PlaceHighlights.swift

func placeHighlights(atTheseLocations rectangles: [CGRect]) {
        
    /// First, we'll remove all the existing highlights in the drawingView
    for highlight in drawingView.subviews {

        /// Fading it out
        UIView.animate(withDuration: 0.3, animations: {
            highlight.alpha = 0
        }) { _ in

            /// Then removing it once it's finished fading
            highlight.removeFromSuperview()
        }
    }

    /// we're going to loop through the rectangles, adding a highlight "in real life" every time
    for rectangle in rectangles {

        /// we make a new UIView
        let highlight = UIView(frame: CGRect(x: rectangle.origin.x, y: rectangle.origin.y, width: rectangle.width, height: rectangle.height))

        /// alpha is 0 right now because we're going to fade it in
        highlight.alpha = 0

        /// we're going to draw a rounded rectangle
        let newLayer = CAShapeLayer()
        let layerRect = CGRect(x: 0, y: 0, width: rectangle.width, height: rectangle.height)
        newLayer.bounds = layerRect
        newLayer.path = UIBezierPath(roundedRect: layerRect, cornerRadius: rectangle.height / 3.5).cgPath
        newLayer.lineWidth = 3

        /// set the position of the rounded rectangle (by default it would appear centered in the upper left corner, we don't want that)
        let x = newLayer.bounds.size.width / 2
        let y = newLayer.bounds.size.height / 2
        newLayer.position = CGPoint(x: x, y: y)

        /// Color of the highlights
        newLayer.fillColor =  #colorLiteral(red: 0.6502560775, green: 0.9603669892, blue: 1, alpha: 0.3)
        newLayer.strokeColor =  #colorLiteral(red: 0, green: 0.6823529412, blue: 0.937254902, alpha: 1)

        /// Add the rounded rectangle to the highlight (a UIView)
        highlight.layer.addSublayer(newLayer)

        /// we'll add the highlight to the drawingView now
        drawingView.addSubview(highlight)

        /// and finally, we'll fade it in!
        UIView.animate(withDuration: 0.3, animations: {
            highlight.alpha = 1
        })
    }
}