Mocking with MockingKit

In this post, I’ll demonstrate how to use MockingKit to mock protocols and classes. We can then use this technique in unit tests and to fake not yet implemented logic.


Mocking is a way to create an interactive, fake implementation of a protocol or class. It’s a great tool when writing unit tests and to fake not yet implemented logic. If you’re unfamiliar with unit testing or mocking, you can read more by following these links.

I come from a .NET background and was fortunate to be around when the community was all about unit testing and how to write testable code. As I later started looking at iOS and Android development, I found that this was not as much discussed and that the mindset around mocking was more about mocking HTTP requests than protocols. The native tools were also very basic and lacked a lot of the power that I had come to expect from Visual Studio and the various .NET test libraries.

As Swift was later introduced, the limited introspection made mocking hard, where the few Swift-based libraries I found were more about generating boilerplate code than interactive mocks. So I made do with manual mocks for many years, writing a lot of manual code to register function invokations, tracking parameters, returning fake results etc. Let’s look at this way of mocking.

Manual mocks

We’e not going to get stuck on manual mocks, but I think it’s good to take a quick look at it to be able to compare this approach to using real mocks.

Consider that we have a simple printer protocol:

protocol Printer {

    func print(_ string: String) -> Bool

A manual mock of this protocol could look something like this:

class MockPrinter: Printer {

    var printInvokeCount = 0
    var printInvokeArgs = [String]()
    var printInvokeResult = false

    func print(_ string: String) -> Bool {
        printInvokeCount += 1
        return printInvokeResult

Now, for any class that needs a Printer:

class MyClass {

    init(printer: Printer) {
        self.printer = printer

    private let printer: Printer

    func doSomething() {
        let result = printer.print("I'm doing it!")

we can now inject a mock printer instead of a real one:

let printer = MockPrinter()
let obj = MyClass(printer: printer)

and inspect how the mock is used by the class like this:

printer.printInvokeCount   // 2
printer.printInvokeArgs    // ["I'm doing it!", "I'm doing it!"]

In lack of other tools, this works fairy well. However, manual mocks are tedious and time-consuming and dulicates the same boilerplate code and logic over and over again.

We can do better.



MockingKit is a Swift-based mocking library that makes it easy to mock mock protocol implementations and classes. It lets you invoke function calls, inspect invokations and register function results.


Before we continue, let’s clarify what this means in detail.

Invokation is to record a function call. It saves information about how many times a function has been called, with which arguments and the returned result.

Inspection is to look at recorded invokation information and use it e.g. in a unit test. For instance, we can verify that a function has been triggered only once, with a certain argument.

Registration is to pre-register function results, based on the arguments with which a function is called.

Creating a mock

To create a mock with MockingKit, you can inherit the Mock base class and implement any protocol that you want to mock. If your mock has to inherit another class (e.g. when mocking system classes like UserDefaults), you can implement the Mockable protocol instead and provide a custom mock.

Mock is basically just a Mockable that uses itself as mock.

In other words, you can either do this:

import MockingKit

class MockStringValidator: Mock {}

or this:

import MockingKit

class MockStringValidator: Mockable {

    let mock = Mock()

The two alternatives are identical except from the mock property. You use them in exactly the same way.

Invoking function calls

To be able to invoke functions, you need to create a MockReference for each function that you want to mock. A mock reference is basically just a reference to a function with a unique identifier.

Let’s implement the Printer protocol and mock the print function:

import MockingKit

class MockPrinter: Mock, Printer {

    lazy var printRef = MockReference(print)

    func print(_ text: String) {
        invoke(printRef, args: (text))

Note how a reference must be lazy, since it refers to an instance function. The printRef now refers to print and is invoked whenever print is called.

Inspecting invokations

As we mentioned before, calling invoke records a function call so that you can inspect it later.

