Tim Fuqua Tim Fuqua - 3 months ago 20
iOS Question

Creating a protocol that represents hashable objects that can be on or off

I'm trying to create a simple protocol that says whether or not an object is in an "on" state or an "off" state. The interpretation of what that is depends on the implementing object. For a

UISwitch
, it's whether the switch is on or off (duh). For a
UIButton
, it could be whether the button is in the
selected
state or not. For a
Car
, it could be whether the car's engine is on or not, or even if it is moving or not. So I set out to create this simple protocol:

protocol OnOffRepresentable {
func isInOnState() -> Bool
func isInOffState() -> Bool
}


Now I can extent the aforementioned UI controls like so:

extension UISwitch: OnOffRepresentable {
func isInOnState() -> Bool { return on }
func isInOffState() -> Bool { return !on }
}

extension UIButton: OnOffRepresentable {
func isInOnState() -> Bool { return selected }
func isInOffState() -> Bool { return !selected }
}


Now I can make an array of these kinds of objects and loop over it checking whether they are on or off:

let booleanControls: [OnOffRepresentable] = [UISwitch(), UIButton()]
booleanControls.forEach { print($0.isInOnState()) }


Great! Now I want to make a dictionary that maps these controls to a
UILabel
so I can change the text of the label associated with the control when the control changes state. So I go to declare my dictionary:

var toggleToLabelMapper: [OnOffRepresentable : UILabel] = [:]
// error: type 'OnOffRepresentable' does not conform to protocol 'Hashable'


Oh! Right! Silly me. Ok, so let me just update the protocol using protocol composition (after all, the controls I want to use here are all Hashable: UISwitch, UIButton, etc):

protocol OnOffRepresentable: Hashable {
func isInOnState() -> Bool
func isInOffState() -> Bool
}


But now I get a new set of errors:

error: protocol 'OnOffRepresentable' can only be used as a generic constraint because it has Self or associated type requirements
error: using 'OnOffRepresentable' as a concrete type conforming to protocol 'Hashable' is not supported


Ok... So I do some stack overflow digging and searching. I find many articles that seem promising, like Set and protocols in Swift, Using some protocol as a concrete type conforming to another protocol is not supported, and I see that there are some great articles out there on
type erasure
that seem to be exactly what I need: http://krakendev.io/blog/generic-protocols-and-their-shortcomings, http://robnapier.net/erasure, and https://realm.io/news/type-erased-wrappers-in-swift/ just to name a few.

This is where I get stuck though. I've tried reading through all these, and I've tried to create a class that will be
Hashable
and also conform to my
OnOffRepresentable
protocol, but I can't figure out how to make it all connect.

Answer

I don't know if I'd necessarily make the OnOffRepresentable protocol inherit from Hashable. It doesn't seem like something that you'd want to be represented as on or off must also be hashable. So in my implementation below, I add the Hashable conformance to the type erasing wrapper only. That way, you can reference OnOffRepresentable items directly whenever possible (without the "can only be used in a generic constraint" warning), and only wrap them inside the HashableOnOffRepresentable type eraser when you need to place them in sets or use them as dictionary keys.

protocol OnOffRepresentable {
    func isInOnState() -> Bool
    func isInOffState() -> Bool
}

extension UISwitch: OnOffRepresentable {
    func isInOnState() -> Bool { return on }
    func isInOffState() -> Bool { return !on }
}

extension UIButton: OnOffRepresentable {
    func isInOnState() -> Bool { return selected }
    func isInOffState() -> Bool { return !selected }
}

struct HashableOnOffRepresentable : OnOffRepresentable, Hashable {

    private let wrapped:OnOffRepresentable
    private let hashClosure:()->Int
    private let equalClosure:Any->Bool

    var hashValue: Int {
        return hashClosure()
    }

    func isInOnState() -> Bool {
        return wrapped.isInOnState()
    }

    func isInOffState() -> Bool {
        return wrapped.isInOffState()
    }

    init<T where T:OnOffRepresentable, T:Hashable>(with:T) {
        wrapped = with
        hashClosure = { return with.hashValue }
        equalClosure = { if let other = $0 as? T { return with == other } else { return false } }
    }
}

func == (left:HashableOnOffRepresentable, right:HashableOnOffRepresentable) -> Bool {
    return left.equalClosure(right.wrapped)
}

func == (left:HashableOnOffRepresentable, right:OnOffRepresentable) -> Bool {
    return left.equalClosure(right)
}

var toggleToLabelMapper: [HashableOnOffRepresentable : UILabel] = [:]

let anySwitch = HashableOnOffRepresentable(with:UISwitch())
let anyButton = HashableOnOffRepresentable(with:UIButton())

var switchLabel:UILabel!
var buttonLabel:UILabel!

toggleToLabelMapper[anySwitch] = switchLabel
toggleToLabelMapper[anyButton] = buttonLabel