Simple demo of dispatch_group_async/dispatch_group_notify/dispatch_group_wait in Swift

DispatchGroupDemo.swift

import Foundation

let notified = dispatch_semaphore_create(0)

let group = dispatch_group_create()
let queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)

for n in 0..<20 {
    dispatch_group_async(group, queue) {
        let timeInterval = Double(arc4random_uniform(1000)) * 0.01
        NSThread.sleepForTimeInterval(timeInterval)
        println("Finish task \(n)")
    }
}

dispatch_group_notify(group, queue) {
    // This block will be executed when all tasks are complete
    println("All tasks complete")
    dispatch_semaphore_signal(notified)
}

// Block this thread until all tasks are complete
dispatch_group_wait(group, DISPATCH_TIME_FOREVER)

// Wait until the notify block signals our semaphore
dispatch_semaphore_wait(notified, DISPATCH_TIME_FOREVER)