You could use a dedicated dispatch queue for all property access and use
dispatch barriers to restrict access to the queue for writes, while still
allowing simultaneous reads.

In -copy:

- (id)copy
{
__block __typeof(self) copy;

 dispatch_async(self.propertyQueue, ^{
copy = [[[self class] alloc] init];
 // Copy properties here
});

return copy;
}

This assumes a property called propertyQueue:

@property (readonly, nonatomic) dispatch_queue_t propertyQueue;

- (dispatch_queue_t)propertyQueue
{
if (!_propertyQueue) {
_propertyQueue = dispatch_queue_create("queue name here",
DISPATCH_QUEUE_CONCURRENT);
 }

return _propertyQueue;
}


In your property getters:

- (int)count
{
__block int count;

dispatch_async(self.propertyQueue, ^{
count = _count;
});

return count;
}


And finally, in your property *setters*:

- (void)setCount:(int)count
{
dispatch_barrier_async(self.propertyQueue, ^{
_count = count;
 });
}


Hope this helps!

(Note: all code was typed in the e-mail, so may not compile.)

Jeff Kelley


On Tue, Sep 3, 2013 at 9:34 AM, Jonathan Taylor <
jonathan.tay...@glasgow.ac.uk> wrote:

> Ah. In my original email I didn't explain *why* it is that "ideally I
> would like to make the copy on a thread other than the main thread". The
> algorithm is doing real-time video processing, and I very much want to
> avoid holding up anything in that code path by synchronizing with the main
> queue. Past experience has shown that that does lead to glitches I am keen
> to avoid. So, while I'm a big fan of such constructions, I'm deliberately
> trying to avoid that here.
>
> Serializing access to MyParameters will work, it's just a shame that there
> isn't such a tidy way of achieving that...
>
>
> On 3 Sep 2013, at 14:18, Robert Vojta wrote:
>
> > Then this should be enough ...
> >
> > - (MyParameters *)copyParameters {
> >     __block MyParameters *parameters;
> >     dispatch_sync( dispatch_get_main_queue(), ^{
> >         parameters = [myObjectHoldingParameters.parameters copy];
> >     });
> >     return parameters;
> > }
> >
> > ... if all your parameters object properties are set on the main thread
> only.
> >
>
>
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