In this talk, I’m introducing Pegomock, a mocking framework for Go, that I have written over the last 2 years. By quickly going through the idea of Dependency Injection, I'm explaining why mocks are useful in general. Afterwards, I'm diving a bit deeper into how to use Pegomock and why I think it is better than its alternatives.

1. Pegomock
A mocking framework for Go
2018
Peter Goetz
1
peter.gtz@gmail.com
github.com/petergtz
twitter.com/petergtz
medium.com/@peter.gtz

2. Unit Tests
2
func sortInts(a []int) { /* ... */ }
ints := []int{8, 3, 6, 2, 7}
sortInts(ints)
Expect(ints).To(Equal([]int{2, 3, 6, 7, 8}))

3. DEPENDENCY INJECTION
3

4. Example: CheckoutService
4
Checkout-
Service
Shipping
-Service
CreditCard-
Service
Order-
Service

5. Example: CheckoutService
5
func (cs *CheckoutService) CheckOut(shoppingCart ShoppingCart) error {
sum := sumItemPricesIn(shoppingCart)
sum += cs.shippingService.GetShippingCosts()
err := cs.creditCardService.Charge(sum)
if err != nil {
return wrapped(err)
}
err = cs.orderService.Submit(shoppingCart)
if err != nil {
return wrapped(err)
}
return nil
}
How to test this method?

6. Example: CheckoutService
6
type CheckoutService struct {
shippingService *ShippingService
creditCardService *CreditCardService
orderService *OrderService
}
func NewCheckoutService() *CheckoutService {
return &CheckoutService{
NewShippingService(...),
NewCreditCardService(...),
NewOrderService(...),
}
}

7. Example: CheckoutService
7
Checkout-
Service
Http-
Shipping
-Service
gRPC-
CreditCard-
Service
Http-
Order-
Service
Imple-
mentation
Interface
Dependency
Legend:
Shipping-
Service
Order-
Service
CreditCard-
Service

8. Dependency Injection
8
type ShippingService interface {
GetShippingCosts() (costs Currency)
}
type CreditCardService interface {
Charge(amount Currency) error
}
type OrderService interface {
Submit(ShoppingCart) error
}
type CheckoutService struct {
shippingService ShippingService
creditCardService CreditCardService
orderService OrderService
}
func NewCheckoutService(
shippingService ShippingService,
creditCardService CreditCardService,
orderService OrderService) *CheckoutService {
return &CheckoutService{
shippingService,
creditCardService,
orderService,
}
}

9. Unit Testing the CheckoutService
9
fakeShippingService := ...
fakeCreditCardService := ...
fakeOrderService := ...
checkoutService := NewCheckoutService(
fakeShippingService,
fakeCreditCardService,
fakeOrderService)
When(fakeShippingService.GetShippingCost()).ThenReturn(5.90)
shoppingCart := ShoppingCart{[]Items{
Item{
Name: "Creation: Life and How to Make it - Steve Grand",
Price: 23.49,
},
}}
checkoutService.CheckOut(shoppingCart)
fakeCreditCardService.VerifyWasCalledOnce().Charge(15.90)
fakeOrderService.VerifyWasCalledOnce().Submit(shoppingCart)

10. Unit Testing the CheckoutService
10
fakeShippingService := NewMockShippingService()
fakeCreditCardService := NewMockCreditCardService()
fakeOrderService := NewMockOrderService()
checkoutService := NewCheckoutService(
fakeShippingService,
fakeCreditCardService,
fakeOrderService)
When(fakeShippingService.GetShippingCost()).ThenReturn(5.90)
shoppingCart := ShoppingCart{[]Items{
Item{
Name: "Creation: Life and How to Make it - Steve Grand",
Price: 23.49,
},
}}
checkoutService.CheckOut(shoppingCart)
fakeCreditCardService.VerifyWasCalledOnce().Charge(15.90)
fakeOrderService.VerifyWasCalledOnce().Submit(shoppingCart)
Generated by Pegomock

11. Pegomock CLI
11
# Install it:
go get github.com/petergtz/pegomock
go get github.com/petergtz/pegomock/pegomock
# Generate mocks:
pegomock generate ShippingService
pegomock generate CreditCardService
pegomock generate OrderService

