On 18 Feb 2012, at 7:41 AM, steven Hooley wrote:

>> The same issue came up again later the same day:
>>
>> __block UIBackgroundTaskIdentifier bti = [[UIApplication sharedApplication]
>> beginBackgroundTaskWithExpirationHandler:
>> ^{
>> [[UIApplication sharedApplication] endBackgroundTask:bti];
>> }];
>>
>> Without __block, bti is invalid, and you won't find that out easily because
>> it's unlikely that you'll actually expire. :) m.
>
> Can this be right?

If I understand why you're mystified, consider this, without the __block
declaration.

- bti starts as junk.

- The block is instantiated, and captures the junk _value_ (because it's not a
__block variable) of bti.

- The beginBackgroundTaskâ method creates the background task identifier.

- That identifier is then assigned to bti. bti is no longer junk, but that does
the block no good, because it's already captured the junk value.


If bti is declared __block, the block captures (notionally) a _reference_ to
bti. It doesn't evaluate bti until it executes, which will be after the
assignment.

â F


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