Service4.swift

// 4. Service that weakly delegates input and output to a client that aggregates (i.e. references and uses) the service (instead of inheriting from it). Allows weak/unowned definition for the delegate reference on service side, ensuring no memory leaks would be caused (even if client side developers don't know about ARC). And through the use of an extension, default implementations may still be provided whenever needed (and when there is no delegate set at all, a fallback implementation can be defined by the service itself too).
public class Service4 {
    public init() { }
    public weak var delegate: Service4Delegate?
    public func doSomething() {
        if delegate?.input() ?? false {
            delegate?.output(text: "done")
        }
    }
}
public protocol Service4Delegate: class {
    func input() -> Bool
    func output(text: String)
}
public extension Service4Delegate {
    func output(text: String) -> Void {
        print(type(of: self), text)
    }
}
class Client4: Service4Delegate {
    var service4 = Service4()
    init() {
        service4.delegate = self
    }
    func input() -> Bool {
        return true
    }
    func doSomething() {
        service4.doSomething()
    }
}
Client4().doSomething() // prints "Client4 done"