Android: How to update an UI from AsyncTask if Asy

问题:

I hate inner class.

I've a main activity who launches a 'short-life' AsyncTask.

AsyncTask is in a separate file, is not an inner class of main activity

I need async task updates a textView from main Activity.

I know i can update a TextView from onProgressUpdate, if AsyncTask is a inner class

But how from an external, indipendent, async task ?

UPDATE: This looks like working :

In acitivty i call the task

backgroundTask = new BackgroundTask(this);
backgroundTask.execute();

In the constructor i've

public BackgroundTask(Activity myContext)
{
    debug = (TextView) myContext.findViewById(R.id.debugText);
}

where debug was a private field of AsyncTask.

So onProgressUpdate I can

debug.append(text);

Thanks for all of you suggestions

回答1:

EDIT I edited the answer to use WeakReference

---

AsyncTask is always separate class from Activity, but I suspect you mean it is in different file than your activity class file, so you cannot benefit from being activity's inner class. Simply pass Activity context as argument to your Async Task (i.e. to its constructor)

class MyAsyncTask extends AsyncTask<URL, Integer, Long> {

    WeakReference<Activity> mWeakActivity;

    public MyAsyncTask(Activity activity) {
       mWeakActivity = new WeakReference<Activity>(activity);
    }

 ...

and use when you need it (remember to NOT use in during doInBackground()) i.e. so when you would normally call

int id = findViewById(...)

in AsyncTask you call i.e.

Activity activity = mWeakActivity.get();
if (activity != null) {
   int id = activity.findViewById(...);
}

Note that our Activity can be gone while doInBackground() is in progress (so the reference returned can become null), but by using WeakReference we do not prevent GC from collecting it (and leaking memory) and as Activity is gone, it's usually pointless to even try to update it state (still, depending on your logic you may want to do something like changing internal state or update DB, but touching UI must be skipped).

回答2:

Using Interface 1) Create one Interface

public interface OnDataSendToActivity {
    public void sendData(String str);
}

2) Implements it in your Activity

public class MainActivity extends Activity implements OnDataSendToActivity{

     @Override
     protected void onCreate(Bundle savedInstanceState) {
          new AsyncTest(this).execute(new String[]{"AnyData"}); // start your task
     }

     @Override
     public void sendData(String str) {
         // TODO Auto-generated method stub

     }

}

3) Create constructor in AsyncTask(Activity activity){} Register your Interface in AsyncTask file and call interface method as below.

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

    OnDataSendToActivity dataSendToActivity;
    public AsyncTest(Activity activity){
        dataSendToActivity = (OnDataSendToActivity)activity;
    }

    @Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);
        dataSendToActivity.sendData(result);
    }

}

Here, your OnPostExecute will call after all task done by AsyncTask and will get "result" as a parameter, returned by doInBackground(){ return "";}.

While "dataSendToActivity.sendData(result);" it will call activity's overrided method "public void sendData(String str) {}".

An edge case to remember: Be sure to pass this, i.e. you current activity's context to AsyncTask and not create another instance of your activity, otherwise your Activity will be destroyed and new one is created.

回答3:

Make an static function in your activity class passing context in it to update your text view and then call this function in your AsynkTask class to update.

In Activity class: public static void updateTextView(){

//your code here }

In AynckTask class call this function.

回答4:

Just pass the context (activity or whatever) to your AsyncTask in a constructor and then in onSuccess or onProgressUpdate call whatever you need on the context.

回答5:

I wrote a small extension to AsyncTask for this kind of scenario. It allows you to keep your AsyncTask in a separate class, but also gives you convenient access to the Tasks's completion:

public abstract class ListenableAsyncTask<Params, Progress, Result> extends AsyncTask<Params, Progress, Result>{

    @Override
    protected final void onPostExecute(Result result) {
        notifyListenerOnPostExecute(result);
    }

    private AsyncTaskListener<Result> mListener;
    public interface AsyncTaskListener<Result>{
        public void onPostExecute(Result result);
    }
    public void listenWith(AsyncTaskListener<Result> l){
        mListener = l;
    }
    private void notifyListenerOnPostExecute(Result result){
        if(mListener != null)
            mListener.onPostExecute(result);
    }

}

So first you extend ListenableAsyncTask instead of AsyncTask. Then in your UI code, make a concrete instance and set listenWith(...).

回答6:

The Question has already been answered, still im posting how it should be done i guess..

Mainactivity class

  public class MainActivity extends Activity implements OnClickListener
    {

        TextView Ctemp;

        @Override
        protected void onCreate(Bundle savedInstanceState)
        {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            Ctemp = (TextView) findViewById(R.id.Ctemp);
            doConv = (Button) findViewById(R.id.doConv);
            doConv.setOnClickListener(this);
        }

        @Override
        public void onClick(View arg0) // The conversion to do
        {
            new asyncConvert(this).execute();
        }
    }

now in the async class

public class asyncConvert extends AsyncTask<Void, Void, String>
{
    SoapPrimitive response = null;
    Context context;

    public asyncConvert(Context callerclass)
    {
        contextGUI = callerclass;
    }
.
.
.
.
protected void onPostExecute(String result)
    {
        ((MainActivity) contextGUI).Ctemp.setText(result); // changing TextView
    }
}

回答7:

    /**
     * Background Async Task to Load all product by making HTTP Request
     * */
     public  static class updateTExtviewAsyncTask extends AsyncTask<String, String, String> {

        Context context;
        ProgressDialog pDialog;
        String id, name;

        String state_id;

        //--- Constructor for getting network id from asking method

        public  updateTExtviewAsyncTask(Context context,String id,String city) 
        {
            context   = context;
            state_id  = id;
            city_name = city;
        }       
        /* *
         * Before starting background thread Show Progress Dialog
         * */
        @Override
        protected void onPreExecute() 
        {
            super.onPreExecute();
            pDialog = ProgressDialog.show(context, "","Please wait...", true, true);
            pDialog.show();

        }

        /**
         * getting All products from url
         * */
        protected String doInBackground(String... args) 
        {
            return null;
        }

        /**
         * After completing background task Dismiss the progress dialog
         * **/
            protected void onPostExecute(String file_url)  {

                     YourClass.UpdateTextViewData("Textview data");
        }
    }

// place this code inside your activity class and also declare updating textview static

    public static void  UpdateTextViewData(String tvData) 
{
   tv.setText(tvData);
}