A C#-style Async/Await implementation in Swift

AsyncAwait.swift

// A much better version of C#-style Async/Await. It can return values!
import Cocoa

struct Await<T> {
    // private
    let group: dispatch_group_t
    let getResult: () -> T

    // public
    func await() -> T { return getResult() }
}

func async<T>(queue: dispatch_queue_t, block: () -> T) -> Await<T> {
    let group = dispatch_group_create()
    var result: T?
    dispatch_group_async(group, queue) { result = block() }
    return Await(group: group, getResult: { dispatch_group_wait(group, DISPATCH_TIME_FOREVER); return result! })
}

let q = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
let t = async(q) { () -> Int in return 2 }

let v = t.await()
println("Value is \(v)")

This is basically the Task<T> bit of async/await, but the truly useful part of await in C# is that it transforms your code into a continuation, so that when you await an async task you aren't still blocking the thread. It essentially allows you to write synchronous-looking code that is still asynchronous. It's not really possible to get that without language/compiler support though.

It’s actually kinda sorta possible in Objective-C. Horrendous, but possible.

https://github.com/nevyn/SPAsync

Mmm, macros.

I quite like the terse nature of this implementation, but it does, as threeve said, suffer from the problem that await blocks. It’s probably worth anyone reading this taking a look at my blog post and the related Async framework, since that results in pretty much the exact behaviour that C# programmers are after, give or take (i.e. non-blocking await).