culprit.swift

var animationDuration: CFTimeInterval { get }

original-shakeable.swift

protocol Shakeable {
    var horizontalShake: CGFloat { get }
    var verticalShake: CGFloat { get }
    var animationDuration: CFTimeInterval { get }
    var repeatCount: Float { get }
    
    func shake()
}

struct Defaults {
    static let horizontalShake: CGFloat = 4.0
    static let verticalShake: CGFloat = 0.0
    static let duration: CFTimeInterval = 0.05
    static let repeatCount: Float = 5
}

extension Shakeable {
    var horizontalShake: CGFloat {
        return Defaults.horizontalShake
    }
    
    var verticalShake: CGFloat {
        return Defaults.verticalShake
    }
    
    var animationDuration: CFTimeInterval {
        return Defaults.duration
    }
    
    var repeatCount: Float {
        return Defaults.repeatCount
    }
}

shake-extension-for-view.swift

extension Shakeable where Self: UIView {
    func shake() {
        let position = "position"
        let animation = CABasicAnimation(keyPath: position)
        
        animation.duration = animationDuration
        animation.repeatCount = repeatCount
        animation.autoreverses = true
        animation.fromValue = NSValue(cgPoint: CGPoint(x: center.x - horizontalShake,
                                                       y: center.y - verticalShake))
        animation.toValue = NSValue(cgPoint: CGPoint(x: center.x + horizontalShake,
                                                     y: center.y + verticalShake))
        
        layer.add(animation, forKey: position)
    }
}

shakeable-views.swift

class ImageView: UIImageView, Shakeable {
    @IBInspectable var horizontalShake: CGFloat = Defaults.horizontalShake
    @IBInspectable var verticalShake: CGFloat = Defaults.verticalShake
}

class Button: UIButton, Shakeable {
    @IBInspectable var horizontalShake: CGFloat = Defaults.horizontalShake
    @IBInspectable var verticalShake: CGFloat = Defaults.verticalShake
}

shakeable.swift

protocol Shakeable {
    var horizontalShake: CGFloat { get }
    var verticalShake: CGFloat { get }
    var duration: CFTimeInterval { get }
    var repeatCount: Float { get }
    
    func shake()
}

shakeables.swift

protocol ShakeablesContainer {
    var shakeables: [Shakeable] { get }
    func shake()
}

extension ShakeablesContainer {
    func shake() {
        shakeables.forEach { $0.shake() }
    }
}

Swift Protocols and the Law of Unintended Consequences.md

Code snippets for the blog post "Swift Protocols and the Law of Unintended Consequences" at http://www.apokrupto.com/blog-1/2016/8/14/swift-protocols-and-the-law-of-unintended-consequences

view-controller-with-shakeables.swift

class ViewController: UIViewController, ShakeablesContainer {
    @IBOutlet weak var imageView: ImageView!
    @IBOutlet weak var button: Button!
    
    @IBAction func shake(_ sender: Button) {
        shake()
    }
    
    var shakeables: [Shakeable] {
        return [imageView, button].flatMap {
            $0 as? Shakeable
        }
    }
}

Since the shake direction is limited to horizontal and vertical in this case, what say if we use enum for the shake direction

enum ShakeDirection {
case horizontal
case vertical
}
protocol Shakeable {
    var shakeDirection: ShakeDirection = .horizontal
    var animationDuration: CFTimeInterval { get }
    var repeatCount: Float { get }  
    func shake()
}