
    } 
    else if(curTab==3){ 
     whereClause=Database.MONTH+" = '"+month+"'"; 
     sort=Database.DAY; 
    } 

    if(DrawerMain.pos==1){ 
     if(curTab==0){ 
      whereClause=Database.TYPE+"='birthday'"; 
     } 
     else{ 
      whereClause=whereClause+" and "+Database.TYPE+"='birthday'"; 
     } 
    } 
    else if(DrawerMain.pos==2){ 
     if(curTab==0){ 
      whereClause=Database.TYPE+"='anniversary'"; 
     } 
     else{ 
      whereClause=whereClause+" and "+Database.TYPE+"='anniversary'"; 
     } 
    } 

    c = database.query(Database.TABLE_EVENT, null, whereClause, null, null, null, sort); 

    String[] fromDB={Database.NAME,Database.MONTH,Database.DAY}; 
    int[] toView={R.id.tvName_lv,R.id.tv_month_lv,R.id.tv_day_lv}; 

    CustomCursorAdapter adapter=new CustomCursorAdapter(getActivity(), c, 0, R.layout.events_list_item,fromDB,toView); 
    lv.setAdapter(adapter); 
    database.close(); 
} 

Answer 1

Вместо этого вы должны использовать AsyncTaskLoader. AsyncLoaders будут обрабатывать изменения ориентации лучше, чем AsyncTasks.

Вы можете найти учебник здесь: http://www.androiddesignpatterns.com/2012/08/implementing-loaders.html

код (скопировали непосредственно из учебника)

public class SampleLoader extends AsyncTaskLoader<List<SampleItem>> { 

    // We hold a reference to the Loader’s data here. 
    private List<SampleItem> mData; 

    public SampleLoader(Context ctx) { 
    // Loaders may be used across multiple Activitys (assuming they aren't 
    // bound to the LoaderManager), so NEVER hold a reference to the context 
    // directly. Doing so will cause you to leak an entire Activity's context. 
    // The superclass constructor will store a reference to the Application 
    // Context instead, and can be retrieved with a call to getContext(). 
    super(ctx); 
    } 

    /****************************************************/ 
    /** (1) A task that performs the asynchronous load **/ 
    /****************************************************/ 

    @Override 
    public List<SampleItem> loadInBackground() { 
    // This method is called on a background thread and should generate a 
    // new set of data to be delivered back to the client. 
    List<SampleItem> data = new ArrayList<SampleItem>(); 

    // TODO: Perform the query here and add the results to 'data'. 

    return data; 
    } 

    /********************************************************/ 
    /** (2) Deliver the results to the registered listener **/ 
    /********************************************************/ 

    @Override 
    public void deliverResult(List<SampleItem> data) { 
    if (isReset()) { 
     // The Loader has been reset; ignore the result and invalidate the data. 
     releaseResources(data); 
     return; 
    } 

    // Hold a reference to the old data so it doesn't get garbage collected. 
    // We must protect it until the new data has been delivered. 
    List<SampleItem> oldData = mData; 
    mData = data; 

    if (isStarted()) { 
     // If the Loader is in a started state, deliver the results to the 
     // client. The superclass method does this for us. 
     super.deliverResult(data); 
    } 

    // Invalidate the old data as we don't need it any more. 
    if (oldData != null && oldData != data) { 
     releaseResources(oldData); 
    } 
    } 

    /*********************************************************/ 
    /** (3) Implement the Loader’s state-dependent behavior **/ 
    /*********************************************************/ 

    @Override 
    protected void onStartLoading() { 
    if (mData != null) { 
     // Deliver any previously loaded data immediately. 
     deliverResult(mData); 
    } 

    // Begin monitoring the underlying data source. 
    if (mObserver == null) { 
     mObserver = new SampleObserver(); 
     // TODO: register the observer 
    } 

    if (takeContentChanged() || mData == null) { 
     // When the observer detects a change, it should call onContentChanged() 
     // on the Loader, which will cause the next call to takeContentChanged() 
     // to return true. If this is ever the case (or if the current data is 
     // null), we force a new load. 
     forceLoad(); 
    } 
    } 

    @Override 
    protected void onStopLoading() { 
    // The Loader is in a stopped state, so we should attempt to cancel the 
    // current load (if there is one). 
    cancelLoad(); 

    // Note that we leave the observer as is. Loaders in a stopped state 
    // should still monitor the data source for changes so that the Loader 
    // will know to force a new load if it is ever started again. 
    } 

    @Override 
    protected void onReset() { 
    // Ensure the loader has been stopped. 
    onStopLoading(); 

    // At this point we can release the resources associated with 'mData'. 
    if (mData != null) { 
     releaseResources(mData); 
     mData = null; 
    } 

    // The Loader is being reset, so we should stop monitoring for changes. 
    if (mObserver != null) { 
     // TODO: unregister the observer 
     mObserver = null; 
    } 
    } 

    @Override 
    public void onCanceled(List<SampleItem> data) { 
    // Attempt to cancel the current asynchronous load. 
    super.onCanceled(data); 

    // The load has been canceled, so we should release the resources 
    // associated with 'data'. 
    releaseResources(data); 
    } 

    private void releaseResources(List<SampleItem> data) { 
    // For a simple List, there is nothing to do. For something like a Cursor, we 
    // would close it in this method. All resources associated with the Loader 
    // should be released here. 
    } 

    /*********************************************************************/ 
    /** (4) Observer which receives notifications when the data changes **/ 
    /*********************************************************************/ 

    // NOTE: Implementing an observer is outside the scope of this post (this example 
    // uses a made-up "SampleObserver" to illustrate when/where the observer should 
    // be initialized). 

    // The observer could be anything so long as it is able to detect content changes 
    // and report them to the loader with a call to onContentChanged(). For example, 
    // if you were writing a Loader which loads a list of all installed applications 
    // on the device, the observer could be a BroadcastReceiver that listens for the 
    // ACTION_PACKAGE_ADDED intent, and calls onContentChanged() on the particular 
    // Loader whenever the receiver detects that a new application has been installed. 
    // Please don’t hesitate to leave a comment if you still find this confusing! :) 
    private SampleObserver mObserver; 
} 

Поместите свой метод fetchData() в loadInBackground(). Закройте свой курсор в методе releaseResources(). В вашем запросе onCreate()

getLoaderManager().initLoader(0, null, this);