Implementing method callback in Android

Question

Currently in my project, I am making Http requests and I want different http response to be sent to different callback methods.

I wrote a quick sample below to show what I want to do. I know it probably wont be possible the way i want it, but are there any clean solutions to achieve the same thing?

Sample:

Activity Class:

public class Main extends Activity{  
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Services service = new Services();
        service.login("user", "password", **onLoginComplete()** );
    }

    public void onLoginComplete(String HTTPResponse){
        // Do something with the response
    }
}

Service Class:

public class Services{  

    public void login(String user, String password, CALLBACK){
        Request request = createLoginRequest(user, password);
        sendRequest(request, CALLBACK);
    }

    public class sendRequest extends AsyncTask{
        @Override
        protected Object doInBackground(Object... params) {
             // Do Http Request
             // Get Response
             CALLBACK(response);
        } 
    }
}

Answer 1

I think I had the same problem as yours.

I was looking for a good answer and this was the implementation that worked for me:

First create an interface which contains your methods; in my case I use the typical onSuccess and onFailure but you can make your own methods:

//MyInterface.java

public interface MyInterface
{
    void onSuccess(String response);
    void onFailure(String response);
}

Then create class Services:

//Services.java

public class Services
{
    public void login(String user, String password, MyInterface myInterface)
    {
        Request request = createLoginRequest(user, password);

        if(/*Request successful*/)
            myInterface.onSuccess("Login succesful");

        else
            myInterface.onFailure("Login failed");
    }
}

And finally call the method on your Activity:

//Main.java

public class Main extends Activity
{  
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Services service = new Services();
        service.login("user", "password", new Myinterface()
        {
            @Override
            public void onSuccess(String response)
            {
                Toast.makeText(getApplicationContext(), response, Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onFailure(String response)
            {
                Toast.makeText(getApplicationContext(), response, Toast.LENGTH_SHORT).show();
            }
        });
    }
}

Answer 2

If I understand you correctly, what you are trying to achieve is recommended to be done via the use of an AsyncTask. It's explained in a very simple way here http://developer.android.com/reference/android/os/AsyncTask.html

Additionally, I share an example of how I execute (doInBackground) GET requests to a site and which result then I read (onPostExecute)... hope it helps!

protected InputStream doInBackground(String... example) {
    JsonComm jc = new JsonComm();
    String baseUrl = "http://www.somewhere.com/get_request.php?data=";
    String jcString = jc.encodeJSON("nowyou","seeme");
    String url = "";

    try {
        url = baseUrl + URLEncoder.encode(jcString, "UTF-8");
    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    }

    HttpResponse response;
    HttpClient httpClient = new DefaultHttpClient();
    HttpGet request = new HttpGet(url);
    try {
        request.setHeader("Accept", "application/json");
        request.setHeader("Content-type", "application/json");
        response = httpClient.execute(request);
    } catch (Exception ex) {
        // handle exception here
    }finally {
        httpClient.getConnectionManager().shutdown();
    }
    return response.getEntity().getContent();
}

protected void onPostExecute(InputStream is) {
    BufferedReader reader = new BufferedReader(new InputStreamReader(is));
    StringBuilder sb = new StringBuilder();

    String line = null;
    try {
        while ((line = reader.readLine()) != null) {
            sb.append(line + "\n");
        }
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        try {
            is.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
     System.out.println(sb.toString());

}

Answer 3

How to implement callbacks in java:

public interface SomeCallbackInterface {
    public void finished(Request req);
}

Then in your class you do:

YourReqeust.instantiateWithCallback(new SomeCallbackInterface() {
   @Override
   public void finished(Request req){
      // do something here
   }
});

This pretty much the same thing your do with any View.OnClickListener

Answer 4

There are several ways you can handle this. You could pass in a Runnable for your callback or you could provide a method to override in your Services class and use an anonymous class derived from Services in your main activity. If you need to pass in a parameter, you could also define an interface for something equivalent to the Runnable where you can define a method with a response parameter.

Answer 5

interface OnLoginCompleteListener {
    void onLoginComplete(String response);
}

And then

public void login(String user, String password, OnLoginComplete listener) {
    mOnCompleteListener = listener;
}

and

protected Object doInBackground(Object... params) {
    mOnCompleteListener.onLoginComplete(response);
}

and finally

service.login("user", "password", new OnLoginCompleteListener() {
    public void onLoginComplete(String response) {
        // Handle your response
    }
});