如何在Go中对结构的特定方法执行单元测试

Question

Suppose that I have a struct Car and it has some methods I want to test. For example IgniteEngine, SwitchGear, and drive. As you can see that drive depends on the other methods. I need a way to mock IgniteEngine and SwitchGear.

I think I am supposed to use interface but I don't quite understand how to accomplish it.

Suppose Car is now an interface

type Car interface {
    IgniteEngine()
    SwitchGear()
    Drive()
}

I can create a MockCar and two mocked functions for IgniteEngine and SwitchGear but now how do I test the source code for Drive?

Do I copy and paste my source code into the mock object? That seems silly. Did I get the idea wrong about how to perform mocking? Does mocking only work when I do dependency injection?

Now what if Drive actually depends on external library like a database or a message broker system?

Thank you

Answer 1

I don't think that the problem is in the interface per se, it is more how the Car is implemented. Good testable code favour composition, so if you have something like:

type Engine interface {
  Ignite()
}

type Clutch interface {
  SwitchGear()
}

Then you can have a Car like this:

type Car struct {
  engine Engine
  clutch Clutch
}

func (c *Car) IgniteEngine() {
  c.engine.Ignite()
}

...

In this way you can substitute engines and clutches in the Car and create mock clutches and engines that produce exactly the behaviour that you need to test your Drive method.