动画UIView(在UIGraphicsGetCurrentContext上绘制时)

我创建了一个类,它绘制一个条形图,当设置一个条形图时,该条形图指示一个百分比值。如何设置条的动画,使其从左向右增长?

动画功能,我已经没有工作,因为它希望它。。。

class EasyLineView : UIView {

    var fillRect = CGRect.init(x: 0, y: 0, width: 0, height: 0)

    var percentValue: CGFloat = 0 {
        didSet {
                self.layoutIfNeeded()
        }
    }

    var orientation: BarOrientation = .vertical

    override init(frame: CGRect) {
        super.init(frame: frame)
        backgroundColor = UIColor.black
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    override func draw(_ rect: CGRect) {

        print("drawing with \(rect.size.width)")

        let r = self.bounds // the view's bounds

        let ctx = UIGraphicsGetCurrentContext() // get the current context

        ctx!.setFillColor(UIColor.yellow.cgColor) // set fill color to the given color if it's provided, else use clearColor

        if (self.orientation == .vertical){
            fillRect = CGRect.init(x: 0, y: 0, width: r.size.width, height: percentValue*r.size.height)

        } else {
            fillRect = CGRect.init(x: 0, y: 0, width: percentValue*r.size.width, height: r.size.height)
        }

        ctx!.fill(fillRect)

    }

// does not work
func animate(){
    UIView.animate(withDuration: 5, delay: 0, animations: {
        self.layoutIfNeeded()
    })
}

}