DispatchGroup Demo

DispatchGroup.swift

/*
 ##: DispatchGroup
- Lets you wait for task to happen and then we can continue to do what we want
- Whenever starting a task, use enter() to denote the start of a task
- Whenever completed, use leave() to denote finishing of the task
- You call wait() to block the current thread while waiting for tasks’ completion.
This waits forever which is fine because the photos creation task always completes.
You can use wait(timeout:) to specify a timeout and bail out on waiting after a specified time.
- To let know when all tasks are done, use notify() which automatically gets triggered when all tasks are done
*/

import UIKit

var items = [Int]()

func getItems() -> [Int] {
    let numberOfItems = arc4random_uniform(10) + 1
    return (0..<numberOfItems).map { _ in  Int(arc4random_uniform(100)) }
}

func run(after delay: Int, tasks: @escaping () -> Void) {
    DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(delay)) {
        tasks()
    }
}

let dispatchGroup = DispatchGroup()

let tasks = {
    items.append(contentsOf: getItems())
    print(items)
    dispatchGroup.leave()
}

dispatchGroup.enter()
run(after: 2) { tasks() }

dispatchGroup.enter()
run(after: 4) { tasks() }

dispatchGroup.enter()
run(after: 6) { tasks() }


dispatchGroup.notify(queue: .main) {
    print("\n## Done")
    print(items)
}