gomock example

gomock is a mocking framework that allows you to replace network calls with mocked network calls in your tests.

File Structure

• gomock_example: The core functionality of this package.
• client.go: Contains the example API client that will be mocked.
• gomock_example_test: The test code implementing gomock.
• mock.go: Code generated by mockgen.

Running this Example

$ go get
$ go test .

About this Example

Gomock relies on files generated by mock.go. These files use interfaces that you define.

The key to setting up mocking is to write interfaces for your objects that are making network calls.

In this example, I created a client.go file with the following content

type ApiClient interface {
	// When called, the API is expected to double the provided value
	ApiDouble(valueToDouble int) (int, error)
}

This is a simple example of a client that makes a network call during its execution. For simplicity, there is no network call here, but the idea is the same. We can imagine that ApiDouble hits some external service to perform its operation. We don't want to include this call in our unit tests to avoid flakiness/eliminate extra dependencies.

In order to generate our mocked code, we first install mockgen with:

# Assumes Go 1.16+
go install github.com/golang/mock/mockgen@v1.6.0

Then, from the root of our project run

mockgen -source client.go mock.go > mock.go

Now we have mock code that we can use. The mock client can be created with NewMockApiClient which returns a MockApiClient which implements our provided interface. This is passed into the object we are testing.

func getMockServer(t *testing.T) (*Server, *MockApiClient, *gomock.Controller) {
	ctrl := gomock.NewController(t)
	cli := NewMockApiClient(ctrl)
	server := NewServer(cli)
	return server, cli, ctrl
}

Anywhere that this server uses its Client, it will use the mock client instead.

More Information

• Mocking Wikipedia Page
• gomock repo