How to mock NSDate in Swift? - unit-testing

Question

I have to test some date calculation but to do so I need to mock NSDate() in Swift. Whole app is written in Swift and I'd like to write test in it as well.
I've tried method swizzling but it doesn't work (or I'm doing something wrong which is more likely).
extension NSDate {
func dateStub() -> NSDate {
println("swizzzzzle")
return NSDate(timeIntervalSince1970: 1429886412) // 24/04/2015 14:40:12
}
}
test:
func testCase() {
let original = class_getInstanceMethod(NSDate.self.dynamicType, "init")
let swizzled = class_getInstanceMethod(NSDate.self.dynamicType, "dateStub")
method_exchangeImplementations(original, swizzled)
let date = NSDate()
// ...
}
but date is always current date.

Answer 1

Disclaimer -- I'm new to Swift testing so this may be a horribly hacky solution, but I've been struggling with this, too, so hopefully this will help someone out.
I found this explanation to be a huge help.
I had to create a buffer class between NSDate and my code:
class DateHandler {
func currentDate() -> NSDate! {
return NSDate()
}
}
then used the buffer class in any code that used NSDate(), providing the default DateHandler() as an optional argument.
class UsesADate {
func fiveSecsFromNow(dateHandler: DateHandler = DateHandler()) -> NSDate! {
return dateHandler.currentDate().dateByAddingTimeInterval(5)
}
}
Then in the test create a mock that inherits from the original DateHandler(), and "inject" that into the code to be tested:
class programModelTests: XCTestCase {
override func setUp() {
super.setUp()
class MockDateHandler:DateHandler {
var mockedDate:NSDate! = // whatever date you want to mock
override func currentDate() -> NSDate! {
return mockedDate
}
}
}
override func tearDown() {
super.tearDown()
}
func testAddFiveSeconds() {
let mockDateHandler = MockDateHandler()
let newUsesADate = UsesADate()
let resultToTest = usesADate.fiveSecondsFromNow(dateHandler: mockDateHandler)
XCTAssertEqual(resultToTest, etc...)
}
}

Answer 2

If you want to swizzle it you need to swizzle a class that is internally used by NSDate and it is __NSPlaceholderDate. Use this only for testing since it is a private API.
func timeTravel(to date: NSDate, block: () -> Void) {
let customDateBlock: #convention(block) (AnyObject) -> NSDate = { _ in date }
let implementation = imp_implementationWithBlock(unsafeBitCast(customDateBlock, AnyObject.self))
let method = class_getInstanceMethod(NSClassFromString("__NSPlaceholderDate"), #selector(NSObject.init))
let oldImplementation = method_getImplementation(method)
method_setImplementation(method, implementation)
block()
method_setImplementation(method, oldImplementation)
}
And later you can use like this:
let date = NSDate(timeIntervalSince1970: 946684800) // 2000-01-01
timeTravel(to: date) {
print(NSDate()) // 2000-01-01
}
As others suggested I would rather recommend introducing a class Clock or similar that you can pass around and get a date from it and you can easily replace it with an alternative implementation in your tests.

Answer 3

Rather than use swizzling you should really design your system to support testing. If you do a lot of data processing then you should inject the appropriate date into the functions which use it. In this way your test injects the dates into these functions to test them and you have other tests which verify that the correct dates will be injected (when you stub the methods that use the dates) for various other situations.
Specifically for your swizzling problem, IIRC NSDate is a class cluster so the method you're replacing is unlikely to be called as a different class will be 'silently' created and returned.