Mock suspend that never returns

2 min read 05-10-2024

Mocking Suspend Functions that Never Return: A Guide for Kotlin Developers

Testing asynchronous code in Kotlin can be a real headache, especially when dealing with suspending functions that might never return. These functions, often found in networking or database interactions, pose a unique challenge for mocking. This article explores the intricacies of mocking suspend functions that never return, providing practical solutions and insights for Kotlin developers.

The Problem: Mocking Functions That Never Return

Imagine you have a suspend function responsible for retrieving data from a remote server:

suspend fun fetchData(): String {
    // Network call
    return "Data from server"
}

You want to test a function that uses fetchData, but the server might be down, causing fetchData to block indefinitely. Traditional mocking approaches, like using Mockito, fall short here because they cannot accurately mimic the non-returning behavior of fetchData.

The Solution: Leveraging Coroutines and Channels

The key to mocking a suspend function that never returns lies in harnessing the power of coroutines and channels. Here's how:

Mock the Function with a Channel:
- Create a Channel<String> to represent the mocked function's return value.
- Replace the actual fetchData function with a mock implementation that uses the channel.

import kotlinx.coroutines.channels.Channel

// Mocked version of fetchData
suspend fun mockedFetchData(channel: Channel<String>): String {
    return channel.receive() // Receive data from the channel
}

// Test function using mockedFetchData
suspend fun testFunction(mockedChannel: Channel<String>) {
    val data = mockedFetchData(mockedChannel)
    // Perform assertions on data
}

Trigger the Mock Function:
- In your test, launch a coroutine that sends a value to the mocked channel.
- This emulates the behavior of a successful response from the server.

import kotlinx.coroutines.launch

// Launch a coroutine to send data to the channel
launch {
    mockedChannel.send("Mock data") // Send the mock data
}

// Call the test function
testFunction(mockedChannel)

Handle Non-returning Behavior:
- To simulate a scenario where fetchData never returns, simply don't send any value to the channel.
- The receive() call in mockedFetchData will then block indefinitely, mimicking the behavior of a function that never returns.

Example: Testing a Network Request

Here's a complete example demonstrating the mocking approach:

import kotlinx.coroutines.*
import kotlinx.coroutines.channels.Channel
import org.junit.jupiter.api.Test
import kotlin.test.assertEquals

class NetworkRequestTest {

    @Test
    fun `testSuccessfulRequest`() = runBlocking {
        val mockedChannel = Channel<String>()

        // Launch a coroutine to send mock data
        launch { mockedChannel.send("Mock data") }

        val result = testNetworkRequest(mockedFetchData(mockedChannel))
        assertEquals("Mock data processed", result)
    }

    @Test
    fun `testFailedRequest`() = runBlocking {
        val mockedChannel = Channel<String>()

        // Do not send any data to the channel

        val result = testNetworkRequest(mockedFetchData(mockedChannel))
        // Assertions for the failed case
    }

    // Test function
    private suspend fun testNetworkRequest(data: String): String {
        return "$data processed"
    }
}

Benefits of This Approach

Accurate Representation: The channel-based mocking effectively replicates the non-returning behavior of the fetchData function.
Flexibility: This approach provides fine-grained control over the mock function's return value, allowing you to test various scenarios.
Testability: You can easily test both successful and failed cases by manipulating the channel's state.

Conclusion

Mocking suspend functions that never return is crucial for writing robust tests for asynchronous Kotlin code. The approach outlined in this article leverages coroutines and channels, providing a powerful and flexible solution for effectively mimicking the behavior of non-returning functions. By mastering this technique, you can confidently test even the most complex asynchronous operations in your Kotlin applications.

Remember to adapt this approach to your specific needs, creating custom mocks and test scenarios to ensure comprehensive test coverage.