`CAGradientLayer` is a special purpose `CALayer` subclass for drawing animatable linear gradients. It's a part of the QuartzCore framework and is available for both iOS (since iOS 3.0) and OS X (since OS X v10.6 Snow Leopard). The gradient is specified using four properties for the `colors`, their `locations` and the `startPoint` and `endPoint` of the layer. All four properties are animatable.
