Almost all apps work with remote APIs for data fetching, uploading and processing. The most commonly used library for this task is Retrofit. As this has becomes very obvious part of application development, it becomes even more critical to test our api layer to verify our set up.

In this article, we’ll explore how can we write unit test cases for our network layer and make it more and more error free. We’ll be using Retrofit as rest client. So let’s begin.

Retrofit set up

The very first step is to add retrofit dependencies along with gson in our build.gradle as follows

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

Now, we define an interface where we list our apis end point calls as follows

Now if we want to access the user data, it’ll only be accessible vis User Repository. Alright! Now we’re done with our layer set up. Let’s move on to testing set up.

Set up for Testing

The first thing to note is that the test cases will resides in test folder and not in androidTest folder. We don’t have any android related dependency in this layer.

Let’s add some dependencies and understand why we need them.

• Coroutine test: It provides us support to test coroutines. We get test scopes and dispatchers to run coroutines.

// Coroutine test
testImplementation "org.jetbrains.kotlinx:kotlinx-coroutines-test:1.5.2"

• JUnit4: It provides us with unit testing framework apis to run tests locally on JVM.

testImplementation 'junit:junit:4.13.2'

• MockWebServer: It provides support for mocking of the server with which we can actually make api calls and get the output.

// Mock web server
testImplementation 'com.squareup.okhttp3:mockwebserver:4.10.0'

So why we are mocking a server? This is because mocking a server would give us control on what all kind of responses and requests we want to test our apis handling. As we’re not running tests on a real device and hence don’t have access to a real server. To make our tests functions as expected we’re mocking a real server and MockWebServer would let us do exactly that.

Cool! Now we’re all set to start writing test cases. So let’s begin.

Writing Test Cases

Test Case 1

Let’s write a test wherein we want to verify that if the server has two users and we hit the get all user api, we would get 2 users in return with http code 200.

We run this test in a test scope because we’ve suspend functions.

Line 3–6: So first, we’ll create a list of users which we expect in the server response.

Line 7–9: Create a mock response for the server to return by setting http code and body. So in this case server will return this response no matter which api we hit.

Line 10: Finally we set this mock response to our server.

Line 12: We hit our api end point.

Line 13: We assert if we get exactly same number of users as we set in our server response. We get a true for this.

Line 14: We assert if the list is exactly the same as we set in our server response. We get a true for this.

Bamn! We verified that our api set up is handling the responses correctly and as expected. Now we can get started and write more test cases for various scenarios and assert if all those are handled correctly.

I would leave that as practice for you to play around and observe the behavior. Let me know if you get stuck somewhere.

Gist For Reference

I’ve added the gist for UserRepositoryImplTest for reference.

Bonus Read

In case you want to read about testing various layers of your app. Do checkout below links.

• ViewModel testing
• Room Database testing
• Local tests in Android

That is all for now! Stay tuned!

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Until next time…

Cheers!