<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">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).<div class=""><br class=""></div><div class="">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:</div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(79, 129, 135);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">public</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">var</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> receivers: [</span>GameControllerReceiverType<span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">] {</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(0, 132, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span>// self.privateReceivers.allObjects as! [GameControllerReceiverType] crashes Swift :(</div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo; color: rgb(79, 129, 135);" class=""><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">return</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">self</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">.</span>privateReceivers<span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #703daa" class="">allObjects</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">.</span><span style="font-variant-ligatures: no-common-ligatures; color: #3d1d81" class="">map</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">({ $0 </span><span style="font-variant-ligatures: no-common-ligatures; color: #bb2ca2" class="">as</span><span style="font-variant-ligatures: no-common-ligatures; color: #000000" class="">! </span>GameControllerReceiverType<span style="font-variant-ligatures: no-common-ligatures; color: #000000" class=""> })</span></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class="">}</div></div><div style="margin: 0px; font-size: 11px; line-height: normal; font-family: Menlo;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">This workaround works, but is undesirable for a number of reasons. Most notably:</div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">• NSHashTable is not a native Swift collection, and is also not in the Foundation Swift port, so it is not portable to other systems.</div><div style="margin: 0px; line-height: normal;" class="">• 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).</div><div style="margin: 0px; line-height: normal;" class="">• 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.</div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">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.</div><div style="margin: 0px; line-height: normal;" class=""><br class=""></div><div style="margin: 0px; line-height: normal;" class="">Thoughts?</div></body></html>