How to render to offscreen bitmap then blit to screen using Core Graphics

You can use a CGBitmapContext. You can generate an image from a CGBitmapContext and draw it during your drawRect.

To render into an offscreen context and save it as a CGImageRef:

void *bitmapData = calloc(height, bytesPerLine);
CGContextRef offscreen = CGBitmapContextCreate(..., bitmapData, ...)
// draw stuff into offscreen
CGImageRef image = CGBitmapContextCreateImage(offscreen);
CFRelease(offscreen);
free(bitmapData);

To draw it on the screen:

- (void)drawRect:(CGRect)rect {
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextDrawImage(context, rect, image);
}

You could also just save the image in the view's layer's contents property (view.layer.contents = image), or use a UIImageView.