Mocking single methods in Go

Question

In Go, how can I mock an interface without having to implement every method? Let's say I have a Car interface and a Corolla struct that implements that interface:

type Car interface {
    changeTire()
    startEngine()
    ....
    refuel()
}

type Corolla struct {
    ...
}

func (c Corolla) changeTire() {...}

func (c Corolla) startEngine() {...}

func (c Corolla) refuel() {...}

Let's say I also have a Garage struct that depends on Car:

type Garage struct {
    MyCar Car
}

func (g Garage) PrepareCarToDrive() {
    g.MyCar.changeTire()
    g.MyCar.refuel()
    g.MyCar.startEngine()
}

And I want to test Garage, so I create a MockCar that implements Car

type MockCar struct {
    ...
}

func (c MockCar) changeTire() {...}

func (c MockCar) startEngine() {...}

func (c MockCar) refuel() {...}

Now I have tests that test PrepareCarToDrive and I use the MockCar:

func TestGarage_PrepareCarToDrive_DoesSomething(t *testing.T) {
    mockCar := MockCar{}
    garageUnderTest := Garage{}
    garageUnderTest.MyCar = mockCar

    // some other setup

    // when Garage calls mockCar.changeTire(), should do X
    ...
}

func TestGarage_PrepareCarToDrive_DoesSomethingElse(t *testing.T) {
    mockCar := MockCar{}
    garageUnderTest := Garage{}
    garageUnderTest.MyCar = mockCar

    // some other setup 
    // when Garage calls mockCar.changeTire(), should do Y
    ...
}

My question is, how can I have mockCar do different things each test? I know that I can create a different mock implementation of Car for each test. But this will get out of hand really quickly as I add more methods to Car.

I'm coming from a Java background so I'm looking for something like Mockito that will let me mock just the methods I need for each test.

What is the best way to do this in Go? Or am I missing something more fundamental?

Answer 1

If you embed the interface type itself in your mock struct, you can then only implement the methods you need. For example:

type MockCar struct {
    Car
    ...
}

func (c MockCar) changeTire() {...}

Even though your struct only implements changeTire explicitly, it still satisfies the interface because the Car field provides the rest. This works as long as you don't try to call any of the unimplemented methods (which will cause a panic because Car is nil)