Skill: gomock

What I do

I provide expertise in using GoMock to create and manage mock implementations of interfaces for unit testing. I focus on defining expectations, verifying call sequences, and isolating components for reliable BDD-style testing.

When to use me

• When writing unit tests for components that depend on interfaces
• When verifying complex interactions between a service and its repository
• When simulating error conditions or specific return values from dependencies

Core principles

1. Isolation: Use mocks to test the logic of a single component without invoking its real dependencies.
2. Expectation setting: Clearly define what calls are expected, what they should return, and how many times they should occur.
3. Verification: Ensure that all expected calls were made by verifying the controller state at the end of the test.
4. Readability: Keep mock setups concise and readable to maintain the focus on the behaviour being tested.

Patterns & examples

Basic mock setup:

func TestUserService(t *testing.T) {
    ctrl := gomock.NewController(t)
    defer ctrl.Finish() // Required to verify expectations

    mockRepo := mocks.NewMockUserRepository(ctrl)
    svc := NewUserService(mockRepo)

    // Set expectations
    mockRepo.EXPECT().
        Get(gomock.Eq(1)).
        Return(&User{ID: 1, Name: "Alice"}, nil).
        Times(1)

    user, err := svc.FindUser(1)
    // Assertions...
}

Using argument matchers: Use gomock.Any() when the specific value doesn't matter, or custom matchers for complex validation.

mockRepo.EXPECT().Save(gomock.Any()).Return(nil)

Stubbing behavior with DoAndReturn:

mockRepo.EXPECT().Get(gomock.Any()).DoAndReturn(func(id int) (*User, error) {
    if id == 0 {
        return nil, errors.New("not found")
    }
    return &User{ID: id}, nil
})

Ordering calls:

gomock.InOrder(
    mockRepo.EXPECT().Get(1).Return(u, nil),
    mockRepo.EXPECT().Save(u).Return(nil),
)

Anti-patterns to avoid

• ❌ Over-mocking: Do not mock internal implementation details. Only mock at interface boundaries.
• ❌ Ignoring ctrl.Finish(): Forgetting to call Finish() (or use the built-in cleanup in newer Go versions) means failed expectations won't cause the test to fail.
• ❌ Brittle expectations: Avoid overly strict ordering or call counts unless they are critical to the system's correctness.

KB Reference

~/vaults/baphled/3. Resources/Knowledge Base/AI Development System/Skills/Testing-BDD/Gomock.md

Related skills

• bdd-workflow: For structuring tests that describe system behaviour
• ginkgo-gomega: For using mocks within a Ginkgo test suite
• code-generation: For automating the creation of mock files using mockgen
• golang: For principles of interface design and composition