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How to mock Retrofit API responses for testing in Kotlin Android

How to mock Retrofit API responses for testing in Kotlin Android.

Here's a detailed step-by-step tutorial on how to mock Retrofit API responses for testing in Kotlin Android.

Step 1: Set up Retrofit​

First, you need to set up Retrofit in your Android project. Add the Retrofit dependency to your build.gradle file:

implementation 'com.squareup.retrofit2:retrofit:2.9.0'

Create an interface that defines your API endpoints. For example:

interface ApiService {
    @GET("users/{username}")
    suspend fun getUser(@Path("username") username: String): User
}

Create a Retrofit instance and initialize it with the base URL and a converter factory. For example:

val retrofit = Retrofit.Builder()
    .baseUrl("https://api.example.com/")
    .addConverterFactory(GsonConverterFactory.create())
    .build()

val apiService = retrofit.create(ApiService::class.java)

Step 2: Set up MockWebServer​

MockWebServer is a library that allows you to mock HTTP responses for testing. Add the MockWebServer dependency to your build.gradle file:

debugImplementation 'com.squareup.okhttp3:mockwebserver:4.9.1'

Create an instance of MockWebServer in your test class:

val server = MockWebServer()

Step 3: Enqueue Mock Responses​

In your test, before making the API request, enqueue the mock response using MockWebServer. For example:

val mockResponse = MockResponse()
    .setResponseCode(200)
    .setBody("{ \"name\": \"John Doe\" }")

server.enqueue(mockResponse)

Step 4: Start MockWebServer​

Start the MockWebServer before making the API request:

server.start()

Get the base URL of the MockWebServer and replace the base URL in your Retrofit instance:

val mockUrl = server.url("/")
val mockBaseUrl = mockUrl.toString()

retrofit = retrofit.newBuilder()
    .baseUrl(mockBaseUrl)
    .build()

Step 5: Make the API Request​

Make the API request using the Retrofit instance:

val response = apiService.getUser("john")

Step 6: Verify the Mock Response​

Verify that the response received is the same as the mock response:

assertEquals(200, response.code)
assertEquals("{ \"name\": \"John Doe\" }", response.body)

Step 7: Stop MockWebServer​

Stop the MockWebServer after the test:

server.shutdown()

That's it! You have successfully mocked Retrofit API responses for testing in Kotlin Android. This allows you to test your app's network requests without making actual API calls.