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

[Jordan Rose]

> On Dec 14, 2015, at 11:54 , John McCall via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> On Dec 13, 2015, at 7:46 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
>>> 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.
> 
> Yes.  This was always part of the long-term vision for weak references.

Hm. I don't know. Allowing arbitrary callbacks during the deinit process is a little scary to me; it means that any API that vends an object publicly can no longer assume that deinitialization is cheap. I mean, deinit can already do arbitrary work, but the creator of the class has full control over what that work is (modulo their superclasses). This feels like something that a class might need to opt into, much like KVO requires opt-in in Swift with 'dynamic'.

If one of the selling points of ARC over GC is "deterministic destruction", there's value in being able to control what happens during that destruction.

Jordan