[Working with multiple tasks in GCD]

gcd-multiple-tasks.swift

let myGroup = DispatchGroup()

var result = [Data]()
for i in 0 ..< 5 {
    myGroup.enter()

    Alamofire.request("https://httpbin.org/get", parameters: ["foo": "bar"]).responseJSON { response in
        print("Finished request \(i)")
        result.append(response.data)
        myGroup.leave()
    }
}

// Synchronously with timer (10 seconds)
myGroup.wait(timeout: DispatchTime(uptimeNanoseconds: 10000000000))

// Asynchronously without timer
myGroup.notify(queue: .main) {
    return result
}

// Synchronousely with timer
myGroup.notifyWait(target: .main,
                   timeout: DispatchTime.now() + 10) {
    return result
}
extension DispatchGroup {
    func notifyWait(target: DispatchQueue, timeout: DispatchTime, handler: @escaping (() -> Void)) {
        DispatchQueue.global(qos: .default).async {
            _ = self.wait(timeout: timeout)
            target.async {
                handler()
            }
        }
    }
}