[Office Hours] is there a way to mock dates in unit tests?

[kkellybonilla]

Related Discussion

Is there a way to properly mock dates in unit tests? I attempted to mock them as such:

extension Date {
	private static var _mockNow: Date?
	
	static var mockNow: Date? {
		get { _mockNow }
		set { _mockNow = newValue }
	}
	
	static var now: Date {
		mockNow ?? Date()
	}
}

@Test("Returns medications the patient is currently taking") mutating func currentMedicationsReturns() {
			Date.mockNow = Date.createDateFromComponents(
						calendar: Calendar.current,
						year: 2025,
						month: 1,
						day: 10)

but i think it may be causing race conditions since sometimes my tests pass and at other times they don't. i remember briefly reading that swift unit tests run concurrently, though i'm not certain.

Reproduction

N/A

Additional context

No response

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[ChenaLee]

I'm guessing you have static dates for your prescription test data and trying to compare that with today?
Option1: Hack
In swift there seem to be nothing that does such constructor mocking, so I ended up just hacking by changing prescription date of my test object every time I run the tests. e.g. I construct date_two_days_ago_from_today if I want the date to be in the past, etc.

Option2: Best practice
Alternatively, you can do follow the best practice that applies across all languages. In source code, instead of doing Date(), you can supply a new custom class like DateFactory, and your source code can take a DateFactory instance as an input variable. So in source code instead of doing today = Date() you'd be doing dateFactory.today() which, under the hood, can be just return Date(). Then for unit test, you can just supply a MockDateFactory that's an extension of DateFactory and make dateFactory.today() return whatever you want.
I didn't go this route because again, I did not wanna spend time figuring out dependency injection framework for swift, and if I do it without such framework too many parts of the codebase will look cluttered. Deadlines too tight; wasn't worth it.

[vishnuravi]

Hi @kkellybonilla,

Along the lines of what @ChenaLee has mentioned in her comment, there is a "simple" dependency injection mechanism you can use here to inject a function that generates the date, as described here.

@PSchmiedmayer do you have any advice here?

[ChenaLee]

Thanks, but that article's just describing how to do manual dependency injection. I was looking for something like Guice or Spring's dependency injection which makes the best practice a realistic option and the reason why everybody uses them. Although, I didn't realize I could supply default values like that to help in this case. That's definitely much more doable

[PSchmiedmayer]

Hi @ChenaLee,

Good question!
Here are multiple great dependency injection frameworks out there that you can use to simplify your testing setup. Pointfree has a gerat library that you can use (swift-dependencies).

Factory and Swinject are also great options.

For simpler elements you might also want to use simple default values or injecting elements in initializers if necessary or follow articles such as the ones that @vishnuravi has linked.