Add Dependecies

Add the following code into the dependencies block in your build.gradle

compile 'io.reactivex:rxkotlin:0.60.0'
compile 'io.reactivex:rxandroid:1.2.1'
compile 'com.squareup.retrofit2:retrofit:2.1.0'
compile 'com.squareup.retrofit2:adapter-rxjava:2.1.0'
compile 'com.squareup.retrofit2:converter-gson:2.1.0'
compile 'com.orhanobut:logger:1.15'

Add internet permission for you application

Open you AndroidManifest.xml file and add the following line above your applicaiton tag.

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

Create the service interface

interface ApiService {
    @GET("/ip")
    fun ip(): Observable<Ip>
}

Create model

data class Ip(val origin: String)

Create Retrofit client

class RestClient {
    val BASE_URL = "http://httpbin.org"

    fun createApi(): ApiService {
        val retrofit = Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addCallAdapterFactory(RxJavaCallAdapterFactory.create())
                .addConverterFactory(GsonConverterFactory.create())
                .build()
        return retrofit.create(ApiService::class.java)
    }
}

Integrate code into your activity

Init ApiService in your class

	 class MainActivity : AppCompatActivity() {
	+    lateinit var apiService: ApiService
	+
	+    private val subscriptions = CompositeSubscription()
	 
	     override fun onCreate(savedInstanceState: Bundle?) {
	         super.onCreate(savedInstanceState)
	@@ -27,25 +32,21 @@ class MainActivity : AppCompatActivity() {
	             Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
	                     .setAction("Action", null).show()
	         }
	+        apiService = RestClient().createApi()
	     } 

Update the code in your onStart and onStop method.

    override fun onStart() {
        super.onStart()
        subscriptions.add(apiService.ip().subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe {
                    Logger.d(it)
                })
    }

    override fun onStop() {
        super.onStop()
        subscriptions.clear()
    }

Run you application you will see the message on console

02-16 15:03:32.529 15858-15858/mingsin.androidkotlinexample D/PRETTYLOGGER: ╔════════════════════════════════════════════════════════════════════════════════════════
02-16 15:03:32.530 15858-15858/mingsin.androidkotlinexample D/PRETTYLOGGER: ║ Thread: main
02-16 15:03:32.530 15858-15858/mingsin.androidkotlinexample D/PRETTYLOGGER: ╟────────────────────────────────────────────────────────────────────────────────────────
02-16 15:03:32.530 15858-15858/mingsin.androidkotlinexample D/PRETTYLOGGER: ║ MainActivity$onStart$1.call  (MainActivity.kt:15)
02-16 15:03:32.530 15858-15858/mingsin.androidkotlinexample D/PRETTYLOGGER: ║    MainActivity$onStart$1.call  (MainActivity.kt:44)
02-16 15:03:32.530 15858-15858/mingsin.androidkotlinexample D/PRETTYLOGGER: ╟────────────────────────────────────────────────────────────────────────────────────────
02-16 15:03:32.530 15858-15858/mingsin.androidkotlinexample D/PRETTYLOGGER: ║ Ip(origin=20x.xx.xx.xx, 20x.xx.xx.xx)
02-16 15:03:32.530 15858-15858/mingsin.androidkotlinexample D/PRETTYLOGGER: ╚════════════════════════════════════════════════════════════════════════════════════════
02-16 15:03:33.025 15858-15858/mingsin.androidkotlinexample D/KeyEvent: obtain, mHwFlags=0

You can find the full version code from here