如何从函数onResponse of Retrofit返回值？

Question

I'm trying to return a value that i get from onResponse method in retrofit call request, is there a way that i can get that value out of the overrided method? here is my code:

public JSONArray RequestGR(LatLng start, LatLng end)
    {
       final JSONArray jsonArray_GR;

        EndpointInterface loginService = ServiceAuthGenerator.createService(EndpointInterface.class);    
        Call<GR> call = loginService.getroutedriver();
        call.enqueue(new Callback<GR>() {
            @Override
            public void onResponse(Response<GR> response , Retrofit retrofit)
            {

                 jsonArray_GR = response.body().getRoutes();
//i need to return this jsonArray_GR in my RequestGR method
            }
            @Override
            public void onFailure(Throwable t) {
            }
        });
        return jsonArray_GR;
    }

i can't get the value of jsonArray_GR because to be able to use it in onResponse method i need to declare it final and i can't give it a value.

Answer 1

The problem is you are trying to synchronously return the value of enqueue , but it is an asynchronous method using a callback so you can't do that. You have 2 options:

1. You can change your RequestGR method to accept a callback and then chain the enqueue callback to it. This is similar to mapping in frameworks like rxJava.

This would look roughly like:

public void RequestGR(LatLng start, LatLng end, final Callback<JSONArray> arrayCallback)
    {

        EndpointInterface loginService = ServiceAuthGenerator.createService(EndpointInterface.class);    
        Call<GR> call = loginService.getroutedriver();
        call.enqueue(new Callback<GR>() {
            @Override
            public void onResponse(Response<GR> response , Retrofit retrofit)
            {

                 JSONArray jsonArray_GR = response.body().getRoutes();
                 arrayCallback.onResponse(jsonArray_GR);
            }
            @Override
            public void onFailure(Throwable t) {
               // error handling? arrayCallback.onFailure(t)?
            }
        });
    }

The caveat with this approach is it just pushes the async stuff up another level, which might be a problem for you.

2. You can use an object similar to a BlockingQueue , Promise or an Observable or even your own container object (be careful to be thread safe) that allows you to check and set the value.

This would look like:

public BlockingQueue<JSONArray> RequestGR(LatLng start, LatLng end)
    {
        // You can create a final container object outside of your callback and then pass in your value to it from inside the callback.
        final BlockingQueue<JSONArray> blockingQueue = new ArrayBlockingQueue<>(1);
        EndpointInterface loginService = ServiceAuthGenerator.createService(EndpointInterface.class);    
        Call<GR> call = loginService.getroutedriver();
        call.enqueue(new Callback<GR>() {
            @Override
            public void onResponse(Response<GR> response , Retrofit retrofit)
            {

                 JSONArray jsonArray_GR = response.body().getRoutes();
                 blockingQueue.add(jsonArray_GR);
            }
            @Override
            public void onFailure(Throwable t) {
            }
        });
        return blockingQueue;
    }

You can then synchronously wait for your result in your calling method like this:

BlockingQueue<JSONArray> result = RequestGR(42,42);
JSONArray value = result.take(); // this will block your thread

I would highly suggest reading up on a framework like rxJava though.