thread.swift

import Foundation

func th() {
    print(Thread.current)
}
th()

func a(_ i: Int) async -> Int {
    th()
    print("sleep \(i)")
    sleep(1)
    print("awake \(i)")
    return i
}

let t = async {
    th()
    print("Task begin")

    let i: Int = await withTaskGroup(of: Int.self) { g in
        th()
        print(g)
        for i in 0..<20 {
            // Add child task to the group
            g.async {
                await a(i)
            }
        }
        var i = 0
        // group conforms `AsyncSequence`
        for await r in g {
            i += r
            print(r)
        }
        return i
    }
    print(i)

    print("Task end")
}
print(t)

sleep(5)
print("Done")