Loggable.swift

protocol Loggable {
    init(filename: String)
    func log()
}

class Logger: Loggable {
    private var filename = ""

    // Required is needed for subclass conforming protocol too.
    // Unless class is marked as final
    required init(filename: String) {
        self.filename = filename
    }

    func log() {
        print("I'm watching you!")
    }
}