Is calling -setNeedsDisplay from a background task asking for trouble?

Question

I have a background task that updates a view. That task calls -setNeedsDisplay to have the view drawn.

This works:

- (void) drawChangesTask;
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    if (pixels) {
     drawChanges((UInt32 *) origPixels, (UInt32 *) pixels, CGBitmapContextGetBytesPerRow(ctx)/4, CGBitmapContextGetHeight(ctx), count--);

     if (count < 0) {
      count = 150;
     }
     else
      [self performSelectorInBackground:@selector(drawChangesTask) withObject:nil ];

     [self performSelectorOnMainThread:@selector(setNeedsDisplay) withObject:nil waitUntilDone:NO ];

    }
    [pool release];
}

This does not work:

- (void) drawChangesTask;
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    if (pixels) {
     drawChanges((UInt32 *) origPixels, (UInt32 *) pixels, CGBitmapContextGetBytesPerRow(ctx)/4, CGBitmapContextGetHeight(ctx), count--);

     if (count < 0) {
      count = 150;
     }
     else
      [self performSelectorInBackground:@selector(drawChangesTask) withObject:nil ];

     [self setNeedsDisplay];

    }
    [pool release];
}

Anyone know why? When I say it doesn't work, I mean that it runs tens of iterations, sometimes I see portions of my image shifted up or down, or entirely blank, and then the deugger give me an “EXC_BAD_ACCESS” somewhere in CoreGraphics.

Also, if I don't handle the autorelease pool myself, then I get leaking error messages. Don't understand why that is either. My drawChanges() doesn't create any new objects. Here's the error:

2009-08-17 11:41:42.358 BlurApp[23974:1b30f] *** _NSAutoreleaseNoPool(): Object 0xd78270 of class NSThread autoreleased with no pool in place - just leaking

Answer 1

UIKit simply isn't thread-safe — you need to call methods that update UIKit controls on the main thread.

I think that this line:

[self performSelectorInBackground:@selector(drawChangesTask) withObject:nil];

Is causing trouble. Have you tried simply calling it again on the current thread? If you need the runloop to execute between the calls, use:

[self performSelector:@selector(drawChangesTask) withObject:nil afterDelay:0.0];

This will call the method on the current thread after the method you're in has finished and the runloop has gone round once.

Answer 2

Problem here is that UIKit is not thread safe, if you tell your UI to do something from a background thread nothign is guaranteed, what you want to do is use the performSelectorOnMainThread method to do updates t o your UI elements