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Re: Am I using NSConditionLock correctly?
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Re: Am I using NSConditionLock correctly?


  • Subject: Re: Am I using NSConditionLock correctly?
  • From: jkp <email@hidden>
  • Date: Mon, 10 Oct 2005 21:37:57 +0100

I had a quick peek at your blog and although i havent read it all im not sure i understand what you are trying to do.

"So the logic is to have the worker thread set the condition to one state when it is running and the other state when it exits."

so you want your worker thread to do this calculation and your main thread to be notified when it is done? Condition locks are great things, but i think you are trying to use them the wrong way around. You want your worker thread to sit sleeping until it has some work to do, then you want it to do its thing and to let the main thread know when it is done.

The way you have described it, you are making your main thread sleep on the condition lock whilst your worker does the work...blocking in the process. the simple question i ask you is...what does this buy you? You might as well just do the calculation on the main thread and simplify your life! If you have the main thread sleep on the lock whilst the worker does its work you are still blocking, so you achieve the same thing.

I recently implemented worker class to use throughout my current application and i learnt a lot in the process. A condition lock is a great way to go since it allows you to have your workers sleep efficiently and for you to exact granular control on them when you need their services.

I think you could do with defining a set of conditions that might apply to your thread at any point during its life cycle. Maybe something like NOWORK, WORKPENDING, DATAPENDING etc...You can use these to notify the worker what it needs to do. The worker can do its thing and when it is done you have a choice of ways to send data / notify the main thread that things are finished. My favorite is NSObject's performSelectorOnMainThread:withObject:waitUntilDone: method. You can use this to call a routine on the main thread and if you want, wait until it is finished before your worker continues on its way. This way your main thread can carry on accepting user input etc, but each time it returns to its runloop it gets a chance to process a pending request from the worker to perform a selector.

Another way you could do it would be to use the condition lock to signal you have data, and maybe use a timer to check the lock from the main thread - again, this way you dont lock the main thread, but when the timer fires and there is data, the main thread does the right thing. I dont like this method myself since it is basically polling, but that would be one way you could use the condition lock in this way if you wanted to.

These are just some thoughts and ideas, but i think you could do with working out exactly what you want your worker thread implementation to be capable of, and then perhaps write a small test app that proves that it can do these things as you want. I'd also take time to stress test the classes you write since you will be suprised what problems can arrise that you hadnt thought of.

If its any comfort it took me 3 iterations to get a pair of classes that do the job for me and that i am happy with. Multithreading is a tough topic so dont be suprised if you need to go back and rehash your design several times before you get something that works for you.

HTH

Jamie

PS - I in no way claim to be an expert on this subject, im just sharing my experiences with you.


On 10 Oct 2005, at 06:53, Steve Weller wrote:


I am using NSConditionLock to delay the completion of a method while another thread completes. I am not sure that I am using it correctly, since my conditional lock hangs up the program.


In my worker thread I do [calculatingLock lock]; [calculatingLock unlockWithCondition:YES] when calculation starts and [calculatingLock lock]; [calculatingLock unlockWithCondition:NO] when it stops.

In my method that depends on the worker thread stopping I do [calculatingLock lockWhenCondition:NO]; [calculatingLock unlockWithCondition:NO]. The idea is that it should wait for the condition to be set to NO before continuing. It hangs at this point. If this is all correct then my problem lies elsewhere.

More information on my blog, URL below.

--
Watch me learn Cocoa  http://homepage.mac.com/bagelturf/



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