Considerations for Creating Custom Controls for your Mac App
The Mac OS X Aqua user interface provides users with a consistent visual and behavioral experience across applications and the operating system, and provides an aesthetically refined use of color, transparency, and animation. If your application requires an element or behavior that doesn’t already exist, you can create your own custom controls, provided that your element or behavior supports Apple’s interface design principles. Review the Extending the Interface section of the Apple Human Interface Guidelines for details.