一、异步

class ViewController: UIViewController {
  override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    DispatchQueue.global().async {
        print("开启子线程:\(Thread.current)")
        }
        DispatchQueue.main.async {
            print("回归主线程:\(Thread.current)")
        }
    }
}

    class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad(
    let dispatchWorkitem = DispatchWorkItem {
            print("创建一个异步任务")
        }
        DispatchQueue.global().async(execute: dispatchWorkitem)
        dispatchWorkitem.notify(queue: DispatchQueue.main) {
            print("回到主线程")
        }
    }
}

二、延迟执行

     let dispatchWorkitem = DispatchWorkItem {
            print("创建一个异步任务")
        }
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now()+3, execute: dispatchWorkitem)
        dispatchWorkitem.notify(queue:DispatchQueue.main) {
            print("回归主线程")
        }

三、多线程-once

dispatch_once在Swift中已经被废弃，可以用类型属性或者全局变量\常量，默认自带lazy + dispatch_once 效果

fileprivate let task1:Void = {
    print("执行一次")
}()

class ViewController: UIViewController {
    static var age:Int = {
        print("once")
        return 10
    }()
    override func viewDidLoad() {
        super.viewDidLoad()
       
        let _ = task1
        let _ = task1
        
        let _ = Self.age
        let _ = Self.age
    }
}