Service2.swift

// 2. Service with default implementations that may be overriden at the client level through inheritance. Clients remain clean, and they can override only some of the open members, but they must have the service as their base class for this (and that's not always acceptable).
open class Service2 {
    public init() { }
    open func input() -> Bool {
        return false
    }
    open func output(text: String) {
        print(type(of: self), text)
    }
    public func doSomething() {
        if input() {
            output(text: "done")
        }
    }
}
class Client2: Service2 {
    override func input() -> Bool { return true }
}
Client2().doSomething() // prints "Client2 done"