moq

Introduction to Moq

What is Moq?

Moq is an interface mocking tool designed specifically for Go's go generate. It assists developers by creating a struct out of an interface, which can be employed in testing as a mock version of the real interface. This makes Moq particularly useful for unit testing in Go, allowing for controlled simulation of interface behaviors.

Installing Moq

To install the most recent version of Moq, simply execute the following command in your terminal:

$ go install github.com/matryer/moq@latest

Please note that this installation method requires Go version 1.18 or higher. For those using earlier Go versions, you can download the pre-compiled binaries from the Moq releases page.

Usage

Moq can be utilized from the command line with flexible parameters to tailor the generation process:

moq [flags] source-dir interface [interface2 [interface3 [...]]]

Key flags include:

• -fmt: Determines the pretty-printing style (gofmt, goimports, or noop), with gofmt as default.
• -out: Sets the output file path, defaulting to stdout if not specified.
• -pkg: Specifies the package name.
• -rm: Removes the existing output file if present.
• -skip-ensure: Skips the mock implementation check to prevent cyclic imports.
• -stub: Specifies that zero values should be returned instead of panicking when no mock implementation is given.
• -version: Displays the current version of Moq.
• -with-resets: Generates extra functions for resetting call history on a mock.

Aliases for mocks can also be specified using the format interface:alias:

moq -pkg different . MyInterface:MyMock

Using Moq

Moq is typically operated from the source directory of the target interface (e.g., . for the current directory). For example, to generate mocks for MyInterface and output them to mocks_test.go, you'd use:

$ moq -out mocks_test.go . MyInterface

Alternatively, integrate Moq within your code using go generate:

package my

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

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

Run go generate in your package directory to automatically invoke Moq and generate test mocks.

How to Use Mocked Structs in Tests

Mocking is a powerful technique in unit testing, allowing developers to control and simulate the expected behavior of dependencies. Moq simplifies testing by generating structs with function fields for each method in the interface. Here is a typical usage example in a test function:

func TestCompleteSignup(t *testing.T) {
    var sentTo string

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

    CompleteSignUp("[email protected]", mockedEmailSender)

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

Here, Moq efficiently creates a mock to observe the interactions within the CompleteSignUp function.

Tips for Effective Mocking

• Restrict mock logic to the particular test using it.
• Mock only necessary methods to maintain simplicity.
• Avoid calling nil functions to prevent panics.
• Naming arguments in interfaces improves test clarity.
• Capture method call details using closed-over variables.
• Utilize .MethodCalls() to monitor method calls.
• Reset mocks using .ResetCalls() when test context changes.
• Use go:generate directive for automatic mock generation.
• In case of formatting errors with Moq, use the -fmt noop option for debugging unformatted output.

Licensing and Acknowledgements

Moq is provided under the MIT License. It was created by Mat Ryer and David Hernandez, with significant contributions from the Go community. The Moq logo, crafted by Chris Ryer, is licensed under the Creative Commons Attribution 3.0 License.