Concurrent drawRect: - objective-c

Question

I have a large array of objects (typically 500 - 2000) that render to the screen. Unfortunately, rendering is not exactly snappy at the moment.
Each object needs to perform some calculations which take up most of the time and finally draw itself to the screen, i.e. currently my drawRect: method looks essentially like this:
(I've left out trivial optimizations like checking bounding rects vs. dirtyRect for the sake of readability)
- (void)drawRect:(NSRect)dirtyRect
{
for (Thing *thing in [self getThings])
{
[thing prepareForDrawing];
[thing draw];
}
}
An obvious candidate for concurrent processing, right?
I couldn't come up with a good approach to decouple preparation from the actual drawing operations, i.e. perform the pre-processing in parallel and somehow queue the drawing commands until all processing is done, then render all in one go.
However, thinking of the goodness that is GCD I came up with the following scheme.
It kind of sounds OK to me but being new to GCD and before running into weird multi-threading issues four weeks after a public release or just using a bad GCD design pattern in general I thought I'd ask for feedback.
Can anybody see a problem with this approach - potential issues, or a better solution?
- (void)drawRect:(NSRect)dirtyRect
{
[[self getThings] enumerateObjectsWithOptions:NSEnumerationConcurrent
usingBlock:^(id obj, NSUInteger idx, BOOL *stop)
{
// prepare concurrently
Thing *thing = (Thing*)obj;
[thing prepareForDrawing];
// always draw in main thread
dispatch_async(dispatch_get_main_queue(), ^{
[thing draw];
});
}
}

Answer 1

That won't work because the invocations of [thing draw] will happen outside of -drawRect: after it has completed. The graphics context will no longer be valid for drawing into that view.
Why are the "things" not prepared in advance? -drawRect: is for drawing, not computation. Any necessary expensive computation should have been done in advance.