ObjectWithCriticalResource.swift

import Foundation

class ObjectWithCriticalResource: NSObject {
	
	private let semaphore = dispatch_semaphore_create(1)
	
	func doTask() {
		
		guard dispatch_semaphore_wait(semaphore, DISPATCH_TIME_NOW) == 0 else {
		// abort this call without doing the task
			return
		}
		
		self.doTaskImpl()
	}
	
	private func doTaskImpl() {
		
	}
}