On 1 Feb 2009, at 4:41 pm, Seth Willits wrote:
When inside of -[NSView drawRect:], what's the difference between
- (void)drawRect:(NSRect)rect;
{
if ([self needsToDrawRect:someRect])
....
}
... and ...
- (void)drawRect:(NSRect)rect;
{
if (NSIntersectsRect(rect, someRect))
....
}
... ?
AFAICT from the documentation, there isn't a difference. Am I
misreading something important?
As others have mentioned, there is potentially a huge difference. If
you invalidate a lot of small rectangles, the update region might be
quite complex in shape; the <rect> passed in -drawRect: is only the
bounds of this complex area.
But there is also a difference in intent between -needsToDrawRect: and
-getRectsBeingDrawn:count:, even though they also appear to amount to
much the same thing. The first method implies you'll be iterating a
list and testing each element against the view to see if it needs to
be drawn or not. In many (most?) cases that's going to work out just
fine. But if you have a huge number of possible objects to draw,
merely iterating the list and testing for every update could incur a
noticeable performance penalty. The second method offers the
possibility of using a different approach that allows you to
efficiently search and locate the objects needing to be drawn without
iterating the entire list to do so. (An analogy would be the
difference between a linear search and a binary search). That can have
great benefits (though as I say, typically only for large numbers of
objects). Algorithms such as BSP, R*-Trees and so forth can be used
with the second approach. My own tests indicate that the number at
which a BSP storage approach starts being worth considering is around
the 1000 object mark, depending on what exactly it is you're drawing.
YMMV.
This is probably more than you need or care to know at this point but
I thought it might be worth mentioning ;-)
--Graham
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