Mock interfaces

Mock interfaces in Go tests by defining a struct that implements the target interface and using it to inject controlled behavior into your code. This allows you to isolate unit tests from external dependencies like databases or network calls.

type MockHandler struct {
    HandleFunc func(ctx context.Context, r Record) error
}

func (m *MockHandler) Handle(ctx context.Context, r Record) error {
    return m.HandleFunc(ctx, r)
}

// Usage in test
mock := &MockHandler{HandleFunc: func(ctx context.Context, r Record) error {
    return errors.New("mock error")
}}
multi := NewMultiHandler(mock)

Mocking an interface means creating a fake version of a component that behaves exactly like the real one but lets you control its output. You use this to test your code in isolation without needing the actual database, API, or file system. It's like using a rubber duck instead of a real duck to practice swimming; the duck doesn't swim, but it lets you focus on your technique.