playground.swift

import UIKit


protocol CanvasItemProtocol
{
    var scale : CGFloat { get set }
}

class CanvasItem : CanvasItemProtocol
{
    public var scale : CGFloat = 1.0
}

class StickerItem: CanvasItem
{
    public var stickerName : String = ""
}

class ShapeItem: CanvasItem
{
    public var color: UIColor = .red
}


class ViewItem<T: CanvasItemProtocol> : UIView
{
    let canvasItem: T
    init(_ item: T)
    {
        canvasItem = item
        super.init(frame: .zero)
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

class CanvasShapeView: ViewItem<ShapeItem> { }

class CanvasStickerView: ViewItem<StickerItem> {}

let shapeItem = ShapeItem()
let stickerItem = StickerItem()
let shapeView = ViewItem<ShapeItem>(shapeItem)
let stickerView = ViewItem<StickerItem>(stickerItem)
let superview = UIView()
superview.addSubview(shapeView)
superview.addSubview(stickerView)

for canvasItemView in superview.subviews.compactMap({$0 as? ViewItem<CanvasItem>}) {
    print(canvasItemView.canvasItem.scale) // **access only the common properties**
}