Re: Implementing a "Synchronous Channel" with dispatch
Re: Implementing a "Synchronous Channel" with dispatch
- Subject: Re: Implementing a "Synchronous Channel" with dispatch
- From: Andreas Grosam <email@hidden>
- Date: Wed, 14 Sep 2011 21:08:55 +0200
On Sep 14, 2011, at 8:00 PM, Dave Zarzycki wrote:
> Dispatch queues exist to solve both the synchronous and asynchronous producer/consumer pattern. If you want the producer to wait until the consumer is done, then use dispatch_sync() instead of dispatch_async():
>
> x = create_something();
> dispatch_sync(consumer_q, ^{
> do_something_with(x);
> });
> // do_something_with() is done
>
> That's it. Easy, huh? :-)
Yes, sure :)
But - unless I miss something - I can't use that - or better I can't use this *directly* with my consumer (see previous post). That is, when the consumer finishes processing one data buffer, it needs to keep its state - through being blocked in its "get_me_more_data" method.
The consumer's state (it's a parser) is the function stack. So, I need an intermediate actor - which is capable to hand off one data object. This is some kind of queue.
>
> A note about dispatch semaphores: While they are powerful, they are not meant to replace queues. They exist for two very specific problems: 1) Refactoring synchronous APIs on top of asynchronous APIs and
> 2) managing finite resource pools.
This (2) is actually what I'm trying to accomplish (well, a bit more - performance for example). Otherwise, I would just safe the 100Mbyte data download on my iPhone in memory ;)
A synchronous queue, would have capacity zero -- which would block producers till a consumer takes the data. This is what I want. Otherwise, if I would asynchronously queue the NSData objects, somehow, NSURLConnection would flood a iPhone with almost 1Mbyte per sec, that is a few seconds before crash.
So, unless the parser is capable to truly parse partial content, and can return and restart while it's state is kept somewhere else, I can't see how to make it "easy" in conjunction with a NSURLConnection :)
So, the questions remains: would the use of a "Synchronous Queue" be effective enough? What are the consequences?
Andreas
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