Implement the contract of the role-specific protocol (NSAccessibilityButton, NSAccessibilityImage, NSAccessibilityGroup, etc) within the NSAccessibility protocol that best matches the behavior of the GUI element being rendered.
Linux / BSD
For GNOME applications, the GNOME Accessibility Implementation Library (GAIL) bridges GNOME widgets and the Accessibility Toolkit (ATK). ATK bridges to the Assistive Technology Service Provider Interface (AT-SPI). AT-SPI is currently used by GTK2, Java and OpenOffice.
Microsoft Windows SDK includes all the tools necessary for MSAA and/or UI Automation. The IAccessibleEx interface the bridges between the two worlds.