Given the MockPrinter above, we could inspect it like this:

let printer = MockPrinter()
let inv = printer.invokations(of: printer.printRef)     // => 1 item
inv[0].arguments                                        // => "Hello!"
printer.hasInvoked(printer.printRef)                    // => true
printer.hasInvoked(printer.printRef, numberOfTimes: 1)  // => true
printer.hasInvoked(printer.printRef, numberOfTimes: 2)  // => false

MockingKit has a bunch of handly inspection alternatives, like checking if a function has been invoked or not, how many times, with what arguments, what it returned etc. Since print has no return value, this information is not available in this case.

Registering return values

Say that we have a protocol that has a function that returns a value:

protocol StringConverter {

    func convert(_ text: String) -> String

A MockingKit mock would look like this:

class MockStringConverter: Mock, StringConverter {

    lazy var convertRef = MockReference(convert)

    func convert(_ text: String) -> String {
        invoke(convertRef, args: (text))

Unlike the void invokation in MockPrinter, this invoke actually returns a value. Since the function body is a one-lines, you can omit return.

If the return value is optional, it is optional to register a return value before invoking the function. Calling invoke before registering a return value will return nil.

If the return value is non-optional, it is mandatory to register a return value before invoking the function. Calling invoke before registering a return value will cause a crash.

Registering a return value is easy:

let mock = MockConverter()
let result = mock.convert("banana") // => Crash!
converter.registerResult(for: mock.convertRef) { input in String(input.reversed()) }
// or, shorter: 
converter.registerResult(for: mock.convertRef) { String($0.reversed()) }
let result = mock.convert("banana") // => Returns "ananab"

Note how we can use the input argument to determine the returned result. This gives function registration a lot of power in MockingKit.

Multiple function arguments

If a mocked function has multiple arguments, inspection behaves a little different, since function arguments are handled as a tuple.

Say that we have a protocol that looks like this:

protocol MyProtocol {

    func doStuff(int: Int, string: String) -> String

A MockingKit mock would look like this:

class MyMock: Mock, MyProtocol {

    lazy var doStuffRef = MockReference(doStuff)

    func doStuff(int: Int, string: String) -> String {
        invoke(doStuffRef, args: (int, string))

Since function arguments are handled as tuples, you now use tuple positions when referring to the arguments:

let mock = MyMock()
mock.registerResult(for: mock.doStuffRef) { args in String(args.1.reversed()) }
let result = mock.doStuff(int: 42, string: "string")    // => "gnirts"
let inv = mock.invokations(of: mock.doStuffRef)         // => 1 item
inv[0].arguments.0                                      // => 42
inv[0].arguments.1                                      // => "message"
inv[0].result                                           // => "gnirts"
mock.hasInvoked(mock.doStuffRef)                        // => true
mock.hasInvoked(mock.doStuffRef, numberOfTimes: 1)      // => true
mock.hasInvoked(mock.doStuffRef, numberOfTimes: 2)      // => false

There is no upper-limit to the number of function arguments you can use in a mocked function.

Multiple functions with the same name

If your mock has multiple methods with the same name:

protocol MyProtocol {
    func doStuff(with int: Int) -> Bool
    func doStuff(with int: Int, string: String) -> String

your must explicitly specify the function signature when creating references:

class MyMock: Mock, MyProtocol {

    lazy var doStuffWithIntRef = MockReference(doStuff as (Int) -> Bool)
    lazy var doStuffWithIntAndStringRef = MockReference(doStuff as (Int, String) -> String)

    func doStuff(with int: Int) -> Bool {
        invoke(doStuffWithInt, args: (int))

    func doStuff(with int: Int, string: String) -> String {
        invoke(doStuffWithIntAndStringRef, args: (int, string))

This is actually nice, since it gives you unique references to each function. It also makes the unit test code easier to write.


In MockingKit, properties can’t be mocked since you really shouldn’t have to. Just set the properties of the mock right away. If you for some reason want to inspect how a property is called and modified (you probably shouldn’t have to), you can just invoke a custom reference in the getter and/or setter.

Async functions

There are currently no built-in tools for working with async functions. An async function is just a void function and any completion blocks you provide it with are just arguments like any others. You must manually call the completions from within your mocks.


MockingKit is a tiny library that simplifies working with mocked logic. If you are into unit testing and mocking, I’d love for you to try it out and tell me what you think.