Prompt Title: Android Kotlin Mock or Mockk: What is relaxed = true?

In unit testing, especially when dealing with a large number of dependencies or complex objects, it can be cumbersome and verbose to specify behavior for every single function call on a mock object. This is where relaxed mocks come in handy, particularly when using the MockK library in Kotlin for Android development.

What is a Relaxed Mock?

A relaxed mock is a type of mock provided by MockK that automatically returns "smart" defaults for all functions without requiring explicit stubbing. This is particularly useful when you need an object to fulfill a dependency but are not concerned with most of its behavior for the scope of certain tests.

• For primitive types: Returns default values (0 for numbers, false for boolean, '\u0000' for char, etc.).
• For reference types: Returns chained mocks (further calls on this mock return more mocks).
• For functions: They return Unit by default unless otherwise specified.

Why Use a Relaxed Mock?

• Simplicity: Reduces boilerplate by not requiring every method to be explicitly stubbed.
• Flexibility: Allows tests to run even if some methods on the mock are not crucial to the test's purpose.
• Focus: Lets you focus on the behavior that is important for your tests, ignoring irrelevant method calls.

When to Use a Relaxed Mock?

Use a relaxed mock when:

• You need to provide a mock as a dependency but only care about specific interactions or outputs.
• You want to ensure a test does not fail simply because a non-critical method returned null or some default value.
• You are writing exploratory tests where not all interactions are known upfront.

How to Use a Relaxed Mock?

Here’s how you can create and use a relaxed mock in Kotlin using MockK:

kotlin
Copy code
import io.mockk.every
import io.mockk.mockk
import org.junit.Test
import kotlin.test.assertEquals

class ExampleService {
    fun computeValue(): Int = 10
    fun fetchDetails(): String = "Details"
}

class ExampleTest {

    @Test
    fun `test with relaxed mock`() {
        // Creating a relaxed mock of ExampleService
        val mockService = mockk<ExampleService>(relaxed = true)

        // Overriding behavior for specific function
        every { mockService.computeValue() } returns 5

        // computeValue is stubbed
        assertEquals(5, mockService.computeValue())

        // fetchDetails is not stubbed, but returns a default non-null value because the mock is relaxed
        assertEquals("", mockService.fetchDetails())
    }
}

In this example:

• ExampleService has two methods: computeValue and fetchDetails.
• A relaxed mock of ExampleService is created, meaning unstubbed methods return default values.
• computeValue() is explicitly stubbed to return 5.
• fetchDetails() is not stubbed, so it returns an empty string (""), which is the default for a non-void returning function in a relaxed mock.

Best Practices

• Use sparingly: Relaxed mocks should be used judiciously. Overuse can lead to tests that pass despite incorrect assumptions about dependencies' behaviors.
• Be explicit when necessary: While relaxed mocks reduce setup, it’s crucial to explicitly define behavior for interactions that are relevant to your test’s purpose.
• Integration Testing: Consider using real instances or less-relaxed mocks in integration tests to ensure interactions are tested more rigorously.

By leveraging relaxed mocks, you can streamline the setup phase of your tests while keeping them focused on the specific behaviors that matter. This approach is particularly effective in complex systems where dependencies have many methods, but only a few are relevant to the tests at hand.