Android, can I put AsyncTask in a separate class and have a callback?

Question

I'm just learning about AsyncTask and want to use it as a separate class, rather then a subclass.

For example,

class inetloader extends AsyncTask<String, Void, String> {


    @Override
    protected String doInBackground(String... urls) {

        String response = "";

            DefaultHttpClient client = new DefaultHttpClient();
            HttpGet httpGet = new HttpGet(urls[0]);
            try {
                HttpResponse execute = client.execute(httpGet);
                InputStream content = execute.getEntity().getContent();

                BufferedReader buffer = new BufferedReader(
                        new InputStreamReader(content));
                String s = "";
                while ((s = buffer.readLine()) != null) {
                    response += s;
                }

            } catch (Exception e) {
                e.printStackTrace();
            }

        return response;
    }


    @Override
    protected void onPostExecute(String result) {
        Log.e("xx",result);
        // how do I pass this result back to the thread, that created me?
    }


}

and the main(ui) thread:

inetloader il = new inetloader();
il.execute("http://www.google.com");
//il.onResult()
//{
    ///do something...
//}

Thanks!

Yes you can put it in a separate class, but most people don't because its purpose is usually a lot more clear if it's an inner class. If you're going to reuse it in lots of activities, go ahead. — keyser, May 26 '12 at 17:46
I plan using it for different purposes, like if I call it from here, on result it should do that and that, if I call it from there, on result it should do something else... therefor if I'll keep it in the same class and have it call a function from that class on result, i'll have to make a new AsyncTask for every kind of action that it has to do onresult... — Roger Travis, May 26 '12 at 17:52
use interface, just implement your activity by a interface and in onPostExcute() of Asyn class call mContext.implementedMethod(). — Anand Tiwari, May 26 '12 at 17:54
Having it in a separate class also helps with version control & change management. — Kris Krause, Dec 29 '13 at 14:53
BTW pls start your class names with a capital letter e.g. "InetLoader" - just basic Java convention. — Richard Le Mesurier, Feb 03 '14 at 15:12

score 34 · Accepted Answer · answered May 26 '12 at 17:54

34

Use a interface. Something like:

interface CallBackListener{
  public void callback();
}

Then do this in your UI thread:

inetloader il = new inetloader();
li.setListener(this);
il.execute("http://www.google.com");

In inetloader, add:

   CallBackListener mListener;

   public void setListener(CallBackListener listener){
     mListener = listener;
   }

then In postExecute(), do:

   mListener.callback();

answered May 26 '12 at 17:54

Xi 张熹

10,492
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Thanks, sound like it's the right direction to go. Especially if I could create a CallBackListener with the inetloader il = new inetloader(); and pass it into the il object. – Roger Travis May 26 '12 at 18:26
Hmm... strange, I don't have a "CallBackListener" in android v3, maybe it goes by some other name? – Roger Travis May 26 '12 at 18:28
4

@RogerTravis Yeah you have to create a CallBackListener interface yourself. And don't forget to add "implements CallBackLisnter" for your main UI class. – Xi 张熹 May 26 '12 at 18:29
ouch, it seems to give just one @Override public void callback() { for the UI thread. Is there a way for me to make multiple callbacks(){} on for each inetloader object I make? – Roger Travis May 26 '12 at 18:33

score 3 · Answer 2 · edited May 26 '12 at 21:19

3

you can pass the activity instance to constructor and call activity function from there...
Like use interface :

public interface ResultUpdatable {

   public void setResult(Object obj);
   }

Implement this in the Activity and pass in the constructor of Async task and update the result from onPostExecute using setResult function.

edited May 26 '12 at 21:19

Daniel Fischer

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answered May 26 '12 at 17:53

Dheeresh Singh

15,643
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score 1 · Answer 3 · answered May 26 '12 at 17:43

1

inetloader il = new inetloader();
il.execute("http://www.google.com");

String result = il.get();//put it in try-catch
                ^^^^^^^^

here you get result which is in onPostExecute(String result)

answered May 26 '12 at 17:43

Samir Mangroliya

39,918
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Not really :( I need some sort of an onResult listener, that will trigger once the AsyncTask finished, because querying some servers might take a while. On the other hand, putting the il.get() into a while(true) loop might sound too primitive, won't it? :) are there any better options? – Roger Travis May 26 '12 at 17:48
2

@Samir : This is a really bad idea. Whilst it does allow retrieving a result from the `AsyncTask`, using the `get()` method will block the UI thread and effectively turns what should be an asynchronous procedure into a synchronous one. The only valid use for the `get()` method I have ever come up with would be for 'chaining' two `AsyncTasks`. – Squonk May 26 '12 at 18:18

Android, can I put AsyncTask in a separate class and have a callback?

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