gistfile1.swift

class SomeClass {
    var _optionalVar: String?
    
    var nonoptionalVar: String? {
        get { return _optionalVar }
        set { _optionalVar = newValue == nil ? "123" : newValue }
    }
    
    init() {
        self.nonoptionalVar = nil
    }
}

let instance = SomeClass()

instance.nonoptionalVar
instance.nonoptionalVar = nil
instance.nonoptionalVar
instance.nonoptionalVar = "newvalue"
instance.nonoptionalVar = nil