更新NSLayoutConstraint动画问题

Question

这是一个简单的测试，所有代码如下，为什么当宽度更改为30时lblA没有动画？但是当宽度更改为300时它具有动画。

import UIKit

class ViewController: UIViewController {
    let lblA = UILabel()
    var lblAWidthConstraint: NSLayoutConstraint?
    var tag = false
//  let lblB = UILabel()

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        self.lblA.backgroundColor = .red

        self.view.addSubview(self.lblA)
        self.lblA.translatesAutoresizingMaskIntoConstraints = false
        self.view.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "|[v]"
            , options: []
            , metrics: nil
            , views: ["v" : self.lblA]))
        self.view.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|[v]|"
            , options: []
            , metrics: nil
            , views: ["v" : self.lblA]))

        let widthConstraint = self.lblA.widthAnchor.constraint(equalToConstant: 30)

        widthConstraint.isActive = true
        self.lblAWidthConstraint = widthConstraint

        //
        let tap = UITapGestureRecognizer(target: self, action: #selector(tapView))

        self.view.addGestureRecognizer(tap)
    }

    @objc func tapView() {
        tag = !tag

        self.lblAWidthConstraint?.constant = tag ? 300 : 30

        UIView.animate(withDuration: 0.3) {
            self.view.layoutIfNeeded()
        }
    }
}

[UPDATE]最后，我发现了这一点：Animating Frame of UILabel smoothly