Grand Central Dispatch (GCD, Dispatch in Swift 3) DispatchGroup and DispatchSemaphore Demo

DispatchDemo.swift

// import Dispatch
import Foundation

let semaphore = DispatchSemaphore(value: 0)
let group = DispatchGroup()
let globalQueue = DispatchQueue.global(qos: .default)

print("Tasks started!!!")
print()

// Run asynchronous closures
for i in 0..<10 {
    globalQueue.async(group: group) {
        let timeInterval = Double(arc4random_uniform(300)) / 100
        Thread.sleep(forTimeInterval: timeInterval)
        print("Task #\(i) finished!!")
        
        // signal and increase the semaphore
        if i == 5 {
            semaphore.signal()
        }
    }
}

// DispatchGroup.notify is an asynchoruous method that return immediately.
group.notify(queue: globalQueue) {
    // This closure will be executed after all tasks done.
    print()
    print("All tasks done!!!")
}


// Wait until the someone signals the semaphore
semaphore.wait() // wait forever, block this thread
print("semaphore count decreased to zero")


// Block this thread until all tasks in the group are complete
// The enum `.distantFuture` is equal to `DISPATCH_TIME_FOREVER` in Objc and Swift 2.3
group.wait(timeout: .distantFuture)
print("group complete!!!")