2020/11/11 Swift 線上讀書會討論 Deadlock

ViewController.swift

import UIKit

class ViewController: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        setMainQueueKey()
    }
    @IBAction func trigger(_ sender: Any) {
        let list = (6...10).map(\.description)
        var captured = [String]()
        let group = DispatchGroup()
        assert(isMainQueue())
        for s in list {
            print(s," start")
            group.enter()
            assert(isMainQueue())
            delayWork { [group] in
                print(s, "finished")
                captured.append(s)
                group.leave()
            }
            assert(isMainQueue())
            group.wait()
        }
        assert(isMainQueue())
        group.notify(queue: .main) {
            print("captured.count == ", captured.count)
        }
    }
}

func delayWork(then: @escaping () -> Void) {
    DispatchQueue.global().asyncAfter(deadline: .now() + 2.0) {
        then()
    }
}
let key = DispatchSpecificKey<UInt>()

func setMainQueueKey() {
    DispatchQueue.main.setSpecific(key: key, value: UInt(1))
}
func isMainQueue() ->Bool {
    DispatchQueue.getSpecific(key: key) == UInt(1)
}

這樣用你的 UI 會卡住