Throttling queue. Useful to discard button clicks until the button action is done executing.

ThrottledQueue.swift

import Foundation

/*
 Execute asynchronous tasks concurrently until reaching a given 'max' number.
 Tasks queued while there is already a 'max' number of concurrent tasks are silently discarded.
 
 Usage: ThrottledQueue(1).queueAction { finish in /* work */ finish() }
 */
struct ThrottledQueue
{
	private let semaphore: dispatch_semaphore_t
	private let queue: dispatch_queue_t = dispatch_queue_create("com.yourcompany.ThrottledQueue", nil)
	
	init(concurrentJobs:Int){
		semaphore = dispatch_semaphore_create(concurrentJobs)
	}
	
	/* The action queued must call completion when it's done processing. */
	func queueAction(action:(completion:()->())->())
	{
		if (dispatch_semaphore_wait(semaphore, DISPATCH_TIME_NOW) == 0) {
			dispatch_async(queue){
				action(){
					dispatch_semaphore_signal(self.semaphore)
				}
			}
		}
	}
}

let queue = ThrottledQueue(concurrentJobs:2)
queue.queueAction { finish in print("1"); finish() }
queue.queueAction { finish in print("2"); finish() }
queue.queueAction { finish in print("3"); finish() } // discarded

// wait 2 seconds
NSRunLoop.currentRunLoop().runMode(NSDefaultRunLoopMode, beforeDate:NSDate(timeIntervalSinceNow:2))