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()

So why not just use an NSTimer?