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.error { color:#AA0000; }</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="bloop_markdown"><p>Hi Matthew,</p>
<p>Thank you for the hint. Indeed, at first glance your proposal is very similar to my thinking. I’ll try to dive deeper into your proposal and find out more about the discussion behind it when I get some more time for that.</p>
<p>We could collaborate on a revision for Swift 5 if you would like to, but we wouldn’t need to rush now, because there is plenty of time for that. </p>
<p>Personally I would love to avoid some kind of a prefix like <code>?</code> if possible.</p>
<p>Brainstorming:</p>
<p><code>let c1 = weak someObject.method</code> convert it to a weak closure.</p>
<p><code>let c2: weak (CLOSURETYPE)? = someObject.method</code> Let the compiler do the job for us.</p>
<p>The latter would be great for APIs to avoid the prefix.</p>
<p><code>.subscribe(onNext: self.bar)</code> <code>weak</code> is inferred and can be omitted </p>
<p></p></div><div class="bloop_original_html"><style>body{font-family:Helvetica,Arial;font-size:13px}</style><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div> <br> <div id="bloop_sign_1497122959888763904" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <br><p class="airmail_on">Am 10. Juni 2017 um 21:09:28, Matthew Johnson (<a href="mailto:matthew@anandabits.com">matthew@anandabits.com</a>) schrieb:</p> <blockquote type="cite" class="clean_bq"><span><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div></div><div>
<title></title>
Hi Adrian, this is pretty similar to the Guarded Closures proposal
I drafted in February. This proposal needs a revision
incorporating discussion feedback and some new ideas. If
you’re interested, here’s a link to the original discussion
thread: <a href="https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170213/032478.html" class="">https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170213/032478.html</a>.
<div class=""><br class="">
<div>
<blockquote type="cite" class="">
<div class="">On Jun 10, 2017, at 12:29 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="">
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<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>
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<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>
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