NSScrollView messes up NSGradient (corruption) - objective-c

Question

I have a custom box that I've made that is a subclass of NSBox. I override the drawRect: method and draw a gradient in it like this (assuming I already have a start & end color):
-(void)drawRect:(NSRect)dirtyRect {
NSGradient *gradient = [[NSGradient alloc] initWithStartingColor:start endingColor:end];
[gradient drawInRect:dirtyRect angle:270];
[gradient release];
}
Now this box is added as a subview of a prototype view for a NSCollectionView. In the view's original state it looks like this:
And after scrolling the view out of sight and back in again, it looks like this:
Why is my gradient getting corrupted like that, and how can I fix it? Thanks!

Answer 1

That dirtyRect argument doesn’t necessarily represent the entire box. If Cocoa decides that only a subframe of the original frame needs (re)drawing, dirtyRect represents only that subframe. If you’ve drawn a gradient for the entire frame and then (re)draw the same gradient for a subframe, it's likely they won't match.
Try:
[gradient drawInRect:[self bounds] angle:270];
instead.
One further note: it looks like your gradient object could be cached instead of being created/released inside -drawRect:.

Answer 2

Your problem is you're drawing in dirtyFrame, not the entire rectangle of the box. I have no idea if this is correct, but try this:
-(void)drawRect:(NSRect)dirtyRect {
NSGradient *gradient = [[NSGradient alloc] initWithStartingColor:start endingColor:end];
[gradient drawInRect:[self bounds] angle:270];
[gradient release];
}