On Feb 15, 2012, at 12:58 PM, Matt Neuburg <m...@tidbits.com> wrote:
> This appears to be working from all points of view (thank you, Instruments!):
>
> MyMandelbrotOperation* op =
> [[MyMandelbrotOperation alloc] initWithSize:self.bounds.size
> center:center zoom:1];
> __block __weak id observer = [[NSNotificationCenter defaultCenter]
> addObserverForName:@"MyMandelbrotOperationFinished"
> object:op queue:[NSOperationQueue mainQueue]
> usingBlock:^(NSNotification *note) {
> MyMandelbrotOperation* op2 = note.object;
> CGContextRef context = [op2 bitmapContext];
> if (self->bitmapContext)
> CGContextRelease(self->bitmapContext);
> self->bitmapContext = (CGContextRef) context;
> CGContextRetain(self->bitmapContext);
> [self setNeedsDisplay];
> [[NSNotificationCenter defaultCenter] removeObserver:observer
> name:@"MyMandelbrotOperationFinished" object:op2];
> }];
> [self.queue addOperation:op];
>
> This is delightful. I'm not leaking self, my operations are being dealloced
> in good order, I'm successfully registering and deregistering, I'm
> trampolining to the main thread without writing a trampoline method, I'm
> avoiding the nightmare of storing my observers in an instance variable, op
> and op2 are the same object - everything about it seems to be okay. My
> questions are:
>
> * Is this really an okay way to talk?
Are you using GC or ARC? I think this is safe with GC but not safe with ARC,
but I'm not an expert in the NSNotificationCenter machinery.
> * Why was I crashing until I said __block?
Without __block, the block object snapshots the value of the observer variable
when the block object is created. But in your code the block object is created
before the call to -addObserverForName:..., and at that point the observer
variable is not yet initialized. Boom.
With __block, the block object always uses the current value of the observer
variable. The variable is initialized before the block object actually
executes, so everything works.
--
Greg Parker gpar...@apple.com Runtime Wrangler
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