Is it possible to anchor a CGPattern-image background color to the center of a CALayer?

Question

I've set up a layer as follows:

CALayer *layer = [CALayer layer];
layer = /* create a CGColorRef with a pattern image */;

This layer is simply displayed in my window. My draw/release functions for the pattern are as follows:

void drawPattern(void *info, CGContextRef c) {
    CGContextDrawImage(c, CGRectMake(0, 0, CGImageGetWidth(info), CGImageGetHeight(info)), info);
}
void releaseInfo(void *info) {
    CGImageRelease((CGImageRef)info);
}

By default, the pattern is anchored at the lower-left corner of the window, so when I resize the window, that point is fixed.

I figured out that I can anchor it at the upper-left corner with two steps:

• set layer.geometryFlipped = YES;
• use CGAffineTransformMakeScale(1, -1) as an argument to CGPatternCreate instead of CGAffineTransformIdentity.

But how can I anchor it at the center? (Changing the layer's anchorPoint doesn't seem to do anything.)

Answer 1

I think you'll have to pass the layer, not the image, as the info parameter, and have your drawing function retrieve the layer's frame and use that to compute a translation to apply.