Gomock Tutorial

I’ve been using mock/Gomock to write tests in my personal project. When you’re building something in a new language, it is hard to prioritize learning every tool in your toolchain. For me, I’ve been writing custom and suboptimal code for Gomock because of a nifty but undocumented API call .Do.

In many cases, I wan to match subsets of a complex object while ignoring irrelevant parts. e.g., verify a function is invoked with a list of User objects, but only verifying the email addresses. To do that in a generic way, I wrote a custom Matcher API that uses text/template to describe what parts of the object to match. Thus, my mock-and-verify code looks like:

ctrl := mock.NewController(t)
defer ctrl.Finish()
mockMessageSender := NewMockMessageSender(ctrl)

matchFrom := ...
// This matcher returns true if the argument is a list whose objects have Email of "recipient1@..." and "recipient2@..."
matchTo := utils.M("{{(index . 0).Email}} {{(index . 1).Email}}", "recipient1@example.com recipient2@example.com")
matchOutMessage := ...
mockMessageSender.EXPECT().Send(matchFrom, matchTo, matchOutMessage).Return(nil).Times(1)

My custom Matcher was only twenty-ish lines of code. But it never felt clean to me. I was looking for something more like Mockito.

ArgumentCaptor<Person> argument = ArgumentCaptor.forClass(Person.class);
verify(mock).doSomething(argument.capture());
assertEquals("John", argument.getValue().getName());

Or

// more complex Java 8 example - where you can specify complex verification behaviour functionally
verify(target, times(1)).receiveComplexObject(argThat(obj -> obj.getSubObject().get(0).equals("expected")));

It turns out there was a similar feature in Gomock that isn’t documented, the Do function. You can use it this way:

import (
	Assert "github.com/stretchr/testify/assert"
)


func TestSend_ok(t *testing.T) {
	assert := Assert.New(t)
	ctrl := gomock.NewController(t)
	defer ctrl.Finish()
	...
	mockSender.EXPECT().Send(gomock.Any()).Do(func(messageGot email.EmailMessage) {
		assert.Equal("foo@user.com", from.Profile.PreferredEmail)
		assert.Equal("TODO Add Name to models.UserDDB", messageGot.From.Name)
		assert.Equal("Subject", messageGot.Subject)
		assert.Equal("Body", messageGot.HtmlContent)
	})
	...
}

Now I can happily go back and delete my own code!

If you are looking to write good tests for Go, I hope this has been helpful.