Building a library that uses Retrofit internally,

I'm trying to build a library that basically wraps our api. Basically, the structure im going for is something like this:

MySDK mySDK = new MySDK("username", "password");

mySDK.getPlaylistInfo("3423", 2323, new CustomCallback<>(){

//on response
//on failure

});

So with vanilla Retrofit, an api call usually looks something like the following:

ApiService api = retrofit.create(ApiService.class);
Call<Response> call = api.getPlaylistInfo()
call.enqueue(new Callback<Response>() {
    @Override
    public void onResponse(Call<Response> call, Response<Response> response) {
        //handle response
    }

    @Override
    public void onFailure(Call<Response> call, Throwable t) {
        //handle failure
    }

});

Basically, how would I wrap retrofits callback system into my own? Note, the reason for needing to do this is to preprocess the data returned from the api before delivering the final response.

I've written something similar so it might help you getting started, this follows an implementation I'v written for Volley, and re-used when I migrated to Retrofit2 so it resembles it (this SO question).

Create a global object (what you would refer to as MySDK) as a singelton class that handles your requests:

create a singleton class, which you instatiate when you're application comes up:

public class NetworkManager
{
    private static final String TAG = "NetworkManager";
    private static NetworkManager instance = null;

    private static final String prefixURL = "http://some/url/prefix/";

    //for Retrofit API
    private Retrofit retrofit;
    private ServicesApi serviceCaller;

    private NetworkManager(Context context)
    {
        retrofit = new Retrofit.Builder().baseUrl(prefixURL).build();
        serviceCaller = retrofit.create(ServicesApi.class);
        //other stuf if you need
    }

    public static synchronized NetworkManager getInstance(Context context)
    {
        if (null == instance)
            instance = new NetworkManager(context);
        return instance;
    }

    //this is so you don't need to pass context each time
    public static synchronized NetworkManager getInstance()
    {
        if (null == instance)
        {
            throw new IllegalStateException(NetworkManager.class.getSimpleName() +
                    " is not initialized, call getInstance(...) first");
        }
        return instance;
    }

    public void somePostRequestReturningString(Object param1, final SomeCustomListener<String> listener)
    {
        String url = prefixURL + "this/request/suffix";

        Map<String, Object> jsonParams = new HashMap<>();
        jsonParams.put("param1", param1);

        Call<ResponseBody> response;
        RequestBody body;

        body = RequestBody.create(okhttp3.MediaType.parse(JSON_UTF), (new JSONObject(jsonParams)).toString());
        response = serviceCaller.thePostMethodYouWant("someUrlSufix", body);

        response.enqueue(new Callback<ResponseBody>()
        {
            @Override
            public void onResponse(Call<ResponseBody> call, retrofit2.Response<ResponseBody> rawResponse)
            {
                try
                {
                  String response = rawResponse.body().string();

                  // do what you want with it and based on that...
                  //return it to who called this method
                  listener.getResult("someResultString");
                }
                catch (Exception e)
                {
                 e.printStackTrace();
                 listener.getResult("Error1...");
                }
            }  
            @Override
            public void onFailure(Call<ResponseBody> call, Throwable throwable)
            {
                try
                {
                 // do something else in case of an error
                 listener.getResult("Error2...");
                }
                catch (Exception e)
                {
                 throwable.printStackTrace();
                 listener.getResult("Error3...");
                }
            }
        });
    }

    public void someGetRequestReturningString(Object param1, final SomeCustomListener<String> listener)
    {
        // you need it all to be strings, lets say id is an int and name is a string
        Call<ResponseBody> response = serviceCaller.theGetMethodYouWant
            (String.valueOf(param1.getUserId()), param1.getUserName());

        response.enqueue(new Callback<ResponseBody>()
        {
            @Override
            public void onResponse(Call<ResponseBody> call, retrofit2.Response<ResponseBody> rawResponse)
            {
                try
                {
                  String response = rawResponse.body().string();

                  // do what you want with it and based on that...
                  //return it to who called this method
                  listener.getResult("someResultString");
                }
                catch (Exception e)
                {
                 e.printStackTrace();
                 listener.getResult("Error1...");
                }
            }  
            @Override
            public void onFailure(Call<ResponseBody> call, Throwable throwable)
            {
                try
                {
                // do something else in case of an error
                 listener.getResult("Error2...");
                }
                catch (Exception e)
                {
                 throwable.printStackTrace();
                 listener.getResult("Error3...");
                }
            }
        });
    }
}

This works with your interface (example with POST and GET request, GET could be without params):

 public interface BelongServicesApi
 {
    @POST("rest/of/suffix/{lastpart}") // with dynamic suffix example
    Call<ResponseBody> thePostMethodYouWant(@Path("lastpart") String suffix, @Body RequestBody params);

    @GET("rest/of/suffix") // with a fixed suffix example
    Call<ResponseBody> theGetMethodYouWant(@Query("userid") String userid, @Query("username") String username);
 }

when your application comes up:

public class MyApplication extends Application
{
  //...

    @Override
    public void onCreate()
    {
        super.onCreate();
        NetworkManager.getInstance(this);
    }

 //...

}

a simple listener interface for your callback (seperate file would do good):

public interface SomeCustomListener<T>
{
    public void getResult(T object);
}

and finally, from wherever you want, the context is already in there, just call:

public class BlaBla
{
    //.....

        public void someMethod()
        {
            //use the POST or GET 
            NetworkManager.getInstance().somePostRequestReturningString(someObject, new SomeCustomListener<String>()
            {
                @Override
                public void getResult(String result)
                {
                    if (!result.isEmpty())
                    {
                     //do what you need with the result...
                    }
                }
            });
        }
}

you can use any object with the listener, just parse the response string to a corresponding object, depending on what you need to receive and you can call that from everywhere (onClicks, etc.), just remember the objects need to match between methods.

Hope this Helps!