Question

I’ve created a CALayer subclass in order to draw a checkerboard background pattern. Everything works well and is rendering correctly, however I’ve discovered that performance takes a nosedive when the CALayer is given a large frame.

It seems fairly obvious that I could optimise by shifting the allocation of my CGColorRef and CGPatternRef outside of the drawLayer:inContext: call, but I’m not sure how to go about this as both rely on having a CGContextRef.

As far as my understanding goes, CALayer’s drawing context is actually owned by its parent NSView and is only passed during drawing. If this is the case, how best can I optimise the following code?

void drawCheckerboardPattern(void *info, CGContextRef context)
{
    CGColorRef alternateColor = CGColorCreateGenericRGB(1.0, 1.0, 1.0, 0.25);
    CGContextSetFillColorWithColor(context, alternateColor);

    CGContextAddRect(context, CGRectMake(0.0f, 0.0f, kCheckerboardSize, kCheckerboardSize));
    CGContextFillPath(context);

    CGContextAddRect(context, CGRectMake(kCheckerboardSize, kCheckerboardSize, kCheckerboardSize, kCheckerboardSize));
    CGContextFillPath(context);

    CGColorRelease(alternateColor);
}
- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)context
{
    CGFloat red = 0.0f, green = 0.0f, blue = 0.0f, alpha = 0.0f;
    NSColor *originalBackgroundColor = [self.document.backgroundColor colorUsingColorSpaceName:NSCalibratedRGBColorSpace];
    [originalBackgroundColor getRed:&red green:&green blue:&blue alpha:&alpha];

    CGColorRef bgColor = CGColorCreateGenericRGB(red, green, blue, alpha);
    CGContextSetFillColorWithColor(context, bgColor);
    CGContextFillRect(context, layer.bounds);

    // Should we draw a checkerboard pattern?
    if([self.document.drawCheckerboard boolValue])
    {
        static const CGPatternCallbacks callbacks = { 0, &drawCheckerboardPattern, NULL };

        CGContextSaveGState(context);
        CGColorSpaceRef patternSpace = CGColorSpaceCreatePattern(NULL);
        CGContextSetFillColorSpace(context, patternSpace);
        CGColorSpaceRelease(patternSpace);

        CGPatternRef pattern = CGPatternCreate(NULL,
                                               CGRectMake(0.0f, 0.0f, kCheckerboardSize*2, kCheckerboardSize*2),
                                               CGAffineTransformIdentity,
                                               kCheckerboardSize*2,
                                               kCheckerboardSize*2,
                                               kCGPatternTilingConstantSpacing,
                                               true,
                                               &callbacks);
        alpha = 1.0f;
        CGContextSetFillPattern(context, pattern, &alpha);
        CGPatternRelease(pattern);
        CGContextFillRect(context, layer.bounds);
        CGContextRestoreGState(context);
    }

    CGColorRelease(bgColor);
}

Answer 1

You can create the pattern outside your drawLayer:inContext: just fine, it doesn’t need a context. So just create a CGPatternRef instance variable and create the pattern. That should already speed up rendering as creating the pattern is expensive. In fact, I would create all CG* instances that don’t need a context outside your drawLayer:inContext: method, so everything up to CGColorCreateGenericRGB and also the CGColorSpaceCreatePattern.