How to draw a gradient arc with Core Graphics/iPhone?

Question

I know how to draw a arc

and I also found how to draw a gradient line from here I have found two function can draw gradient:CGContextDrawLinearGradient and CGContextDrawRadialGradient.but how can I draw a gradient arc? I want to realize like this picture:

Answer 1

I spent a long time searching for how to do this, too, so I thought I'd post the way I ended up doing it. It turns out both answers are in the excellent answer to this question:

Draw segments from a circle or donut

For my purposes, I only used the drawing and gradient parts of that answer. The structure looks more or less like this...

CGContextRef context = UIGraphicsGetCurrentcontext();

CGFloat arcStartAngle = M_PI;
CGFloat arcEndAngle = 2 * M_PI;

CGPoint startPoint = CGPointMake(...);
CGPoint endPoint = CGPointMake(...);

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

CGFloat colors[] =
{
    1.0, 0.0, 0.0, 1.0,   //RGBA values (so red to green in this case)
    0.0, 1.0, 0.0, 1.0    
};

CGGradientRef gradient = CGGradientCreateWithColorComponents(colorSpace, colors, NULL, 2);
//Where the 2 is for the number of color components. You can have more colors throughout //your gradient by adding to the colors[] array, and changing the components value.

CGColorSpaceRelease(colorSpace);

//Now for the arc part...

CGMutablePathRef arc = CGPathCreateMutable();

CGPathMoveToPoint(arc, NULL, startPoint.x, startPoint.y);


//Here, the CGPoint self.arcCenter is the point around which the arc is placed, so maybe the
//middle of your view. self.radius is the distance between this center point and the arc.
CGPathAddArc(arc, NULL, self.arcCenter.x, self.arcCenter.y, self.radius, 
             arcStartAngle, arcEndAngle, YES);


//This essentially draws along the path in an arc shape
CGPathRef strokedArc = CGPathCreateCopyByStrokingPath(arc, NULL, 5.0f, 
                                                      kCGLineCapButt, kCGLineJoinMiter, 10);


CGContextSaveGState(context);

CGContextAddPath(context, strokedArc);
CGContextClip(context);

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

CGContextDrawPath(context, kCGPathFillStroke);

CGGradientRelease(gradient);
CGContextRestoreGState(context);

//This all draws a gradient that is much larger than the arc itself, but using
//CGContextClip, it clips out everything EXCEPT the colors in the arc. Saving and Restoring
//the state allows you to preserve any other drawing going on. If you didn't use these,
//then all other drawing would also be clipped.

I hope this helps. If any of this is unclear, I recommend you check out the question link above. The answer to that questions contains everything I used in this answer and a few more cool and useful drawing tips.