title: How to draw a color wheel in objective-c
body: <p>I am trying to draw a color wheel for an iPhone, but I can't get the gradient to rotate around a point.  I am trying to use gradients but objective-c provides a linear gradient, which draws a gradient in a straight line like this:
  <img src="https://i.stack.imgur.com/Pl5AB.jpg" alt="linear gradient"></p>
  
  <p>and a radial gradient, which draws the gradient starting at point and radiating out in all directions like this:
  <img src="https://i.stack.imgur.com/R8iTZ.png" alt="radial gradient"></p>
  
  <p>I want to draw a linear gradient that rotates around a point like this:</p>
  
  <p><img src="https://i.stack.imgur.com/nprwi.jpg" alt="colorwheel"></p>