Retrofit — Synchronous and Asynchronous Requests

Retrofit — Synchronous and Asynchronous Requests

Within the previously published tutorials, we walked you through the setup of Retrofit. This tutorial shows you how to perform the actual requests in either a synchronous or a asynchronous way.

Retrofit Series Overview

  1. Introduction to Call Adapters (Coming soon)
  2. Custom Call Adapter to Separate OnResponse Callback (Coming soon)
  3. How to Integrate RxJava 1.x Call Adapter (Coming soon)
  4. How to Integrate RxJava 2.x Call Adapter (Coming soon)
  5. How to Integrate Guava Call Adapter (Coming soon)
  6. Custom Call Adapter to Separate Network and Gson Errors (Coming soon)
  1. Callbacks (Coming soon)
  2. Annotations (Coming soon)
  3. Fluent Interface with Builders (Coming soon)

TL;DR
Retrofit supports synchronous and asynchronous request execution. Users define the concrete execution by setting a return type (synchronous) or not (asynchronous) to service methods.

Synchronous Requests

Synchronous requests with Retrofit 1.9 are declared by defining a return type. The example below expects a list of tasks as the response when executing the method getTasks.

Retrofit 2

public interface TaskService {  
    @GET("/tasks")
    Call<List<Task>> getTasks();
}

Retrofit 1.9

public interface TaskService {  
    @GET("/tasks")
    List<Task> getTasks();
}

Within Retrofit 2, every request is wrapped into a Call object. The actual synchronous or asynchronous request is executed differently using the desired method on a later created call object. However, the interface definition for synchronous and asynchronous requests are the same within Retrofit 2.

Synchronous methods are executed on the main thread. That means the UI blocks during request execution and no interaction is possible for this period.

Synchronous methods provide the ability to use the return value directly, because the operation blocks everything else during your network request.

For non-blocking UI, you have to handle the request execution in a separated thread by yourself. That means, you can still interact with the app itself while waiting for the response.

Get Results from Synchronous Requests

Let’s move to the point where we execute the actual request. This behavior changed from Retrofit v1 to v2. The following code snippets illustrate the synchronous request execution with Retrofit and assume that you’re familiar with the ServiceGenerator class introduced in the tutorial on how to create a sustainable Android client.

Retrofit 2

TaskService taskService = ServiceGenerator.createService(TaskService.class);  
Call<List<Task>> call = taskService.getTasks();  
List<Task>> tasks = call.execute().body();  

Retrofit 1.9

TaskService taskService = ServiceGenerator.createService(TaskService.class);  
List<Task> tasks = taskService.getTasks();  

Using the .execute() method on a call object will perform the synchronous request in Retrofit 2. The deserialized response body is available via the .body() method on the response object.

Asynchronous Requests

Additionally to synchronous calls, Retrofit supports asynchronous requests out of the box. Asynchronous requests in Retrofit 1.9 don’t have a return type. Instead, the defined method requires a typed callback as last method parameter.

Retrofit 2

public interface TaskService {  
    @GET("/tasks")
    Call<List<Task>> getTasks();
}

Retrofit 1.9

public interface TaskService {  
    @GET("/tasks")
    void getTasks(Callback<List<Task>> cb);
}

Retrofit performs and handles the method execution in a separated thread. The Callback class is generic and maps your defined return type. Our example returns a list of tasks and the Callback does the mapping internally.

As already mentioned above: the interface definition in Retrofit 2 is the same for synchronous and asynchronous requests. The desired return type is encapsulated into a Call object and the actual request execution defines its type (synchronous/asynchronous).

Get Results from Asynchronous Requests

Using asynchronous requests forces you to implement a Callback with its two callback methods: success and failure. When calling the asynchronous getTasks() method from a service class, you have to implement a new Callback and define what should be done once the request finishes. The following code snippet illustrates an exemplary implementation.

Retrofit 2

TaskService taskService = ServiceGenerator.createService(TaskService.class);  
Call<List<Task>> call = taskService.getTasks();  
call.enqueue(new Callback<List<Task>>() {  
    @Override
    public void onResponse(Call<List<Task>> call, Response<List<Task>> response) {
        if (response.isSuccessful()) {
            // tasks available
        } else {
            // error response, no access to resource?
        }
    }

    @Override
    public void onFailure(Call<List<Task>> call, Throwable t) {
        // something went completely south (like no internet connection)
        Log.d("Error", t.getMessage());
    }
}

Retrofit 1.9

TaskService taskService = ServiceGenerator.createService(TaskService.class);  
taskService.getTasks(new Callback<List<Task>>() {  
    @Override
    public void success(List<Task> tasks, Response response) {
        // here you do stuff with returned tasks
    }

    @Override
    public void failure(RetrofitError error) {
        // you should handle errors, too
    }
});

Get Raw HTTP Response

In case you need the raw HTTP response object, just define it as the method’s return type. The Response class applies to both methods like any other class.

Retrofit 2

The way you can receive the raw response body in Retrofit 2 has change to v1 the same way you define the request type (sync/async). Namely, you don’t need to define the Response class as the return type, but you can grab it within the onResponse() callback method. Let’s look at the following code snippet to illustrate how you can get the raw response:

call.enqueue(new Callback<List<Task>>() {  
    @Override
    public void onResponse(Call<List<Task>> call, Response<List<Task>> response) {
        // get raw response
        Response raw = response.raw();
    }

    @Override
    public void onFailure(Call<List<Task>> call, Throwable t) {}
}

Retrofit 1.9

// synchronous
@GET("/tasks")
Response getTasks();

// asynchronous
@GET("/tasks")
void getTasks(Callback<Response> cb);  

Happy coding! If you run into problems or need help, please find us on Twitter @futurestud_io.


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