This one is on my wish list, because Siesta has a compelling use case for it. A common mistake when using Siesta is to do this:
resource.addObserver(owner: self, observer: self.fooChanged)
This innocent code creates a retain cycle under Siesta’s memory rules. No need to understand the context or fiddly details here; the fundamental problem is that self.fooChanged retains self, and the way Siesta works, we don’t want that.
Folks have suggested weak closures and/or weak method references (e.g. here, here), but another alternative that requires no new syntax and little new discussion — just the proposal at hand here — would be to alter Siesta’s observer API to use a keypath:
resource.addObserver(owner: self, observer: \.fooChanged) // keypath doesn’t retain self!
Since the idea being pitched in this thread sounds feasible, I’d be happy to help write it up as a formal proposal.