Most mobile applications nowadays fetch data from remote APIs. When building a similar application in Android, you might wonder what library to use for this purpose. One popular option you might want to consider is Retrofit.

In this Note, I'll show the most basic example of using Retrofit in a newly created Android (Kotlin) project.

We will be making a simple app that displays a list of todos taken from this API. Follow the instructions below to start.

1. Create a new Android project with Empty Activity. Make sure to set the language to Kotlin and enable Use androidx.* artifacts option.
2. Import Retrofit and its complementary libraries (Gson and Gson converter) to the project by modifying the module-level build.gradle like this:
   

   dependencies {
       ... other dependencies
   
       implementation 'com.squareup.retrofit2:retrofit:2.6.0'
       implementation 'com.squareup.retrofit2:converter-gson:2.6.0'
       implementation 'com.google.code.gson:gson:2.8.5'
   }​

   When writing this, the latest Retrofit version is 2.6.0. Make sure to check the latest version here and always use the latest version. The same applies to the rest of the libraries.

3. Add permission to use the Internet in AndroidManifest.xml:
   

   <?xml version="1.0" encoding="utf-8"?>
   <manifest xmlns:android="http://schemas.android.com/apk/res/android"
             package="com.hanmajid.kotlintutorialboilerplate">
   
       <uses-permission android:name="android.permission.INTERNET"/>
   
       <application
               android:allowBackup="true"
               android:icon="@mipmap/ic_launcher"
               android:label="@string/app_name"
               android:roundIcon="@mipmap/ic_launcher_round"
               android:supportsRtl="true"
               android:theme="@style/AppTheme">
           <activity android:name=".MainActivity">
               <intent-filter>
                   <action android:name="android.intent.action.MAIN"/>
   
                   <category android:name="android.intent.category.LAUNCHER"/>
               </intent-filter>
           </activity>
       </application>
   
   </manifest>​​

4. Create an XML layout for todo item named todo_item.xml like this:
   

   <?xml version="1.0" encoding="utf-8"?>
   <androidx.constraintlayout.widget.ConstraintLayout 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="wrap_content">
   
       <TextView
               android:layout_width="wrap_content"
               android:layout_height="wrap_content"
               tools:text="Title"
               android:id="@+id/title" app:layout_constraintTop_toTopOf="parent"
               android:layout_marginTop="16dp" app:layout_constraintStart_toStartOf="parent"
               android:layout_marginLeft="16dp" android:layout_marginStart="16dp"
               android:textSize="24sp"/>
       <TextView
               tools:text="Completed"
               android:layout_width="wrap_content"
               android:layout_height="wrap_content" android:id="@+id/completed"
               android:layout_marginTop="8dp"
               app:layout_constraintTop_toBottomOf="@+id/title"
               app:layout_constraintStart_toStartOf="@+id/title"
       />
       <View
               android:id="@+id/divider"
               android:layout_width="match_parent"
               android:layout_height="1dp"
               android:background="?android:attr/listDivider"
               tools:layout_editor_absoluteX="-16dp" app:layout_constraintBottom_toBottomOf="parent"
               app:layout_constraintTop_toBottomOf="@+id/completed" android:layout_marginTop="16dp"
       />
   </androidx.constraintlayout.widget.ConstraintLayout>​

   The layout will look like this:

   
   
   
5. Change the content of activity_main.xml to be like this:
   

   <?xml version="1.0" encoding="utf-8"?>
   <androidx.constraintlayout.widget.ConstraintLayout
           xmlns:android="http://schemas.android.com/apk/res/android"
           xmlns:tools="http://schemas.android.com/tools"
           xmlns:app="http://schemas.android.com/apk/res-auto"
           android:layout_width="match_parent"
           android:layout_height="match_parent"
           tools:context=".MainActivity">
       <androidx.recyclerview.widget.RecyclerView
               tools:listitem="@layout/todo_item"
               android:layout_width="match_parent"
               android:layout_height="match_parent" android:id="@+id/recycler_view"/>
   </androidx.constraintlayout.widget.ConstraintLayout>​

   Here's what activity_main.xml looks like now:
   

6. For simplicity's sake, the rest of the code will be declared inside MainActivity.kt. In a real application, you might want to separate every new class (Todo, TodoAdapter, TodoService) into a new file.
   Here's the content of the MainActivity.kt:
   

   package com.hanmajid.kotlintutorialboilerplate
   
   import android.os.Bundle
   import android.view.LayoutInflater
   import android.view.View
   import android.view.ViewGroup
   import android.widget.TextView
   import androidx.appcompat.app.AppCompatActivity
   import androidx.recyclerview.widget.LinearLayoutManager
   import androidx.recyclerview.widget.RecyclerView
   import retrofit2.Call
   import retrofit2.Callback
   import retrofit2.Response
   import retrofit2.Retrofit
   import retrofit2.converter.gson.GsonConverterFactory
   import retrofit2.http.GET
   
   class MainActivity : AppCompatActivity() {
       private lateinit var recyclerView: RecyclerView
       private var data: ArrayList<Todo> = ArrayList()
       private lateinit var adapter: RecyclerView.Adapter<*>
       private lateinit var manager: RecyclerView.LayoutManager
   
       override fun onCreate(savedInstanceState: Bundle?) {
           super.onCreate(savedInstanceState)
           setContentView(R.layout.activity_main)
   
           var retrofit: Retrofit = Retrofit.Builder()
               .baseUrl("https://jsonplaceholder.typicode.com")
               .addConverterFactory(GsonConverterFactory.create())
               .build()
   
           manager = LinearLayoutManager(this)
           adapter = TodoAdapter(data)
   
           recyclerView = findViewById(R.id.recycler_view)
           recyclerView.layoutManager = manager
           recyclerView.adapter = adapter
   
           val todoService: TodoService = retrofit.create(TodoService::class.java)
           val list = todoService.listTodos()
           list.enqueue(object: Callback<List<Todo>> {
               override fun onFailure(call: Call<List<Todo>>, t: Throwable) {
   
               }
   
               override fun onResponse(call: Call<List<Todo>>, response: Response<List<Todo>>) {
                   if(response.code() === 200) {
                       data.addAll(response.body()!!)
                       recyclerView.adapter!!.notifyDataSetChanged()
                   }
               }
   
           })
       }
   }
   
   class TodoAdapter(private var data: List<Todo>): RecyclerView.Adapter<TodoAdapter.ViewHolder>() {
   
       class ViewHolder(view: View): RecyclerView.ViewHolder(view) {
           var title: TextView = view.findViewById(R.id.title)
           var completed: TextView = view.findViewById(R.id.completed)
       }
   
       override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
           return ViewHolder(
               LayoutInflater.from(parent.context).inflate(R.layout.todo_item, parent, false)
           )
       }
   
       override fun getItemCount() = data.size
   
       override fun onBindViewHolder(holder: ViewHolder, position: Int) {
           holder.title.text = data[position].title
           holder.completed.text = if(data[position].completed) "Completed" else "Not Completed"
       }
   
   }
   
   interface TodoService {
       @GET("todos")
       fun listTodos(): Call<List<Todo>>
   }
   
   class Todo (
       val id: Integer,
       val userId: Integer,
       val title: String,
       val completed: Boolean
   )​

7. That's it! Now run your application and see the result for yourself. It should look like this:
   

That's it for now!