A function that creates and starts a timer dispatch source.

RepeatingTimer.swift

//
// RepeatingTimer.swift
//

import Foundation

enum TimerError: ErrorType {
    /// The timer could not be created.
    case CouldNotCreate
}

/**
Create and start a timer dispatch source.
This is best used for short running timers. Be sure to call `dispatch_source_cancel()` on the timer to invalidate it.
- Parameters:
- interval: The interval for the timer.
- leeway:   The amount of time that the system can defer the timer.
- start:    The start time of the timer. Defaults to DISPATCH_TIME_NOW
- queue:    The dispatch queue to which the action closure is submitted. Defaults to the main queue.
- action:   The closure to submit to the queue. Has the signature of () -> Void
- Returns: A dispatch source timer that has already been started.
- Throws: A `TimerError.CouldNotCreate` error if the timer could not be created.
*/
@warn_unused_result
func repeatingTimerWithInterval(
    interval: NSTimeInterval,
    leeway: NSTimeInterval,
    start: dispatch_time_t = DISPATCH_TIME_NOW,
    queue: dispatch_queue_t = dispatch_get_main_queue(),
    action: dispatch_block_t)
    throws -> dispatch_source_t {

        let timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue)
        guard timer != nil else { throw TimerError.CouldNotCreate }

        func nanoseconds(seconds: NSTimeInterval) -> UInt64 {
            enum Static { static let Multiplier = Double(NSEC_PER_SEC) }

            return UInt64(seconds * Static.Multiplier)
        }

        dispatch_source_set_event_handler(timer, action)
        dispatch_source_set_timer(timer, start, nanoseconds(interval), nanoseconds(leeway))

        dispatch_resume(timer)

        return timer
}

How do you stop the timer?