Extension method accessible only inside class

Currently Swift 3 has some problems due to Private accessibility within the Extensions. In swift4 it will be possible. You can try with Xcode 9 beta.

What you're asking is not possible to do at the moment, and I agree that it would be very helpful in some cases.

Let's say we have this protocol:

protocol HidableViewed {    var hidableView: UIView}

This protocol is used to signify that some view controller has a view that can be hidden (which makes sense in its business case).

So what do we want with hidable views? We want to hide them of course, but hiding operation is same in each instance of this protocol. So to avoid rewriting the hiding method in each view controller, we want to implement it once in our protocol.

extension HidableViewed {    func hideView() {        self.hidableView.isHidden = true    }}

Very nice, how convenient!

There's one problem though: if a view controller implements this protocol, any other class will be able to hide its view. And we certainly don't want that in some cases, which brings us to the original question.