<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=""><font color="#323333" class="">There's a test case demonstrating what I'm about to talking about: <a href="https://gist.github.com/jemmons/6f668006fa2712a84807" class="">https://gist.github.com/jemmons/6f668006fa2712a84807</a></font><div class=""><font color="#323333" class=""><br class=""></font></div><div class=""><font color="#323333" class="">But here's the nut of it. Given a class like:</font></div><div class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;"><br class=""></span></div><div class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;"><span class="pl-k" style="box-sizing: border-box;">class</span><span style="color: rgb(51, 51, 51); background-color: rgb(255, 255, 255);" class=""> Foo {</span> </span></div><div class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;"> </span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">var</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""> handler: (()</span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">-></span><span class="pl-c1" style="box-sizing: border-box; color: rgb(0, 134, 179); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">Void</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class="">)?</span></div><div class=""><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;"> func</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""> </span><span class="pl-en" style="box-sizing: border-box; color: rgb(121, 93, 163); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">noop</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class="">(){}</span></div><div class=""><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""> ...</span></div><div class=""><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class="">}</span></div><div class=""><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""><br class=""></span></div><div class=""><span style="color: rgb(51, 51, 51); white-space: pre; background-color: rgb(255, 255, 255);" class="">If somewhere in this class I do something like:</span></div><div class=""><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""><br class=""></span></div><div class=""><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class="">handler </span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">=</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""> { </span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">self</span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">.</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class="">noop() }</span></div><div class=""><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""><br class=""></span></div><div class=""><span style="color: rgb(51, 51, 51); white-space: pre; background-color: rgb(255, 255, 255);" class="">I've created a retain cycle because <font face="Menlo" class="">handler</font> holds a strong reference to a closure which holds a strong reference to <font face="Menlo" class="">self</font> which holds a strong reference to a closure that…</span></div><div class=""><span style="color: rgb(51, 51, 51); white-space: pre; background-color: rgb(255, 255, 255);" class=""><br class=""></span></div><div class=""><font color="#333333" class=""><span style="white-space: pre; background-color: rgb(255, 255, 255);" class="">In fact, my understanding is Swift's requirement that </span></font><span style="color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;" class="">self</span><span style="color: rgb(51, 51, 51); white-space: pre; background-color: rgb(255, 255, 255);" class=""> be called out here explicitly is to highlight this danger. </span></div><div class=""><span style="color: rgb(51, 51, 51); white-space: pre; background-color: rgb(255, 255, 255);" class=""><br class=""></span></div><div class=""><span style="color: rgb(51, 51, 51); white-space: pre; background-color: rgb(255, 255, 255);" class="">Swift makes it easy to fix the problem with a capture list:</span></div><div class=""><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""><br class=""></span></div><div class=""><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class="">handler </span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">=</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""> { [</span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">unowned</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""> </span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">self</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class="">] </span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">in</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""> </span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">self</span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">.</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class="">noop() }</span></div><div class=""><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""><br class=""></span></div><div class=""><font color="#333333" class=""><span style="white-space: pre; background-color: rgb(255, 255, 255);" class="">So far, so good. But what if I do something daft like assign <font face="Menlo" class="">noop</font> directly:</span></font></div><div class=""><font color="#333333" class=""><span style="white-space: pre; background-color: rgb(255, 255, 255);" class=""><br class=""></span></font></div><div class=""><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class="">handler </span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">=</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""> noop</span></div><div class=""><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""><br class=""></span></div><div class=""><font color="#333333" class=""><span style="white-space: pre; background-color: rgb(255, 255, 255);" class="">This causes a retain loop for all the same reasons as our original example, but there's no </span></font><span style="color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;" class="">self</span><span style="white-space: pre; background-color: rgb(255, 255, 255); color: rgb(51, 51, 51);" class=""> call-out to warn us of impending doom, and no support for some kind of capture list to fix the issue.</span></div><div class=""><span style="white-space: pre; background-color: rgb(255, 255, 255); color: rgb(51, 51, 51);" class=""><br class=""></span></div><div class=""><span style="background-color: rgb(255, 255, 255);" class=""><font color="#333333" class=""><span style="white-space: pre;" class="">Is there any possibility of requiring "self" here? As in: "</span></font></span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class="">handler </span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">=</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class=""> </span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">self</span><span class="pl-k" style="box-sizing: border-box; color: rgb(167, 29, 93); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre;">.</span><span style="color: rgb(51, 51, 51); font-family: Consolas, 'Liberation Mono', Menlo, Courier, monospace; white-space: pre; background-color: rgb(255, 255, 255);" class="">noop" </span><span style="color: rgb(51, 51, 51); white-space: pre; background-color: rgb(255, 255, 255);" class="">for example? And is there any way of specifying how we want self to be retained here?</span></div><div class=""><span style="color: rgb(51, 51, 51); white-space: pre; background-color: rgb(255, 255, 255);" class=""><br class=""></span></div><div class=""><span style="color: rgb(51, 51, 51); white-space: pre; background-color: rgb(255, 255, 255);" class=""><br class=""></span></div><div class=""><span style="color: rgb(51, 51, 51); white-space: pre; background-color: rgb(255, 255, 255);" class="">Cheers,</span></div><div class=""><span style="color: rgb(51, 51, 51); white-space: pre; background-color: rgb(255, 255, 255);" class="">-jemmons</span></div></body></html>