DirsAndNetworkAvailabilityImpl.java

public class CustomApplication extends Application {

    @Override
    public void onCreate() {
        super.onCreate();
        // ...
    }

    private static class DirsImpl implements Dirs {
        private WeakReference<Context> ctxRef;
        DirsImpl(Context ctx) {
            ctxRef = new WeakReference<Context>(ctx);
        }
        @Override
        public File getCacheDir() {
            Context ctx = null;
            if ((ctx=ctxRef.get())!=null)
                return ctx.getCacheDir();
            else return null;
        }
    }

    private static class NetAvailability implements NetworkAvailability {
        private WeakReference<Context> ctxRef;
        NetAvailability(Context ctx) {
            ctxRef = new WeakReference<Context>(ctx);
        }
        @Override
        public boolean check() {
            Context ctx = null;
            if ((ctx=ctxRef.get())!=null) {
                ConnectivityManager connectivityManager =
                        (ConnectivityManager) ctx.getSystemService(Context.CONNECTIVITY_SERVICE);
                NetworkInfo networkInfo = connectivityManager.getActiveNetworkInfo();
                return networkInfo != null && networkInfo.isConnectedOrConnecting();
            } else {
                return false;
            }
        }
    }
}

RetrofitOkHttpCached.java

    public void buildRetrofit(String apiUrl,
                                 final NetworkAvailability availability,
                                 Dirs dirs) {

        // https://github.com/square/retrofit/issues/1259
        OkHttpClient.Builder okHttpClientBuilder = new OkHttpClient().newBuilder();
        okHttpClientBuilder.cache(new Cache(dirs.getCacheDir(), MAX_CACHE_SIZE))
                            .addInterceptor(new ApplicationOkHttpInterceptor(availability))
                            .addNetworkInterceptor(new NetworkOkHttpInterceptor());
        OkHttpClient okHttpClient = okHttpClientBuilder.build();

        Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(apiUrl)
                .callFactory(okHttpClient)
                .build();
        aRestAPI = retrofit.create(RestAPI.class);
    }

    private static final int MAX_AGE = 3600;
    private static final int MAX_STALE = 3600 * 48;
    private static final int MAX_CACHE_SIZE = 10*1024*1024; // 10MB

    private static class ApplicationOkHttpInterceptor implements Interceptor {
        private final NetworkAvailability availability;

        public ApplicationOkHttpInterceptor(NetworkAvailability availability) {
            this.availability = availability;
        }

        @Override
        public Response intercept(Chain chain) throws IOException {
            Request request = chain.request();
            if (availability.check()) {
                request = request.newBuilder().header("Cache-Control", "public, max-age=" + MAX_AGE).build();
            } else {
                request = request.newBuilder().header("Cache-Control", "public, only-if-cached, max-stale=" + MAX_STALE).build();
            }
            return chain.proceed(request);
        }
    }

    private static class NetworkOkHttpInterceptor implements Interceptor {
        @Override
        public Response intercept(Chain chain) throws IOException {
            Response response = chain.proceed(chain.request());
            response = response.newBuilder().header("Cache-Control", "public, max-age=" + MAX_AGE).build();
            return response;
        }
    }