Using semaphore in SWIFT

Semaphore.playground

import UIKit

/// -------------------------------------------------------------------

private func example1() {
    let queue = DispatchQueue.global(qos: DispatchQoS.QoSClass.default)
    for i in 0..<10 {
        queue.async() {
            print("Start: \(i)")
            sleep(3)
            print("End: \(i)")
        }
    }
}

/** Logs:
Start: 0
Start: 1
Start: 2
Start: 3
Start: 4
Start: 5
Start: 6
Start: 7
Start: 8
Start: 9
End: 4
End: 8
End: 2
End: 5
End: 1
End: 6
End: 7
End: 3
End: 0
End: 9
*/

/// -------------------------------------------------------------------

private func semaphoreExample1() {
    let semaphore = DispatchSemaphore(value: 3)
    let queue = DispatchQueue.global(qos: DispatchQoS.QoSClass.default)
    for i in 0..<10 {
        queue.async() {
            semaphore.wait(timeout: DispatchTime.distantFuture)
            print("Start: \(i)")
            sleep(3)
            print("End: \(i)")
            semaphore.signal()
        }
    }
}

/** Logs:
Start: 2
Start: 1
Start: 0
End: 2
End: 1
End: 0
Start: 4
Start: 5
Start: 3
End: 4
End: 5
End: 3
Start: 6
Start: 7
Start: 8
End: 6
End: 7
End: 8
Start: 9
End: 9
*/

/// -------------------------------------------------------------------

private func semaphoreExample2() {
    let semaphore = DispatchSemaphore(value: 0)
    let queue = DispatchQueue.global(qos: DispatchQoS.QoSClass.default)
    queue.async() {
        print("Running async task...")
        sleep(3)
        print("Async task completed")
        semaphore.signal()
    }
    print("Waiting async task...")
    semaphore.wait(timeout: DispatchTime.distantFuture)
    print("Continue!")
}

/** Logs:
Running async task...
Waiting async task...
Async task completed
Continue!
*/

/// -------------------------------------------------------------------

private func semaphoreExample3() {
    let semaphore = DispatchSemaphore(value: 0)
    let queue = DispatchQueue.global(qos: DispatchQoS.QoSClass.default)
    let n = 9
    for i in 0..<n {
        queue.async() {
            print("\(i): Running async task...")
            sleep(3)
            print("\(i): Async task completed")
            semaphore.signal()
        }
    }
    print("Waiting async task...")
    for i in 0..<n {
        print("Start waiting \(i)")
        semaphore.wait(timeout: DispatchTime.distantFuture)
        print("\(i + 1)/\(n) completed")
    }
    print("Continue!")
}

/** Logs:
0: Running async task...
1: Running async task...
2: Running async task...
3: Running async task...
4: Running async task...
5: Running async task...
6: Running async task...
7: Running async task...
8: Running async task...
Waiting async task...
Start waiting 0
2: Async task completed
1: Async task completed
6: Async task completed
5: Async task completed
4: Async task completed
0: Async task completed
3: Async task completed
1/9 completed
Start waiting 1
2/9 completed
Start waiting 2
3/9 completed
Start waiting 3
4/9 completed
Start waiting 4
5/9 completed
Start waiting 5
6/9 completed
Start waiting 6
7/9 completed
Start waiting 7
8: Async task completed
7: Async task completed
8/9 completed
Start waiting 8
9/9 completed
Continue!
*/

/// -------------------------------------------------------------------

private func semaphoreExample4() {
    let queue = DispatchQueue.global(qos: DispatchQoS.QoSClass.default)
    let group = DispatchGroup()
    let n = 9
    for i in 0..<n {
        queue.async(group: group, execute: DispatchWorkItem.init(block: {
            print("\(i): Running async task...")
            sleep(3)
            print("\(i): Async task completed")
        }))
    }
    print("Waiting async task...")
    group.wait(timeout: DispatchTime.distantFuture)
    print("Continue!")
}

/** Logs:
0: Running async task...
1: Running async task...
2: Running async task...
3: Running async task...
4: Running async task...
5: Running async task...
6: Running async task...
7: Running async task...
Waiting async task...
8: Running async task...
8: Async task completed
6: Async task completed
3: Async task completed
5: Async task completed
0: Async task completed
4: Async task completed
1: Async task completed
7: Async task completed
2: Async task completed
Continue!
*/

/// -------------------------------------------------------------------