How can I make my Objective-C class conform to Swift's `Equatable` protocol?

Question

I have an Objective-C class (that happens to be a button, but that is not important), and at another part of my (mixed language) project, I have an array of these buttons and I'd like to get the index of a button using the find() method. Like so:

func doSomethingWithThisButtonIndex(index:Int)
{
    let buttons = [firstButton, secondButton, thirdButton]
    if index == find(buttons, firstButton)
    {
        // we've selected the first button
    }
}

but I'm getting the

Type 'ImplicitlyUnwrappedOptional' does not conform to protocol equatable

Okay, so lets go to Objective-C and have ButtonThing implement <Equatable>. But it doesn't recognize that.

So what am I to do? For now I'm building around it, forcing the array to be an NSArray and using indexOfObject. But this is ugly. And frustrating.

Marko

If my answer doesn't satisfy you, then the only other answer is "It's impossible." You can't make an Objective-C class conform to a Swift protocol unless the protocol is annotated as @objc or if you do it from the Swift end. Equatable is not annotated as such and it can't possibly make sense for it to be so annotated. – Outcrop

Answer 1

First, in Swift write a custom == operator function for your class.

Second, also in Swift, write a class extension that adds the Equatable protocol conformance.

Perhaps, for example:

func == (lhs: YourClass, rhs: YourClass) -> Bool {
    // whatever logic necessary to determine whether they are equal
    return lhs === rhs
}

extension YourClass: Equatable {}

And now your class conforms to Equatable, which is Swift specific. You can not do this on the Objective-C end because you can not write custom operators for Objective-C.

Outcrop

@Marko Read the last line of the answer. You cannot do this for Objective-C. Swift's Equatable protocol requires a custom operator (specifically, ==), and there is no way to do this with Objective-C. Perhaps more importantly, you should be trying to do this for Objective-C anyway. Objective-C's == is equivalent to Swift's === for object references. We shouldn't want to alter that 30-year-old Objective-C behavior... – Outcrop

Additionally, the Equatable protocol is not defined as being an @objc protocol, and therefore the protocol cannot be used from Objective-C at all. Equatable is a purely Swift protocol. The way to make an Objective-C class conform to this protocol is by extending the class from within Swift. Nothing else can make sense. You're effectively asking "How can I override the default behavior of == in Objective-C?" – Outcrop

@Alix no. Swift's === is roughly equivalent to Objective-C's ==. It's a reference comparison. – Outcrop

so in objective-C if I want to test the equality between 2 objects how am I suppose to do it? – Does

@Honey [obj1 isEqual:obj2];. Some classes, such as NSString, have other methods as well, such as [str1 isEqualToString:str2];. In this specific case, isEqualToString: and isEqual: are equivalent. And all objects inherit isEqual: from NSObject, but the default implementation is simply the == reference comparison. – Outcrop

Thanks. I think I wrote my question poorly... in Swift you have struct Home { var rooms : Int var address: String var city : String} and then to implement equality you have do a func == (lhs : Home, rhs : Home) {lsh.rooms == rhs.rooms && lhs.address == rhs.address && lhs.city == rhs.city} How do you implement such in Objective-c? – Does

Implement the isEqual: method. You can't override operators in Objective-C (note the last sentence of this answer). – Outcrop

ohhk...so you mean just like any other method that you write yourself write one which its name is isEqual: which returns a bool. That's simple. So basically in Swift you are mentioning it in a more explicit way and also have the benefit of using a more readable way ie == – Does

Answer 2

If your Objective C class is an NSObject, implement isEqual:

- (BOOL)isEqual:(_Nullable id)other;

This worked for me for Array.index(of: myobject) and == comparisons. NSObject is already Equatable so using a Swift extension does not work.

Discourteous