<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class="">Being able to specify things about closure capturing at the API level could do wonders for Siesta — though its problems may be beyond the scope of what’s proposed here (or what’s workable at all).</div><div class=""><br class=""></div><div class="">Siesta exposes this API call:</div><div class=""><br class=""></div><div class=""><div class=""> someResource.addObserver(owner: self) { // (1)</div><div class=""> [weak self] resource, event in</div><div class=""> …</div><div class=""> self.bar()</div><div class=""> …</div><div class=""> }</div></div><div class=""><br class=""></div><div class="">…but it’s easy to forget that [weak self], and because of Siesta’s observer ownership rules:</div><div class=""><br class=""></div><div class=""> <a href="http://bustoutsolutions.github.io/siesta/guide/memory/" class="">http://bustoutsolutions.github.io/siesta/guide/memory/</a></div><div class=""><br class=""></div><div class="">…omitting it creates a retain cycle. This same problem also makes this otherwise lovely pattern untenable:</div><div class=""><br class=""></div><div class=""> someResource.addObserver(owner: self, closure: self.fooUpdated) // (2)</div><div class=""><br class=""></div><div class="">To solve this problem, addObserver would need to be able to specify that the object passed as the owner should be weakly captured by the closure. It’s just that one specific •owner• object; everything else should be captured as usual. So it’s not a property of the closure itself, either in the type system or as an argument attribute; it’s a relationship between the closure and other args. Ouch!</div><div class=""><br class=""></div><div class="">At a glance, it looks like Matthew’s guarded closure proposal might solve this in practice for (2) but not for (1)?</div><div class=""><br class=""></div><div class=""><div class="">In the absence of cycle detection, some way around this pitfall sure would be nice.</div></div><div class=""><br class=""></div><div class="">Cheers, P</div><div class=""><br class=""></div><br class=""><div><blockquote type="cite" class=""><div class="">On Jun 10, 2017, at 12:49 PM, Gor Gyolchanyan via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><meta http-equiv="Content-Type" content="text/html charset=utf-8" class=""><div style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">The benefit that I can see here is the ability to guarantee memory safety on API level, by way of specifying weak closure members. Previously, there way no conceivable way of knowing that because a closure can essentially capture whatever it wants (even the very object it's stored in), so this would be a deterministic way of resolving *all* circular references caused by closures.<div class=""><br class=""></div><div class="">The downside is that it would require some heavy-duty closure capture analysis on the compiler's part, so I'd expect it to be deferred to Swift 5 or something.</div><div class="">However, this does play really nicely with the ownership concept that Swift is going for.</div><div class=""><br class=""></div><div class="">I say, let's think this one through very thoroughly and write a very very detailed proposal (including details on how would the core team get around implementing this) and see what it looks like before submitting it.<br class=""><div class=""><br class=""><blockquote type="cite" class=""><div class="">On Jun 10, 2017, at 8:29 PM, Adrian Zubarev via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="bloop_markdown" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(254, 254, 254);"><p style="margin: 15px 0px; -webkit-margin-before: 0px;" class="">Hello Evolution,</p><p style="margin: 15px 0px;" class="">I’d like to pitch a new idea and see where it would go. Recently I tapped into a small trap and just now realized that even that<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">non-escaping</code><span class="Apple-converted-space"> </span>should have been the default for closures (SE–0103) there is an exception for that. Apparently generics don’t follow that rule and a closure like<span class="Apple-converted-space"> </span></p><p style="margin: 15px 0px;" class=""><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">Optional<() -> Void></code><span class="Apple-converted-space"> </span>or simply<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal;" class="">(() -> Void)?</code></p><p style="margin: 15px 0px;" class="">is still escaping by default. But that was the half of the story yet. As we all know and “love” reference lists inside closures, methods don’t have any and we have to wrap method calls into a weak referenced closure<span class="Apple-converted-space"> </span></p><p style="margin: 15px 0px;" class=""><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">{ [weak self] in self.foo() }</code></p><p style="margin: 15px 0px;" class="">to avoid strong reference cycles. Maybe you already guess it, I accidentally didn’t and tapped into the land of strong reference cycles yet again on my journey.</p><p style="margin: 15px 0px;" class="">I’d like to pitch a new way, more like a new type behavior, for closures on how they could be used differently in order to avoid strong reference cycles but also providing the ability to use methods without any need to wrap them.</p><p style="margin: 15px 0px;" class="">Here is a simple code snippet using RxSwift, which will recreate my issue:</p><pre style="margin: 15px 0px; font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(204, 204, 204); overflow: auto; padding: 4px 8px; word-break: normal; word-wrap: normal;" class=""><code class="swift" style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 0px; margin: 0px; padding: 0px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;">import RxSwift
