Adım Adım Retrofit - https://www.youtube.com/watch?v=kETK1hS68BQ

activity_main.xml

<?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="match_parent"
    tools:context="com.example.retrofit.MainActivity">

    <TextView
        android:id="@+id/txtPost"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginHorizontal="32dp"
        android:textSize="24sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/txtError"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Bir hata oluştu"
        android:visibility="gone"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <ProgressBar
        android:id="@+id/progress"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

build.gradle

 // ktx
    implementation "androidx.activity:activity-ktx:1.6.1"

    // Retrofit
    implementation 'com.squareup.retrofit2:retrofit:2.9.0'
    implementation 'com.squareup.retrofit2:converter-gson:2.9.0'

    // Coroutines
    implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.6'
    implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-android:1.6.4'
    implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:2.5.1"

MainActivity.kt

import android.os.Bundle
import android.widget.ProgressBar
import android.widget.TextView
import androidx.activity.viewModels
import androidx.appcompat.app.AppCompatActivity
import androidx.core.view.isVisible

class MainActivity : AppCompatActivity() {

    private val mainViewModel: MainViewModel by viewModels()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        mainViewModel.post.observe(this) { myPost ->
            findViewById<TextView>(R.id.txtPost).text = myPost.title
        }
        mainViewModel.isLoading.observe(this) { isLoading ->
            findViewById<ProgressBar>(R.id.progress).isVisible = isLoading
        }
        mainViewModel.hasError.observe(this) { hasError ->
            findViewById<TextView>(R.id.txtError).isVisible = hasError
        }
    }
}

MainViewModel.kt

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import kotlinx.coroutines.delay
import kotlinx.coroutines.launch

class MainViewModel : ViewModel() {

    private val _post = MutableLiveData<MyPost>()
    val post: LiveData<MyPost>
        get() = _post

    private val _isLoading = MutableLiveData<Boolean>()
    val isLoading: LiveData<Boolean>
        get() = _isLoading

    private val _hasError = MutableLiveData<Boolean>()
    val hasError: LiveData<Boolean>
        get() = _hasError

    init {
        fetchPost()
    }

    fun fetchPost() {
        viewModelScope.launch {
            _isLoading.value = true
            delay(2000)
            val response = RetrofitInstance.api.fetchPost()
            if (response.isSuccessful) {
                response.body()?.let { post ->
                    _post.value = post
                    _hasError.value = false
                } ?: run {
                    _hasError.value = true
                }
            } else {
                _hasError.value = true
            }
            _isLoading.value = false
        }
    }
}

RetrofitInstance.kt

import retrofit2.Retrofit
import retrofit2.converter.gson.GsonConverterFactory

object RetrofitInstance {

    //https://jsonplaceholder.typicode.com/posts/1
    private val retrofit by lazy {
        Retrofit.Builder()
            .baseUrl("https://jsonplaceholder.typicode.com")
            .addConverterFactory(GsonConverterFactory.create())
            .build()
    }

    val api: SimpleApi by lazy {
        retrofit.create(SimpleApi::class.java)
    }

}

Simpleapi.kt

import retrofit2.Response
import retrofit2.http.GET

interface SimpleApi {

    @GET("posts/1")
    suspend fun fetchPost(): Response<MyPost>
}

data class MyPost(
    val title: String,
    val body: String
)