Question

I have a view that receives new data from a secondary thread. Every time it does, it should redraw itself. However, it doesn’t play nice with the run loop, and after some time (it’s non-deterministic), I end up getting <Error>: kCGErrorIllegalArgument: CGSUnionRegionWithRect : Invalid region messages in the console.

I’m not sure what’s the right way to synchronize the calls to [view setNeedsDisplay:YES] across threads; can you help me?

To clarify a little, thread B (actually a dispatch queue) gives new contents to a view by calling this:

-(void)setImageBuffer:(unsigned char*)buffer
{
    /* image handling stuff; thread-safe */

    [self setNeedsDisplay:YES]; // but this is not thread-safe
}

And then thread A, on which runs the run loop, should redisplay the view.

Answer 1

-(void)setImageBuffer:(unsigned char*)buffer
{
    /* image handling stuff; thread-safe */

    [self performSelectorOnMainThread:@selector(induceRedraw)
                           withObject:nil
                                      // Don't just copy this; pick one...
                        waitUntilDone:YES or NO];
}

-(void)induceRedraw
{
    [self setNeedsDisplay:YES]; // but this is not thread-safe
}