Allow implicit self for weak self captures, after self is unwrapped — SE-0365

從 Xcode 14.3 的 Swift 5.8 開始，Apple 在 SE-0365 推出了貼心的新功能，當 weak self 已經被 unwrapped 後，我們可以在接下來的程式省略 self。

以底下 Timer 的 closure 程式為例，在 Swift 5.8 之前，我們必須使用 self 才能存取 controller 的 property view & label。

class ViewController: UIViewController {

    @IBOutlet weak var label: UILabel!
    var timer: Timer?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        
        timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { [weak self] timer in
            guard let self else { return }
            self.view.backgroundColor = UIColor(red: .random(in: 0...1), green: .random(in: 0...1), blue: .random(in: 0...1), alpha: 1)
            self.label.text = "\(Int.random(in: 1...100))"
        }
    }
    
    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        
        timer?.invalidate()
    }
    
    
}

沒有加 self 時會出現錯誤 Reference to property xxx in closure requires explicit use of ‘self’ to make capture semantics explicit。

在 Swift 5.8 不用再寫那麼多 self 了，只要 weak self 被 unwrapped，我們即可在接下來的程式省略 self。

• 寫法1: 使用 guard let 做 unwrap。

timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { [weak self] timer in
    guard let self else { return }
    view.backgroundColor = UIColor(red: .random(in: 0...1), green: .random(in: 0...1), blue: .random(in: 0...1), alpha: 1)
    label.text = "\(Int.random(in: 1...100))"
}

• 寫法2: 使用 if let 做 unwrap。

timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { [weak self] timer in
    if let self {
        view.backgroundColor = UIColor(red: .random(in: 0...1), green: .random(in: 0...1), blue: .random(in: 0...1), alpha: 1)
        label.text = "\(Int.random(in: 1...100))"
    }     
}

swift-evolution/0365-implicit-self-weak-capture.md at main · apple/swift-evolution