Implementing a simple string cache

SimpleStringCache.swift

class StringCache: NSCache<NSString, String> {
    
    static let shared = ImageCache()
    
    func set(_ text: String, for keyString: String) {
        self.setObject(text, forKey: keyString as NSString)
    }
    
    func textFrom(_ keyString: String?) -> String? {
        guard let keyString = keyString else {
            return nil
        }
        return self.object(forKey: keyString as NSString)
    }
}