[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