[swift-evolution] Proposal: Give weak references the ability to notify reference-holders when they transition to nil

[Chris Lattner]

> On Dec 13, 2015, at 6:24 PM, Michael Henson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> The use-case for this comes first from proposals to have a weak-reference version of collection types. Implementing a notification signal of some sort to weak reference-holders when the reference becomes nil would make implementing those more straightforward.

+1.  This is very useful for various kinds of APIs, like a weak hashtable that wants to remove the keys when/if they get deallocated.

> SInce this is the germ of an idea, I'll avoid suggesting a syntax for the mechanism so the discussion focuses on whether or not this is useful enough as a feature to be implemented.

IMO, ideally, this would be more of a runtime API than a language feature.

-Chris