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

[Michael Henson]

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.

It would also enable implementing cascading-weakness (or propagating
weakness):

class Thing {
  var text: String
}

class ThingProxy {
  weak var thing: Thing?
}

class ContrivedExample {
  weak var proxy: ThingProxy?
}

var example = ContrivedExample()

such that when example.proxy.thing becomes nil, and the ThingProxy instance
is no longer meaningful, example.proxy becomes nil as well.

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.

Mike
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