How To Use Retrofit Tutorial in Kotlin

What is Retrofit?

Ohh! Such a tough definition

Let us simplify this – As an Android developer at one time you want your app to communicate with the internet, i.e receive something or send something to the internet. So to make this communication to send or receive data on the Internet we use Retrofit.

So Retrofit makes it easier to communicate with the Internet and it sends and receives data in JSON (JavaScript Object Notation)

Prerequisite

We are considering that you have basic knowledge of android and Kotlin and that’s Enough.

So Much Theory…

Retrofit Tutorial in Kotlin [Complete Code]

We will be building an Android app to display a list of superheroes in our App. Here is an example

Before getting started we should know that we make mainly four types of request on RESTful API.

GET Request ( For Receiving data )
POST Request ( For Sending data )
PUT Request ( For Updating data )
DELETE Request ( For Deleting data )

In this example, we will be using the Get request.

We will be using this API

https://simplifiedcoding.net/demos/marvel/

Creating a new Project

Let’s first create a new Android Studio Project. I have created a project named RetrofitExample.
Once the project is created we need to add the following two libraries to our project.
- Retrofit -> For the network calls
- Gson -> For easy parsing of JSON data

// Firstly add these dependencies change these according to the latest version
dependencies {
...
// Retrofit
    implementation 'com.squareup.retrofit2:retrofit:2.5.0'

    // GSON
    implementation 'com.squareup.retrofit2:converter-gson:2.5.0'
}

We also need Internet Permission so we will declare that in Manifest File.

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

In Retrofit we use annotation or ‘@’ Symbol to denote the type of request

Create a new Kotlin Interface file of name superheroAPI

interface superheroAPI {
   @GET("marvel") // making get request at marvel end-point
    fun getHeroes(): Call<List<Heros?>?>?
}

In the above code, we are making a GET request to the API and the “marvel” is the End-Point.

Then we are declaring a function getHeroes() which will return a List of Hero.

We will face an error because we have not declared our Heros data class, So let’s make it.

// Create a new Kotlin data class of name Heros

data class Heros(var name: String, var realname: String, var team: String, var firstappearance: String, var createdby: String, var publisher: String, var imageurl: String, var bio: String)

Make sure your variable names match the JSON attributes. As we have the following JSON objects in our response.

{
name: "Captain America",
realname: "Steve Rogers",
team: "Avengers",
firstappearance: "1941",
createdby: "Joe Simon",
publisher: "Marvel Comics",
imageurl: "https://www.simplifiedcoding.net/demos/marvel/captainamerica.jpg",
bio: " Steven Rogers was born in the Lower East Side..... "
}

We can see that the data class contain the same name as of the API.

Now we will make the API call

In activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.jayant.Retrofit.MainActivity">
 
    <ListView
        android:id="@+id/listView"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
 
</RelativeLayout>

In MainActivity.kt

// Explaination below
class MainActivity : AppCompatActivity() {
      override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_dashboard){
      
        var retrofit = Retrofit.Builder()
                .baseUrl("https://simplifiedcoding.net/demos/")
                .addConverterFactory(GsonConverterFactory.create()) 
                .build();  // Part 1

         val api = retrofit.create(superheroAPI::class.java); //Part 2

          api.getHeroes().enqueue(object : Callback<List<Hero?>?>() {
            fun onResponse(call: Call<List<Hero?>?>?,
                response: Response<List<Hero?>?>) {
                val heroList: List<Hero> = response.body() // Now we can use 
                this heroList for getting the data from API. 

                //Creating an String array for the ListView
                val heroes = arrayOfNulls<String>(heroList.size)

                //looping through all the heroes and inserting the names 
                  inside the string array
                for (i in heroList.indices) {
                    heroes[i] = heroList[i].name
                }


                //displaying the string array into listview
                listView.setAdapter(
                    ArrayAdapter(
                        applicationContext,
                        android.R.layout.simple_list_item_1,
                        heroes
                    )
                ) //Part Extra
            }

            fun onFailure(call: Call<List<Hero?>?>?, t: Throwable) {
                Toast.makeText(applicationContext, t.message, 
                Toast.LENGTH_SHORT).show()
            }
        }) //Part 3

      }
}

Explaination of different Parts of MainActivity.kt

Part 1

We are creating an instance of the Retrofit Builder class.
Then we are adding the base URL i.e. “https://simplifiedcoding.net/demos/”.
Then adding the converter factory as a Gson converter factory( It converts Java/kotlin objects to JSON and vice versa).
And at last, we are calling the build method.

Part 2

We are calling the create method for the interface that we created earlier.

Part 3

Firstly we are calling the getHeros() function from the interface
Then calling the enqueue function which will override two methods – onResponse and onFailure
In onResponse, we will write up for the response using the ‘response.body()’ method
In onFailure, we will handle up for any failure like – There is no Internet.

Part Extra

We are showing the response in form of a list so we are creating an adapter for the list.

And That’s all Folks !!