Implementing Accessible Objects

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Implementation Rules

There are some rules to observe when implementing the UNO accessibility API that go beyond simply following the specifications in the IDL files of the accessibility API's interfaces. These rules have to do with what kind of data ATs expect from an application.

One such rule is that only objects that are visible on the screen are included into the accessibility tree. If, for example, you have a text document with a large number of pages, usually only parts of one or two pages are visible, and only accessibility objects for these parts should be part of the accessibility hierarchy. Another closely related rule is that the bounding boxes of objects are clipped to the visible area.

However, there are exceptions to these rules. For reasons of consistency with the behavior of Java tables represented through the interface, access is granted to all of their cells regardless of whether they are visible or not. Menus are another example. The whole menu structure is represented, even when only the menu bar is visible.

Another rule is that bounding boxes of accessibility objects as returned by getBounds() must not overlap the bounding boxes of their parents. This is crucial to enable ATs to find the accessibility object that lies under a given screen coordinate, such as the mouse position. With properly nesting bounding boxes, ATs can prune whole sub-trees from the search when the bounding box of the root object does not contain the point.


There are only two services in the accessibility API, which have to be supported by any accessible object.

The service contains the interface and must be supported by every UNO object that is accessible.

The service contains the interface and, optionally, the interface. This service must be supported by every accessible object that is returned by the getAccessibleContext() function.

Content on this page is licensed under the Public Documentation License (PDL).
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