On Oct 8, 2008, at 7:46 PM, John Zorko wrote:
Hello, all ...If a certain flag is set, my app needs to advance to the next item in a list to process it when the previous item is done. However, I want the user to still be able to stop it, so I don't want to hang the UI. In effect, if this flag is set, I want the app to act as if the user had manually selected the next item to process.In Win32, one way I could do this is by using PostMessage() -- when the current item is finished processing, it would post a message to the relevant threads' message queue and it would get processed, just as if the message was put there as a result of a GUI action. How do I do this in Cocoa, or is there a better way?Regards, John
Greetings, John,If you can afford to target Leopard-only, you might look into NSOperation and NSOperationQueue. They provide a way to manage sequential (or even concurrent) thread operations in a relatively simple fashion. There's a great tutorial at <http://www.macresearch.org/node/4567 > about multithreading using those classes. Despite that the particular article's title starts out with "Cocoa for Scientists," you needn't worry if you're not in the field of science: you can easily adapt their code for your own ends.
Apple's documentation on "Managing Concurrency with NSOperation" is also a good read, and you can find it at <http://developer.apple.com/cocoa/managingconcurrency.html >.
Granted, it sounds as though you *might* (just might) be porting a Windows app, in which case using NSOperation might not be ideal, since it might very easily require some major refactoring. If you have the option of using it, however, it will very likely make things simpler for you -- one item after another will be executed from the queue, and you can make each item dependent upon its predecessor, if you like, in order to share or accumulate data (I believe that the Apple link that I gave you has an example that does this).
Another option would be NSNotification and NSNotificationCenter (and possible NSNotificationQueue, although I've never used that particular class myself). This is more along the lines of what you are describing, except for this (from the NSNotificationCenter documentation): "In a multithreaded application, notifications are always delivered in the thread in which the notification was posted, which may not be the same thread in which an observer registered itself."
Thus you won't be able to communicate *between* threads using NSNotificationCenter.
One method, which may be a lead for you, is- (void)performSelectorOnMainThread:(SEL)aSelector withObject: (id)argwaitUntilDone:(BOOL)wait
This can be used send a message to the main thread, to signal that it needs to start processing another item.
Hopefully at least some of this will prove relevant!
Cheers,
Andrew
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