Interface mocking tool for go generate.

By Mat Ryer and David Hernandez, with ideas lovingly stolen from Ernesto Jimenez.

What is Moq?

Moq is a tool that generates a struct from any interface. The struct can be used in test code as a mock of the interface.

above: Moq generates the code on the right.

You can read more in the Meet Moq blog post.

Installing

To start using Moq, just run go get:

$ go get github.com/matryer/moq

Usage

moq [flags] source-dir interface [interface2 [interface3 [...]]]
	-fmt string
		go pretty-printer: gofmt, goimports or noop (default gofmt)
	-out string
		output file (default stdout)
	-pkg string
		package name (default will infer)
	-stub
		return zero values when no mock implementation is provided, do not panic
	-skip-ensure
		suppress mock implementation check, avoid import cycle if mocks 
		generated outside of the tested package

Specifying an alias for the mock is also supported with the format 'interface:alias'

Example: moq -pkg different . MyInterface:MyMock

NOTE: source-dir is the directory where the source code (definition) of the target interface is located. It needs to be a path to a directory and not the import statement for a Go package.

In a command line:

$ moq -out mocks_test.go . MyInterface

In code (for go generate):

package my

//go:generate moq -out myinterface_moq_test.go . MyInterface

type MyInterface interface {
	Method1() error
	Method2(i int)
}

Then run go generate for your package.

How to use it

Mocking interfaces is a nice way to write unit tests where you can easily control the behaviour of the mocked object.

Moq creates a struct that has a function field for each method, which you can declare in your test code.

In this example, Moq generated the EmailSenderMock type:

func TestCompleteSignup(t *testing.T) {

	var sentTo string

	mockedEmailSender = &EmailSenderMock{
		SendFunc: func(to, subject, body string) error {
			sentTo = to
			return nil
		},
	}

	CompleteSignUp("me@email.com", mockedEmailSender)

	callsToSend := len(mockedEmailSender.SendCalls())
	if callsToSend != 1 {
		t.Errorf("Send was called %d times", callsToSend)
	}
	if sentTo != "me@email.com" {
		t.Errorf("unexpected recipient: %s", sentTo)
	}

}

func CompleteSignUp(to string, sender EmailSender) {
	// TODO: this
}

The mocked structure implements the interface, where each method calls the associated function field.

Tips

• Keep mocked logic inside the test that is using it
• Only mock the fields you need
• It will panic if a nil function gets called
• Name arguments in the interface for a better experience
• Use closured variables inside your test function to capture details about the calls to the methods
• Use .MethodCalls() to track the calls
• Use go:generate to invoke the moq command
• If Moq fails with a go/format error, it indicates the generated code was not valid. You can run the same command with -fmt noop to print the generated source code without attempting to format it. This can aid in debugging the root cause.

License

The Moq project (and all code) is licensed under the MIT License.

The Moq logo was created by Chris Ryer and is licensed under the Creative Commons Attribution 3.0 License.