201806282115.swift

enum Type: String {
    // 直接指定 API 回傳的 String
    case regular = "regular"
    case system = "system"
    case advertise = "advertise"
    
    var image: UIImage? {
        // 一行搞定
        return UIImage(named: self.rawValue)
    }
}

let imageView = UIImageView()

let json = ["type": "regular"]

if let typeString = json["type"] {
    imageView.image = Type(rawValue: typeString)?.image
}