Service3.swift

// 3. Service that requests input and output from a client as @escaping or optional closures (i.e. the functional approach). No more inheritance, and the service can still provide default implementations whenever needed. But the clients need to capture weak/unowned self if they want to use their current state within the body of the custom closures passed to the service (in order to avoid memory leaks, i.e. they need to know how ARC works).
public class Service3 {
    public init(inputFunction: @escaping InputFunction, outputFunction: OutputFunction? = nil) {
        self.inputFunction = inputFunction
        self.outputFunction = outputFunction
    }
    private var inputFunction: InputFunction
    private var outputFunction: OutputFunction?
    public func doSomething() {
        if inputFunction() {
            let text = "done"
            if let outputFunction = outputFunction {
                outputFunction(text)
            } else {
                print(type(of: self), text)
            }
        }
    }
}
public typealias InputFunction = () -> Bool
public typealias OutputFunction = (String) -> Void
class Client3 {
    var input = true
    func doSomething() {
        let service3 = Service3(
            inputFunction: { [unowned self] in
                return self.input
            })
        service3.doSomething()
    }
}
Client3().doSomething() // prints "Service3 done"