threaded NSProgressIndicator problem

Question

I'm trying to figure out how to update an indeterminate NSProgressIndicator in the UI using a secondary thread while the main thread does some heavy lifting, just like dozens of apps do.This snippet is based on Apple's "Trivial Threads" example using Distributed Objects (DO's):

// In the main (client) thread...
- (void)doSomethingSlow:(id)sender
{ 
 [transferServer showProgress:self];

 int ctr;
 for (ctr=0; ctr <= 100; ctr++)
  {
  [NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.1]];
  NSLog(@"running long task...");
  }
}

// In the secondary (server) thread...
- (oneway void)showProgress:(Controller*)controller
{
 [controller resetProgressBar];

 float ticks;
 for (ticks=0; ticks <= 100; ticks++)
  {
  [NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.1]];
  [controller updateProgress:ticks];
  NSLog(@"updating progress in UI...");
  }
}

Unfortunately however, there's no way I can get both threads to run concurrently. Either the secondary thread will run and the main thread waits until it's finished OR the main thread runs followed by the secondary thread -- but not both at the same time.

Even if I pass a pointer to the server thread and ask it to update the progress bar directly (without calling the main thread back) it makes no difference. It seems that once the main thread enters a loop like this it ignores all objects sent to it. I'm still a novice with Obj-C and I'd really appreciate any help with this.

Answer 1

You might want to try switching your threads. In general, UI updates and user input are handled on the main thread, and heavy tasks are left for secondary threads.

Answer 2

AppKit isn't thread-safe, at all. You have to update the UI from the main thread, or all sorts of crazy stuff will happen (or it just won't work).

The best way is to do your work on the secondary thread, calling back to the main thread when you need to update the UI:

-(void)doSomethingSlow:(id)sender {

    [NSThread detachNewThreadSelector:@selector(threadedMethod) toTarget:self withObject:nil];

    // This will return immediately. 
}

-(void)threadedMethod {

    int ctr;
    for (ctr=0; ctr <= 100; ctr++) {
        NSLog(@"running long task...");

        [self performSelectorOnMainThread:@selector(updateUI)];
    }
}

-(void)updateUI {
    // This will be called on the main thread, and update the controls properly.
    [controller resetProgressBar];
}