lazy.swift

enum YesNo {
    case Yes, No
}


class Foo {
    
    var bar: YesNo? = .None {
        didSet {
            self.baz = calculateValueForBaz()
        }
    }
    
    private func calculateValueForBaz() -> String {
        switch (self.bar) {
        
        case .Some(.Yes):
            return "Ja"
        case .Some(.No):
            return "Nein"
        default:
            return "Möglich"

        }
    }
    
    
    lazy var baz: String = {
        self.calculateValueForBaz()
    }()
}


let foo = Foo()

foo.bar         // -> nil
foo.baz         // -> "Möglich"

foo.bar = .Yes
foo.baz         // -> "Ja"

foo.bar = .No
foo.baz         // -> "Nein"