Re: ^Block statement considered harmful for callbacks?
Re: ^Block statement considered harmful for callbacks?
- Subject: Re: ^Block statement considered harmful for callbacks?
- From: Marcel Weiher <email@hidden>
- Date: Thu, 25 Apr 2013 23:47:50 +0200
Hi Jens,
On Apr 25, 2013, at 18:10 , Jens Alfke <email@hidden> wrote:
> On Apr 25, 2013, at 1:20 AM, Oleg Krupnov <email@hidden> wrote:
>
>> This breaks encapsulation of objects with block properties (e.g.
>> MyAnimation.completionBlock)
>
> I understand the problem you're describing (and yes, I've had a couple of memory leaks resulting from it) but I don't understand how you think it's breaking encapsulation.
Seems pretty obvious to me: blocks capture variables from their lexical scope implicitly and then let them escape this scope. I explain this (briefly) in my HOM paper: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.65.4687
Sometimes that is exactly what you want, but often not. Especially for multi-threading applications you want to explicitly control what you share, not have it implicitly captured.
>> It seems to me that it's much better to drop the convenience of blocks
>> in favor of safety and full control of retain/assign relationships
>> between my objects.
>
> It's a subjective decision, but for what it's worth, I disagree. Blocks are so useful that it's not worth giving them up. In any case, you're talking about only one use of blocks — as a way to tell an object an action to perform later, a replacement for delegates or target/action pairs.
I'd agree that these are application areas that blocks are not really well suited for. For processing something later, I do think the HOM approach is simpler: [[self afterDelay:0.1] pollSearchResults];
Target-action also seems ill-suited to blocks, and IMHO if Objective-C had had blocks at the time, IB would have been much less useful.
> There are plenty of other good uses for blocks that don't have these issues.
Agreed! For example "around" processing. No more -lockFocus / unlockFocus, gsave/grestore etc.
> In my code, most of the places I use a block as an onXXX property value it's going to be called exactly once. What I do then is, in the caller, set the corresponding _onXXX ivar to nil after calling through it, to break cycles.
>
> - (void) xxxHappened {
> if (_onXXX) {
> _onXXX();
> _onXXX = nil;
> }
> }
Interesting. I can see how it works, not quite seeing how that would be a common idiom...but curios!
Cheers,
Marcel
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