CGContextClipToDrawing is a CGContext function for clipping with drawing in a block.

Example

- (void)drawRect:(CGRect)drawRect
{
	CGContextRef context = UIGraphicsGetCurrentContext();
    
    CGContextSaveGState(context);
    CGContextClipToDrawing(context, self.bounds, ^(CGContextRef maskContext, CGRect rect){
        UIGraphicsPushContext(maskContext);
        
        [[UIColor whiteColor] setFill];
        CGContextFillRect(maskContext, rect);
        
        [[UIColor blackColor] setFill];
        [@"Clear" drawInRect:rect withFont:[UIFont boldSystemFontOfSize:20.0]];
        
        UIGraphicsPopContext();
    });
    
    [[UIColor greenColor] setFill];
    CGContextFillRect(context, self.bounds);
    
    CGContextRestoreGState(context);
}

Function

//white gets drawn and black does not with gray being semi transparent

void CGContextClipToDrawing(CGContextRef context, CGRect rect, void(^maskDrawing)(CGContextRef maskContext, CGRect rect))
{
    //the block draws into a context that is rendered to a grayscale CGImage which is then used to mask the calling context
    
    CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceGray();
	
	//the mask context needs to be adjusted for scale, especially when considering retina screens
    CGAffineTransform transform = CGContextGetCTM(context);
    CGSize maskSize = CGSizeApplyAffineTransform(rect.size, transform);
    maskSize.height = fabs(maskSize.height);
    maskSize.width = fabs(maskSize.width);
	
    CGContextRef maskContext = CGBitmapContextCreate(NULL, maskSize.width, maskSize.height, 8, maskSize.width, colorspace, 0);
	
	//we want to ignore other transforms becasue they will be applied automatically
    CGContextScaleCTM(maskContext, maskSize.width / rect.size.width, maskSize.height / rect.size.height);
    
    
    maskDrawing(maskContext, CGRectMake(0.0, 0.0, rect.size.width, rect.size.height));
    CGImageRef alphaMask = CGBitmapContextCreateImage(maskContext);
    CGContextClipToMask(context, rect, alphaMask);
    
    
    CGImageRelease(alphaMask);
    CGContextRelease(maskContext);
    CGColorSpaceRelease(colorspace);
}