CGContextDrawRadialGradientDemo.m

- (void)drawRect:(CGRect)rect
{
  CGContextRef context = UIGraphicsGetCurrentContext();
  
  CGContextSaveGState(context);
  {
    size_t num_locations            = 2;
    CGFloat locations[2]            = {0.1, 0.9};  // 突出一个聚焦点
    CGFloat colorComponents[8]      = { 68.0/255.0, 68.0/255.0, 68.0/255.0, 1.0, 
                                        32.0/255.0, 36.0/255.0, 41.0/255.0, 1.0 };
    CGColorSpaceRef myColorspace    = CGColorSpaceCreateDeviceRGB();
    CGGradientRef gradient          = CGGradientCreateWithColorComponents (myColorspace, colorComponents, locations, num_locations);
    
    CGPoint centerPoint             = CGPointMake(self.bounds.size.width / 2.0,
                                                  self.bounds.size.height / 2.0);
    
    // Draw the gradient
    // 最后一个参数表明会自动填充终点圆的外部区域
    CGContextDrawRadialGradient(context, gradient, centerPoint, 0, centerPoint, rect.size.width / 2,  (kCGGradientDrawsAfterEndLocation));
    
    CGGradientRelease(gradient);
    CGColorSpaceRelease(myColorspace);
  }
  CGContextRestoreGState(context);
  
  // Blend the noize texture to the background
  CGSize textureSize                  = [noizeImage size];
  CGContextDrawTiledImage(context, CGRectMake(0, 0, textureSize.width, textureSize.height), noizeImage.CGImage);
}