Default variable logic for swift protocol

Question

I have a protocol

protocol Example: class {
    var value: Bool { get set }
    func foo()
    func bar()
}

And extension:

extension Example {

//    var value: Bool { // Error: Extensions must not contain stored properties
//        didSet {
//            switch value {
//            case true:
//                foo()
//            case false:
//                bar()
//            }
//        }
//    }

    func foo() {
        // logic...
    }
    func bar() {
        // logic...
    }
}

• When value is set to true, I want foo() to be called
• When value is set to false, I want bar() to be called

However, I do not want to redundantly implement didSet{ } logic into every class that conforms to Example

But, if I try to add didSet{ } logic into the extension, Xcode says "Extensions must not contain stored properties".

What is the best practice for adding default property-observing logic without having to copy/paste into every conforming class?

The Goal:

I want any subclass of UIView to conform to my protocol Expandable. The requirements of my protocol are isExpanded: Bool, expand(), and collapse. I want isExpanded = true to call expand(), and isExpanded = false to call collapse() (much like the behavior of setting isHidden). But for every subclass of UIView, I don't want to have rewrite any logic. I'd like to just make the class conform to Expandable, and jump right in to setting isExpanded.

Answer 1

You don't need observers for what you're describing. You just need some storage for your state. Since you know this is an NSObject, you can do that with the ObjC runtime.

// Since we know it's a UIView, we can use the ObjC runtime to store stuff on it
private var expandedProperty = 0

// In Xcode 10b1, you can make Expandable require this, but it's probably
// nicer to still allow non-UIViews to conform.
extension Expandable where Self: UIView {
    // We'll need a primitive form to prevent infinite loops. It'd be nice to make this private, but
    // then it's not possible to access it in custom versions of expand()
    var _isExpanded: Bool {
        get {
            // If it's not know, default to expanded
            return objc_getAssociatedObject(self, &expandedProperty) as? Bool ?? true
        }
        set {
            objc_setAssociatedObject(self, &expandedProperty, newValue, .OBJC_ASSOCIATION_ASSIGN)
        }
    }
    var isExpanded: Bool {
        get {
            return _isExpanded
        }
        set {
            _isExpanded = newValue
            if newValue { expand() } else { collapse() }
        }
    }
    func expand() {
        _isExpanded = true  // Bypassing the infinite loop
        print("expand")

    }
    func collapse() {
        _isExpanded = false
        print("collapse")
    }
}

If you didn't know that this were an NSObject, you can get the same thing with a global (private) dictionary that maps ObjectIdentifier -> Bool. It just leaks a tiny amount of memory (~16 bytes per view that you collapse).

That said, I wouldn't do it this way. Having two ways to do the same thing makes everything much more complicated. I would either have just isExpanded as settable, or have isExpanded as read-only and a expand and collapse. Then you don't need the extra _isExpanded.

Answer 2

You have to implement getter, setter explicitly:

protocol Example {
   var value: Bool { get set }
   func foo()
   func bar()
}

extension Example {
   var value: Bool {
       get { return value }

       set(newValue) {
           value = newValue
           value ? foo() : bar()
        }
    }


   func foo() {
       print("foo")
   }

   func bar() {
       print("bar")
   }
}