Mocking in Swift

Question

How do you mock an object in Swift?

The Mirror protocol sounded promising, but it doesn't do much right now.

So far the only approach I found is to subclass and override all methods of the mocked class. This is of course not a true mock, far from ideal, and a lot of work.

Any other ideas?

Why not OCMock?

From the source:

Can I use OCMock using the language bridge functionality?

Yes, but with limitations. If you are brave. As of now this is highly experimental. There's no guarantee that OCMock will ever fully support Swift.

Known limitations:

• Tests have to be written in Objective-C
• Objects that should be mocked must inherit from NSObject
• No stubbing/expecting/verifying of class methods

Answer 1

NSHipster touches on language features in Swift which make an external mocking library less necessary:

In Swift, classes can be declared within the definition of a function, allowing for mock objects to be extremely self-contained. Just declare a mock inner-class, override and [sic] necessary methods:

func testFetchRequestWithMockedManagedObjectContext() {     class MockNSManagedObjectContext: NSManagedObjectContext {         override func executeFetchRequest(request: NSFetchRequest!, error: AutoreleasingUnsafePointer<NSError?>) -> [AnyObject]! {             return [["name": "Johnny Appleseed", "email": "[email protected]"]]         }     }      ... } 

The ability to create a subclass of your external dependency in the local scope plus the addition of XCTestExpectation solve a lot of the same problems as OCMock.

The one thing that a library such as OCMock provides that is very useful are its "verify" methods to ensure that the mock classes were called. It's possible to manually add this, but the automatic addition is nice.

Answer 2

I create my mock classes by wrapping everything in a protocol. I hand roll a mock class to conform to the protocol in question, like so:

protocol Dog: class {     var name: String { get }      func bark() }  class DogImpl: Dog {     var name: String      init(name: String) {         self.name = name     }      func bark() {         print("Bark!")     } }  class DogMock: Dog {     var name = "Mock Dog"     var didBark = false      func bark() {         didBark = true     } } 

I use this in conjunction with dependency injection to achieve full test coverage. It's a lot of boilerplate, but I haven't had any issues with this approach so far.

Regarding subclass mocking, you'll run into trouble with final classes, or if they have non-trivial initializers.