Accessibility is the process of ensuring your application can be used by people with various disabilities. Disabilities come in many forms: visual impairments, movement impairments, hearing impairments, cognitive and language impairments, and seizure disorders. Many people have some sort of disability, and making your application accessible will allow more people to use it effectively, even if they are not disabled.
GNOME provides support for accessibility devices using the ATK framework, which stands for Accessibility Tool Kit. This framework defines a set of interfaces to which graphical interface components adhere. This allows, for instance, screen readers to read the text of an interface and interact with its controls. ATK support is built into GTK+ and the rest of the GNOME platform, so any application using GTK+ will have reasonable accessibility support for free.
ATK solves the problem of plugging many different kinds of widgets (text entries, text areas, buttons, menus) with many different types of accessibility technologies (screen readers, braille displays, sip-and-puff control devices). Instead of writing NxM interfaces, from each widget to each accessibility device, one just has to expose a widget through ATK's interfaces. Accessibility devices will in turn use these interfaces to query accessible widgets. For example, an aural screen reader and a braille display will both use a standard "get the text contents" interface in ATK for all widgets with textual information.
Nonetheless, you should be aware of accessibility issues when when developing your applications. Although GTK+ interfaces provide reasonable accessibility by default, you can often improve how well your program behaves with accessibility tools by providing additional information to ATK. If you develop custom widgets, you should ensure that they expose their properties to ATK. You should also avoid using sound, graphics, or color as the sole means of conveying information to the user; have redundant ways of providing this information instead of relying on the user being able to perceive a particular medium.
The GNOME desktop ships with a number of accessibility tools which enable users with disabilities to take full advantage of their desktop and applications. Applications that fully implement ATK will be able to work with the accessibility tools. These include a screen reader, a screen magnifier, an on-screen keyboard, and Dasher, an innovative predictive text entry tool.
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