Retrofit is a HTTP client for Android and Java, it turns your HTTP API into a Java interface. This time I will show you how to create a basic project using Retrofit.
Setup
Create a new project in android studio with no Activity, default options and set the following dependencies in build.gradle.kts
implementation("com.squareup.retrofit2:retrofit:$retrofitVersion")
implementation("com.squareup.retrofit2:converter-gson:$retrofitVersion")
Here we are using Gson converter to transform the JSON responses to the model classes.
In this example we are going to use a Fruitypedia API to get smoothie categories: https://fruitypedia.josdem.io/categories/en
This is the JSON output
[
{
"id": 5,
"name": "Healing"
},
{
"id": 6,
"name": "Energy"
},
{
"id": 7,
"name": "Healthy"
},
{
"id": 8,
"name": "Boost"
}
]
Let’s create a basic model data Category class
package com.josdem.retrofit.model
data class Category(
val id: Int,
val name: String,
)
Now we can create the FruitypediaService interface that will embody our HTTP response.
package com.josdem.retrofit.service
import com.josdem.retrofit.model.Beverage
import com.josdem.retrofit.model.Category
import retrofit2.Response
import retrofit2.http.GET
import retrofit2.http.Path
interface FruityService {
@GET("/categories/{language}")
suspend fun getCategories(
@Path("language") language: String,
): Response<List<Category>>
}
Retrofit generates an implementation of the FruityService interface, however we need to create a Retrofit builder using a base URL.
package com.josdem.retrofit.service
import retrofit2.Retrofit
import retrofit2.converter.gson.GsonConverterFactory
object RetrofitHelper {
private const val BASE_URL = "https://fruitypedia.josdem.io/"
fun getInstance(): Retrofit {
return Retrofit.Builder().baseUrl(BASE_URL)
.addConverterFactory(GsonConverterFactory.create())
.build()
}
}
Finally we are going to call asynchronously using Kotlin coroutines. To make it simple from our MainActivity
package com.josdem.retrofit
import android.os.Bundle
import android.util.Log
import androidx.activity.ComponentActivity
import com.josdem.retrofit.service.FruityService
import com.josdem.retrofit.service.RetrofitHelper
import kotlinx.coroutines.MainScope
import kotlinx.coroutines.launch
import java.util.Locale
class MainActivity : ComponentActivity() {
private val language: String = Locale.getDefault().language
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val fruityService = RetrofitHelper.getInstance().create(FruityService::class.java)
MainScope().launch {
val result = fruityService.getCategories(language)
Log.d("categories: ", result.body().toString())
}
}
}
That’s it when you run the project, you will see the categories in the logcat Android Monitor.
[Category(id=5, name=Healing), Category(id=6, name=Energy), Category(id=7, name=Healthy), Category(id=8, name=Boost)]
Here is the test case:
package com.josdem.retrofit
import com.josdem.retrofit.model.Category
import com.josdem.retrofit.service.FruityService
import com.josdem.retrofit.service.RetrofitHelper
import kotlinx.coroutines.test.runTest
import org.junit.Assert.assertEquals
import org.junit.Assert.assertTrue
import org.junit.Test
import retrofit2.Response
const val LANGUAGE = "en"
class FruityServiceTest {
private val fruityService: FruityService = RetrofitHelper.getInstance().create(FruityService::class.java)
private suspend fun getCategories(): Response<List<Category>> {
return fruityService.getCategories(LANGUAGE)
}
@Test
fun shouldGetCategories() =
runTest {
val response = getCategories()
val categories: List<Category>? = response.body()
assertTrue(response.isSuccessful)
assertEquals(4, categories?.size)
}
}
Here is the complete API just in case you want to take a look: https://fruitypedia.josdem.io/swagger-ui.html
To browse the code go here, to download the code:
git clone git@github.com:josdem/android-retrofit-workshop.git