Hello,
I'd like to re-visit a proposal from Riley Testut about weak
containers. Since no conclusion/outcome was achieved (AFAICS from
looking at the archives and the repository)
and since I just would have needed this again too... I found it a good
time to re-propose this :D
---
"
In multiple places in my projects, I essentially recreate the
“multiple observer” pattern used by NSNotificationCenter. Originally
this was implemented by simply maintaining an array of observers, and
adding to/removing from it as necessary. However, this had the
unintended side effect of maintaining a strong reference to the
observers, which in many cases is undesirable (for the same reasons
it’s common to mark delegate properties as weak).
Now, I’m using a private NSHashTable instance, and expose the
observers as public API by creating a public computed property which
essentially returns an array derived from the NSHashTable like so:
public var receivers: [GameControllerReceiverType] {
// self.privateReceivers.allObjects as!
[GameControllerReceiverType] crashes Swift :(
return self.privateReceivers.allObjects.map({ $0 as!
GameControllerReceiverType })
}
This workaround works, but is undesirable for a number of reasons.
Most notably:
• NSHashTable is not a native Swift collection, and is also not in the
Foundation Swift port, so it is not portable to other systems.
• It also has not yet been annotated with generics, so it loses the
nice type safety of other Swift collections. Because of this, I have
to map the objects to the appropriate type before returning the
allObjects array, which runs in O(n) time instead of O(1).
• It’s repetitive. For every type that wants to implement this
pattern, they must maintain both a public computed method and a
private NSHashTable instance. This gets worse when this should be part
of a protocol; there’s no way to enforce that each type conforming to
it has a NSHashTable, while also keeping that information private from
the consumer of the API.
I think native swift collections with support for weak references for
their contents would be very useful, and in more places than just
listed above. I don’t think Array could be easily extended to support
it (what happens if a value is released? does everything shift down?
do they keep their indices?), but Set and Dictionary (where the keys
and/or values could be weak, akin to NSMapTable) would be good
candidates IMO.
Thoughts?"
--reference to last message thread:
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151207/001579.html
(last messages were 'arguing' how to implement it)
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