Mutex built on dispatch_semaphore

semaphore.swift

import Foundation

// a mutex built on a semaphore
// see http://www.cocoawithlove.com/blog/2016/06/02/threads-and-mutexes.html
struct DispatchSemaphore {
    let s = dispatch_semaphore_create(1)
    func sync<R>(@noescape f: () throws -> R) rethrows -> R {
        _ = dispatch_semaphore_wait(s, DISPATCH_TIME_FOREVER)
        defer { _ = dispatch_semaphore_signal(s) }
        return try f()
    }
}
let semaphore = DispatchSemaphore()

// incement and print
var i: Int = 0
var f = { i = i+1; print("\(i)") } 

// repeat 10x asynchronously
let queue: dispatch_queue_t = dispatch_queue_create("com.blah.myqueue", DISPATCH_QUEUE_CONCURRENT)
for _ in 0..<10 {
    dispatch_async(queue){ 
        true ? semaphore.sync(f) : f() // try false to disable the mutex
    }
}

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