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Re: Blocking loops
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Re: Blocking loops


  • Subject: Re: Blocking loops
  • From: Shawn Erickson <email@hidden>
  • Date: Fri, 16 Jun 2006 07:20:48 -0700


On Jun 16, 2006, at 6:24 AM, email@hidden wrote:

I've got an application that does a fair bit of processing in a while loop...

<Code>
NSString* currentObj;
NSEnumerator* allObjects = [myArray objectEnumerator];
while (currentObj = [allObjects nextObject]) {
[progressBar setDoubleValue:((objIndex / [[fileController content] count]) *
100)];
[progressBar setNeedsDisplay:YES];
// Lots of processing here
} // while
</Code>


Unfortunately, the progress bar doesn't actually move (and the App is
unresponsive) until the loop completes. Is there a simple one line fix to this
(I'm thinking along the lines of Visual Basic's DoEvents() and Delphi's
Application.ProcessMessages()), or do I need to do some weird stuff involving
Notifications, and do away with (no pun intended) the loop?


Thanks for any help offered,

Consider throwing that work off to a secondary thread (NSThread) and in that loop (running in the secondary thread) use -[NSObject performSelectorOnMainThread:withObject:waitUntilDone:] to update your user interface (your progress bar).


Another option is to intermix you processing with a nested runloop, see NSRunLoop.

Finally you could break your work up and process it in steps using an NSTimer or a delayed selector call (once you process one object "schedule" the next one to be processed).

Personally I usually throw such stuff off to a secondary thread or possibly schedule a sub-unit of work on the main runloop (usually via a delayed selector call, -[NSObject performSelector:withObject:afterDelay:]).

An quick example of the later...
...
NSEnumerator* objectEnumerator = [myArray objectEnumerator];
[self processObjects:objectEnumerator];
...

- (void) processObjects:(NSEnumerator*)objectEnumerator
{
	id object = [objectEnumerator nextObject];
	if (object != nil) {
		... process object ...

... update UI ...

[self performSelector:@selector(processObjects:) withObject:objectEnumerator afterDelay:0.0];
}
}


-Shawn
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  • Follow-Ups:
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