Main queue?.swift

let mainQueueA = dispatch_get_global_queue(qos_class_main(), 0)
let mainQueueB = dispatch_get_main_queue()

// This works fine
dispatch_apply(3, mainQueueA) { i in
    println("A: \(i)")
}
println("Yep")

// This never calls the block and hangs the main thread
dispatch_apply(3, mainQueueB) { i in
    println("B: \(i)")
}
println("Never called")

// The printout is of running this code on OS X 10.10 or iOS 8.0 is (something like):

// AAA:::   201
//
//
// Yep

// ... and then nothing :/
// 1. Zero CPU usage
// 2. Pausing the debugger show that the main thread hangs in semaphore_wait_trap ()
// 3. It turns out dispatch_get_global_queue(Int(qos_class_main().value), 0) does NOT(!) return the main queue, 
//    but the concurrent user-nteractive queue.

// Lesson to be learned: If you want the main queue, use dispatch_get_main_queue() 
// and NOT(!) dispatch_get_global_queue(Int(qos_class_main().value), 0)