Multiple workers in Swift Command Line Tool

I mistakenly interpreted the locking thread for a hanging program. The work will execute just fine without a run loop. The code in the question will run fine, and blocking the main thread until the whole group has finished.

So say chunks contains 4 items of workload, the following code spins up 4 concurrent workers, and then waits for all of the workers to finish:

let group = DispatchGroup()
let queue = DispatchQueue(label: "", attributes: .concurrent)

for chunk in chunk {
    queue.async(group: group, execute: DispatchWorkItem() {
        do_work(chunk)
    })
}

_ = group.wait(timeout: .distantFuture)

Just like with an Objective-C CLI, you can make your own run loop using NSRunLoop.

Here's one possible implementation, modeled from this gist:

class MainProcess {
    var shouldExit = false

    func start () {
        // do your stuff here
        // set shouldExit to true when you're done
    }
}

println("Hello, World!")

var runLoop : NSRunLoop
var process : MainProcess

autoreleasepool {
    runLoop = NSRunLoop.currentRunLoop()
    process = MainProcess()

    process.start()

    while (!process.shouldExit && (runLoop.runMode(NSDefaultRunLoopMode, beforeDate: NSDate(timeIntervalSinceNow: 2)))) {
        // do nothing
    }
}

As Martin points out, you can use NSDate.distantFuture() as NSDate instead of NSDate(timeIntervalSinceNow: 2). (The cast is necessary because the distantFuture() method signature indicates it returns AnyObject.)

If you need to access CLI arguments see this answer. You can also return exit codes using exit().

Swift 3 minimal implementation of Aaron Brager solution, which simply combines autoreleasepool and RunLoop.current.run(...) until you break the loop:

var shouldExit = false
doSomethingAsync() { _ in
    defer {
        shouldExit = true
    }
}
autoreleasepool {
    var runLoop = RunLoop.current
    while (!shouldExit && (runLoop.run(mode: .defaultRunLoopMode, before: Date.distantFuture))) {}
}