A simple protocol for creating a 'Card' style component in Swift.

CardDecorating.swift

import UIKit

/// Decorates a view as a `Card`. Requires that the view is embedded in some `contentView`
public protocol CardDecorating {
    /// View that will be styled as a card. Should hold the `contentView`.
    var cardView: UIView { get }
    /// Holds the main content. Subviews should be added to this view.
    var contentView: UIView { get }
}

/// Defines the various styling attributes to apply to the `Card`
public struct CardStyle {
    public var backgroundColor: UIColor
    public var cornerRadius: CGFloat
    public var shadowColor: UIColor
    public var shadowOpacity: Float
    public var shadowOffset: UIOffset
    public var shadowRadius: CGFloat
}

public extension CardDecorating {

    /// Applies the specified `style` to the `Card` view and its `contentView`
    func applyCardDecoration(style: CardStyle) {
        cardView.backgroundColor = style.backgroundColor
        cardView.layer.masksToBounds = false
        cardView.layer.cornerCurve = .continuous
        cardView.layer.cornerRadius = style.cornerRadius
        cardView.layer.shadowColor = style.shadowColor.cgColor
        cardView.layer.shadowOffset = CGSize(width: style.shadowOffset.horizontal, height: style.shadowOffset.vertical)
        cardView.layer.shadowRadius = style.shadowRadius
        cardView.layer.shadowOpacity = style.shadowOpacity
        cardView.layer.rasterizationScale = UIScreen.main.scale
        cardView.layer.shouldRasterize = true

        contentView.layer.cornerCurve = .continuous
        contentView.layer.cornerRadius = cardView.layer.cornerRadius
    }

}

/// Convenience type to simplify the usage of the `CardDecorating` protocol
struct CardBuilder: CardDecorating {
    let cardView: UIView
    let contentView: UIView
}

extension UITableViewCell: CardDecorating {
    public var cardView: UIView { return self }
}

extension UICollectionViewCell: CardDecorating {
    public var cardView: UIView { return self }
}