main_test.go

//.....

const sumMethodName = "Sum"
const multiplyMethodName = "Multiply"

type AppHandlerFake struct {
	// method name -> call -> params
	Calls map[string][][]interface{}
}

func (a *AppHandlerFake) Sum(x, y int) (r Result) {
	b := a.Calls[sumMethodName]
	c := []interface{}{x, y}
	a.Calls[sumMethodName] = append(b, c)

	r.Value = 7
	return
}

func (a *AppHandlerFake) Multiply(x, y int) (r Result) {
	b := a.Calls[multiplyMethodName]
	c := []interface{}{x, y}

	a.Calls[multiplyMethodName] = append(b, c)

	r.Value = 10
	return
}

//.....