let test = PublishSubject<Void>()
class A {
let disposeBag = DisposeBag()
func foo() {
test.asObservable()
.subscribe(onNext: self.bar) // The issue is here
.disposed(by: self.disposeBag)
}
func bar() { print("works") }
}
let a = A()
a.foo()
test.onNext(()) // Testing if it works
test.onCompleted() // Some RxSwift stuff
</code></pre><p style="margin: 15px 0px;" class="">In this case by passing directly the method<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">self.bar</code><span class="Apple-converted-space"> </span>we’re capturing<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal;" class="">self</code>, which in this situation isn’t our intention at all. To avoid this issue we can simply wrap the method call into closure:</p><p style="margin: 15px 0px;" class=""><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">.subscribe(onNext: { [unowned self] in self.bar() })</code></p><p style="margin: 15px 0px;" class="">(It’s safe to make it unowned because the dispose bag is a member of<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">self</code>.)</p><hr style="height: 0.2em; border: 0px; color: rgb(204, 204, 204); background-color: rgb(204, 204, 204); display: inherit;" class=""><p style="margin: 15px 0px;" class="">What if we had the ability for weak or unowned closures? By that I don’t mean weak/unowned references to the closures themselves, because they are also reference types, but an invalidation behavior for the whole closure based on the _captured_ references. For instance:</p><p style="margin: 15px 0px;" class=""><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">let closure1: weak (() -> Void)? = { self.doWhatever() }</code></p><p style="margin: 15px 0px;" class=""><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">let closure2: weak (() -> Void)? = self.doWhatever</code></p><p style="margin: 15px 0px;" class="">If one would now try to call the<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">closure</code>, first it will check if all the captured objects are still available or not, if not the whole closure in this case will simply become<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal;" class="">nil</code><span class="Apple-converted-space"> </span>and won’t execute. In case of unowned closures it will trap. Furthermore it will support the general meaning of<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal;" class="">weak/unowned</code><span class="Apple-converted-space"> </span>and will not increase the reference counter for *captured objects*.</p><p style="margin: 15px 0px;" class="">As you have already noticed, in this case the convention is slightly different because we must carry the behavior directly with the type.</p><p style="margin: 15px 0px;" class=""><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">func subscribe(onNext: weak ((Swift.E) -> Void)?)</code></p><p style="margin: 15px 0px;" class="">If the way of my thinking is correct this idea _could maybe_ fade out the very common<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">[weak self] in guard let strongSelf = self …</code><span class="Apple-converted-space"> </span>pattern.<span class="Apple-converted-space"> </span></p><hr style="height: 0.2em; border: 0px; color: rgb(204, 204, 204); background-color: rgb(204, 204, 204); display: inherit;" class=""><p style="margin: 15px 0px;" class="">I personally cannot tell all the technical difficulties this idea might have, but that’s what the evolution list is for, to collaboratively flesh out the ideas if they are worth it.</p><hr style="height: 0.2em; border: 0px; color: rgb(204, 204, 204); background-color: rgb(204, 204, 204); display: inherit;" class=""><p style="margin: 15px 0px;" class="">If something like this could be possible it’s probably worth noting that we might also be able to introduce something like<span class="Apple-converted-space"> </span><code style="font-family: Menlo, Consolas, 'Liberation Mono', Courier, monospace; font-size: 10pt; border-top-left-radius: 3px; border-top-right-radius: 3px; border-bottom-right-radius: 3px; border-bottom-left-radius: 3px; background-color: rgb(248, 248, 248); color: inherit; border: 1px solid rgb(234, 234, 234); margin: 0px 2px; padding: 0px 5px; word-break: normal; word-wrap: normal; -webkit-margin-before: 0px;" class="">@autoclosure(weak/unowned)</code><span class="Apple-converted-space"> </span>to Swift for consistency.</p><div style="margin: 15px 0px;" class=""><br class="webkit-block-placeholder"></div></div><div class="bloop_original_html" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(254, 254, 254);"><div id="bloop_customfont" style="font-family: Helvetica, Arial; font-size: 13px; margin: 0px;" class=""><br class=""></div><br class=""><div id="bloop_sign_1497112926484879104" class="bloop_sign"><div style="font-family: helvetica, arial; font-size: 13px;" class="">-- <br class="">Adrian Zubarev<br class="">Sent with Airmail</div></div></div><div class="bloop_markdown" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(254, 254, 254);"><div style="margin: 15px 0px; -webkit-margin-before: 0px;" class=""><br class="webkit-block-placeholder"></div></div><span style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(254, 254, 254); float: none; display: inline !important;" class="">_______________________________________________</span><br style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; 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