Loop Cupcakes. Network call with Retrofit

AndroidManifest.xml

<uses-permission android:name="android.permission.INTERNET" />

build.gradle

// build.gradle (:app)

android {
  //..
  compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
  }
}

dependencies {
  //.. https://square.github.io/retrofit/
  implementation 'com.squareup.retrofit2:retrofit:2.9.0'
  implementation 'com.squareup.retrofit2:converter-moshi:2.9.0'
}

MainActivity.kt

class MainActivity : AppCompatActivity() {
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    /* Creates an instance of the UserService using a simple Retrofit builder using Moshi 
     * as a JSON converter, this will append the endpoints set on the UserService interface
     * (for example '/api', '/api?results=2') with the base URL set here, resulting on the 
     * full URL that will be called: 'https://randomuser.me/api' */
    val service = Retrofit.Builder()
      .baseUrl("https://randomuser.me/")
      .addConverterFactory(MoshiConverterFactory.create())
      .build()
      .create(UserService::class.java)

    /* Calls the endpoint set on getUsers (/api) from UserService using enqueue method 
     * that creates a new worker thread to make the HTTP call */
    service.getUsers().enqueue(object : Callback<UserResponse> {

      /* The HTTP call failed. This method is run on the main thread */
      override fun onFailure(call: Call<UserResponse>, t: Throwable) {
        Log.d("TAG_", "An error happened!")
        t.printStackTrace()
      }

      /* The HTTP call was successful, we should still check status code and response body
       * on a production app. This method is run on the main thread */
      override fun onResponse(call: Call<UserResponse>, response: Response<UserResponse>) {
        /* This will print the response of the network call to the Logcat */
        Log.d("TAG_", response.body())
      }
    })
  }
}

/* Kotlin data/model classes that map the JSON response, we could also add Moshi 
 * annotations to help the compiler with the mappings on a production app */
data class UserResponse(val results: List<User>)
data class User(val email: String, val phone: String)

/* Retrofit service that maps the different endpoints on the API, you'd create one
 * method per endpoint, and use the @Path, @Query and other annotations to customize 
 * these at runtime */
interface UserService {
  @GET("/api")
  fun getUsers(): Call<UserResponse>
}