Re: ARC and blocks
Re: ARC and blocks
- Subject: Re: ARC and blocks
- From: Greg Parker <email@hidden>
- Date: Thu, 26 Jan 2012 19:40:02 -0800
On Jan 26, 2012, at 6:04 PM, Marco Tabini wrote:
> On 2012-01-26, at 6:09 PM, Jeff Kelley wrote:
>> Without ARC, you would use __block to prevent the block from retaining the object and causing the retain cycle. With ARC, however, the object is retained when you put it into the variable, so to avoid a retain cycle, you have to declare it like so:
>>
>> __unsafe_unretained __block MyViewController *myController = …
>>
>> It looks awful, yes, but without the first piece we were having extreme memory leaks.
>
> Maybe I'm reading this wrong, but I think this might be a bit too unsafe. If you declare something as __unsafe_unretained, ARC won't try to track the variable through its lifetime, so if for some reason that variable is deallocated and then your block gets called, your app will crash. The OP's code feels a bit safer to me: it retains the variable strongly, then Nils it at the end of the block to force a release. There's no retain cycle or memory leak, and the __block variable is guaranteed to stick around until your block is done with it.
The solutions in ARC from best to worst are:
1. Use `__block` and set it to nil when you are done. This works fine for one-shot callbacks, for example. There is a temporary retain cycle but the cycle is broken by the assignment to nil.
2. Use `__block __weak`. The weak pointer breaks the retain cycle.
3. Use `__block __unsafe_unretained`. The unretained pointer breaks the retain cycle. This is more fragile than `__block __weak`, but may be necessary if you are deploying to iOS older than 5.0 or OS X older than 10.7.
--
Greg Parker email@hidden Runtime Wrangler
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