swift_static_variable_thread_lock.swift

import UIKit

enum Hello {
    static let foo: Int = {
        var result: Int?
        
        if Thread.isMainThread {
            result = 300
        } else {
            DispatchQueue.main.sync {
                result = 100
            }
        }
        
        return result!
    }()
}

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        DispatchQueue.global().async {
            _ = Hello.foo
        }
        
        DispatchQueue.main.async {
            _ = Hello.foo
        }
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        view.backgroundColor = .systemOrange // Never Called ㅠㅠ
    }
}