Clock Controller

ClockController.swift

import Foundation

// Controller that evokes its callback every minute
class ClockController {
	private let minuteCallback: NSDate -> ()
	
	init(minuteCallback: NSDate -> ()) {
		self.minuteCallback = minuteCallback
		callbackAndReschedule()
	}
	
	func callbackAndReschedule() {
		let now = NSDate()
		minuteCallback(now)
		schedule(nextMinuteAfter: now)
	}
	
	func schedule(nextMinuteAfter date: NSDate) {
		var spec = timespec(tv_sec: Int(date.timeIntervalSince1970), tv_nsec: 0)

		// Adjust the time for exactly on the next minute
		spec.tv_sec -= spec.tv_sec % 60
		spec.tv_sec += 60

		let time = withUnsafePointer(&spec) { dispatch_walltime($0, 0) }

		dispatch_after(time, dispatch_get_main_queue()) { [weak self] in
			self?.callbackAndReschedule()
		}
	}
}

let controller = ClockController { date in
	print("Hi \(date)") // Will run on init and on the minute, every minute thereafter
}
// If the object is deallocated, the callback will no longer be called.
// So keep a strong reference to the object, and cancel it by deallocating it.

dispatch_main()

Note that this doesn't handle changes to the system clock. We can handle that by adding an observer to the NSSystemClockDidChangeNotification notification in init (and removing the observer in dealloc). When we receive a notification, we should cancel our last dispatch and schedule a new one. Note that dispatch_after does not support canceling blocks, so you'll have to use the lower level dispatch source API.

So why not just use an NSTimer?