Swift property 'shouldSet'

.swift

// -------------------------------------
// Want to do this
// -------------------------------------

var selectable = false {
    willSet {
        if !newValue {
            selected = false
        }
    }
}

var selected: Bool {
    shouldSet {
        return selectable
    }
    didSet {
        // ...
    }
}



// -------------------------------------
// Would settle for this
// -------------------------------------

var selectable = false {
    willSet {
        if !selectable {
            selected = false
        }
    }
}

var selected: Bool {
    willSet {
        if !selectable {
            newValue = oldValue
            return
        }
    }
    didSet {
        // ...
    }
}




// -------------------------------------
// Must do this
// -------------------------------------

var selectable = false {
    willSet {
        if !newValue {
            selected = false
        }
    }
}

fileprivate var _selected = false
var selected: Bool {
   get {
      return _selected
   }
   set {
      guard selectable else { return }
      if _selected != newValue {
         _selected = newValue
         // ...
      }
   }
}