12. PEGOMOCK DSL
12

13. Argument Matchers
Verification:
13
display := NewMockDisplay()
// Calling mock
display.Show("Hello again!")
// Verification:
display.VerifyWasCalledOnce().Show(AnyString())

14. Argument Matchers
Stubbing:
14
phoneBook := NewMockPhoneBook()
// Stubbing:
phoneBook.GetPhoneNumber(AnyString()).ThenReturn("123-456-789")
// Prints "123-456-789":
fmt.Println(phoneBook.GetPhoneNumber("Dan"))
// Also prints "123-456-789":
fmt.Println(phoneBook.GetPhoneNumber("Tom"))

15. Verifying Number of Invocations
15
display := NewMockDisplay()
// Calling mock
display.Show("Hello")
display.Show("Hello, again")
display.Show("And again")
// Verification:
display.VerifyWasCalled(Times(3)).Show(AnyString())
// or:
display.VerifyWasCalled(AtLeast(3)).Show(AnyString())
// or:
display.VerifyWasCalled(Never()).Show("This one was never called")

16. Verifying in Order
16
display1 := NewMockDisplay()
display2 := NewMockDisplay()
// Calling mocks
display1.Show("One")
display1.Show("Two")
display2.Show("Another two")
display1.Show("Three")
// Verification:
inOrderContext := new(InOrderContext)
display1.VerifyWasCalledInOrder(Once(), inOrderContext).Show("One")
display2.VerifyWasCalledInOrder(Once(), inOrderContext).Show("Another two")
display1.VerifyWasCalledInOrder(Once(), inOrderContext).Show("Three")

17. Stubbing with Callbacks
17
phoneBook := NewMockPhoneBook()
// Stubbing:
When(phoneBook.GetPhoneNumber(AnyString())).Then(func(params []Param) ReturnValues {
return []ReturnValue{fmt.Sprintf("1-800-CALL-%v", strings.ToUpper(params[0]))}
}
// Prints "1-800-CALL-DAN":
fmt.Println(phoneBook.GetPhoneNumber("Dan"))
// Prints "1-800-CALL-TOM”:
fmt.Println(phoneBook.GetPhoneNumber("Tom"))

18. Verifying with Argument Capture
18
display := NewMockDisplay()
// Calling mock
display.Show("Hello")
display.Show("Hello, again")
display.Show("And again")
// Verification and getting captured arguments
text := display.VerifyWasCalled(AtLeast(1)).Show(AnyString()).getCapturedArguments()
// Captured arguments are from last invocation
Expect(text).To(Equal("And again"))

19. Verifying with Argument Capture
19
display := NewMockDisplay()
// Calling mock
display.Show("Hello")
display.Show("Hello, again")
display.Show("And again")
// Verification and getting all captured arguments
texts := display.VerifyWasCalled(AtLeast(1)).Show(AnyString()).getAllCapturedArguments()
// Captured arguments are a slice
Expect(texts).To(ConsistOf("Hello", "Hello, again", "And again"))

20. Why Pegomock, why not X?
• https://github.com/golang/mock
• https://github.com/maxbrunsfeld/counterfeiter
• Any others
20
Where X is:

21. GoMock
21
GoMock:
Use the mock in a test:
func TestMyThing(t *testing.T) {
mockCtrl := gomock.NewController(t)
defer mockCtrl.Finish()
mockObj := something.NewMockMyInterface(mockCtrl)
mockObj.EXPECT().SomeMethod(4, "blah")
// pass mockObj to a real object and play with it.
}
Pegomock:
func TestUsingMocks(t *testing.T) {
pegomock.RegisterMockTestingT(t)
mockObj := NewMyInterfaceMock()
// pass mockObj to a real object and play with it.
}

22. Counterfeiter
22
Counterfeiter:
fake.DoThings("stuff", 5)
Expect(fake.DoThingsCallCount()).To(Equal(1))
str, num := fake.DoThingsArgsForCall(0)
Expect(str).To(Equal("stuff"))
Expect(num).To(Equal(uint64(5)))
Pegomock:
fake.DoThings("stuff", 5)
fake.VerifyWasCalledOnce().DoThings("stuff", 5)

23. PEGOMOCK DEMO
23

24. Q & A
24
https://github.com/petergtz/pegomock
peter.gtz@gmail.com
github.com/petergtz
twitter.com/petergtz
medium.com/@peter.gtz