Question

I want to draw a horizontal line with a gradient at loc 550.

CGFloat colors[6] = { 
    138.0f / 255.0f, 1.0f,
    162.0f / 255.0f, 1.0f,
    206.0f / 255.0f, 1.0f};
CGFloat locations[3] = { 0.05f, 0.45f, 0.95f };
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
CGGradientRef gradient = CGGradientCreateWithColorComponents(colorSpace, colors, locations, 3);
CGColorSpaceRelease(colorSpace);

CGPoint startPoint = CGPointMake(0, 0);
CGPoint endPoint   = CGPointMake(0, 550);

 CGContextRef context = UIGraphicsGetCurrentContext(); 
 CGContextSaveGState(context);
 CGContextAddRect(context, CGRectMake(0, 550, self.view.bounds.size.width, 1));
 CGContextClip(context);

CGContextDrawLinearGradient (context, gradient, startPoint, endPoint, 0);
CGContextRestoreGState(context);           

No line is drawn in this code and the Xcode gives me this error:

            CGContextAddRect: invalid context 0x0

Answer 1

You need to place this code in your drawRect: method of a UIView object to make sure a current context exists. Otherwise you need to use UIGraphicsPushContext to get a current context.

From the documentation for UIGraphicsGetCurrentContext():

The current graphics context is nil by default. Prior to calling its
drawRect: method, view objects push a valid context onto the stack,
making it current. If you are not using a UIView object to do your
drawing, however, you must push a valid context onto the stack
manually using the UIGraphicsPushContext function.

You should call this function from the main thread of your application
only.