I'm trying to define a Scalable protocol for some of my classes that need to observe a particular scale value on a view controller. The protocol would require a 'scale' variable and a 'bind(to:)' function, like this:

protocol Scalable {
    var scale: CGFloat { get set }
    func bind(to: Observable<CGFloat>)
}

I would like to give a default implementation of the bind(to:) function in a protocol extension of Scalable as follows...

  extension Scalable {
    func bind(to observable: Observable<CGFloat>) {
        observable.valueChanged = { [weak self] newValue in
            self?.scale = newValue ?? 1
        }
    }
}

But I'm getting the following error on [weak self]:

'weak' must not be applied to non-class-bound 'Self'; consider adding a protocol conformance that has a class bound.

I'm not clear on how to interpret this error. Any ideas?

I am using class in these cases and it works great

protocol Scalable: class {

}

Got it now, thanks!