What language level support (if any) does Swift have for asynchronous programming?

Question

Asynchronous programming is a must for responsive user interfaces when application have to communicate over unpredictable networks (e.g. smart phone applications). The user i

Answer 1

Swift's approach to asynchronous programming is the same as Objective C's: use Grand Central Dispatch. You can pass closures to gcd dispatch_ functions, just as in ObjC. However, for aesthetic reasons, you can also pass your closure (block) after the close parentheses:

dispatch_async(dispatch_get_main_queue()) {
    println("async hello world")
}

Answer 2

While it isn't a built-in language feature, it may be interesting to note that it's possible to implement C# style async/await for Swift, and that because of the special syntax afforded to the last closure argument of a function call, it even looks like it might be part of the language.

If anyone is interested, you can get code for this on Bitbucket. Here's a quick taster of what's possible:

let task = async { () -> () in
  let fetch = async { (t: Task<NSData>) -> NSData in
    let req = NSURLRequest(URL: NSURL.URLWithString("http://www.google.com"))
    let queue = NSOperationQueue.mainQueue()
    var data = NSData!
    NSURLConnection.sendAsynchronousRequest(req,
                                            queue:queue,
      completionHandler:{ (r: NSURLResponse!, d: NSData!, error: NSError!) -> Void in
        data = d
        Async.wake(t)
      })
    Async.suspend()
    return data!
  }

  let data = await(fetch)
  let str = NSString(bytes: data.bytes, length: data.length,
                     encoding: NSUTF8StringEncoding)

  println(str)
}

Also, if you want something like @synchronized, try this:

func synchronized(obj: AnyObject, blk:() -> ()) {
  objc_sync_enter(obj)
  blk()
  objc_sync_exit(obj)
}

var str = "A string we can synchronise on"

synchronized(str) {
  println("The string is locked here")
}