How to implement a listener

Question

I have a design issue. I need to implement a listener. I saw the following SO question: How to create our own Listener interface in android?

But in the link it provides in the answer, author creates a listener which just extends the system-defined listener. E.g onClick, you would do some validation & then call another method called "whenValidatedListener"

I need to define listeners which are not linked to existing event listeners. Basically there would be some processing going on in native(C/C++) code & in the Android code I need a listener to respond to certain messages from it.

I think I could do this using handlers. But AsyncTask is the recommended approach for multithreading.

Is there a way to implement a user-defined-listener using AsyncTask?

score 52 · Accepted Answer · edited Jun 21 '13 at 18:37

52

AsyncTask has nothing to do with implementing a listener.

Here's a listener:

public interface TheListener {
    public void somethingHappened();
}

Call it however you want. For example, here's a class doing something like View:

public class Something {
    private TheListener mTheListener;

    public void setTheListener(TheListener listen) {
        mTheListener = listen;
    }

    private void reportSomethingChanged() {
        if (mTheListener != null) {
            mTheListener.somethingHappened();
        }
    }
}

You can make this as complicated as you want. For example, instead of a single listener pointer you could have an ArrayList to allow multiple listeners to be registered.

Calling this from native code also has nothing to do with implementing a listener interface. You just need to learn about JNI to learn how native code can interact with Java language code.

edited Jun 21 '13 at 18:37

Brandon Romano

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answered May 09 '11 at 20:27

hackbod

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3

Hi, I'm a little confused as to what you set the listener to in setTheListner() – Jon Wells Nov 24 '11 at 10:25
1

The class you want to be told about something happening implements the TheListener interface. You pass in the object implementing that interface to setTheListener(). – hackbod Nov 28 '11 at 03:27
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@hackbod Should I call `new Something().setTheListener(this)` in the Class which implements **TheListener**? – Archie.bpgc Jul 04 '13 at 08:16
1

wow im confused...where does the definition for somethingHappened() go?? – mafiOSo Nov 15 '13 at 12:07

score 5 · Answer 2 · edited Apr 09 '18 at 06:08

5

Just to clear things up;

you do exactly what @hackbod said and add this :

the activity which encloses the class (with the method setListener(Listener listen)), implements Listener and in it's oncreate or onResume or where-ever you call yourClass.setListener(this).

edited Apr 09 '18 at 06:08

MHSaffari

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answered Feb 21 '14 at 23:44

MacD

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How to implement a listener

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