问为什么在UIViewPropertyAnimator中使用“无主”

EN

animator = UIViewPropertyAnimator(duration: 2.0, curve: .easeInOut, animations: { 
        [unowned self, redBox] in
        redBox.center.x = self.view.frame.width
        redBox.transform = CGAffineTransform(rotationAngle: CGFloat.pi).scaledBy(x: 0.001, y: 0.001)
    })

import UIKit

class ViewController: UIViewController {

    var animator: UIViewPropertyAnimator!

    override func viewDidLoad() {
        super.viewDidLoad()

        //redBox

        let redBox = UIView(frame: CGRect(x: 10, y: 100, width: 100, height: 100))
        redBox.translatesAutoresizingMaskIntoConstraints = false// lar oss redigere posisjon og sånn selv, uten at xcode setter posisjon/størrelse i stein.
        redBox.backgroundColor = .red
        redBox.center.y = view.center.y

        view.addSubview(redBox)

        animator = UIViewPropertyAnimator(duration: 2.0, curve: .easeInOut, animations: { 
            [unowned self, redBox] in
            redBox.center.x = self.view.frame.width
            redBox.transform = CGAffineTransform(rotationAngle: CGFloat.pi).scaledBy(x: 0.001, y: 0.001)
        })

        // slider

        let slider = UISlider()
        slider.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(slider)
        slider.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
        slider.widthAnchor.constraint(equalTo: view.widthAnchor).isActive = true
        slider.addTarget(self, action: #selector(sliderChanged), for: .valueChanged)

    }
    func sliderChanged(_ sender: UISlider){
        animator.fractionComplete = CGFloat(sender.value)
    }

}