Document representation guidelines
Last modification on October 31st 2001.
The following list contains a set of guidelines for the representation of StarOffice/OpenOffice.org documents with the UNO Accessibility API (UAA). The intention of these guidelines is to define--in an application unspecific way--what kind of information of a document should expose over the UAA. It thus complements the syntactic specification made by the actual UAA definition.
It is not the goal of these guidelines to give a detailed description of how to represent documents of the individual applications. That is done at another place.
- View based approach
In order to let people that use different interfaces--for example the OpenOffice.org GUI or a Braille display--help each other with working on a document all of them need to have access to an equivalent representation of that document. We think that the best way to achieve this goal is to use the current GUI as a reference and to expose all information over the UAA that is available over the GUI.
We are, of course, aware of the fact that the information available over the UAA will not be completely equivalent to that available over the GUI. Content of graphical objects for example can be exposed only rudimentary.
This approach is called view based as opposed to a structure or content based approach that would focus not on the graphical appearance on the screen, but on the structure of a document's content.
Consequences of employing the view based approach are for instance that in a Writer document a text paragraph may be split into several portions according to its layout on the screen. Another example is the exposure of how text is broken into lines.
A further reason to favor the view based approach over the content based one is the better consistency with the WYSIWYG paradigm.
- Keep it simple
The idea behind this rule is to keep the structure of the UAA document representation as simple as possible while at the same time give access to as much of a document's semantical content as possible. One way to achieve that is to remove nodes from the representation tree that expose no content by themselves but serve only as container for other objects. Such container nodes should be used only in selected places.
The reason for this is a pragmatically one. Current AT devices can't cope with information with a structure that is overly complex. Moreover, no AT vendor will invest much time and money into supporting an API that differs too much from their current source of information.
- Don't represent objects that lie completely off-screen
There are two reasons for this rule. The first one is to keep the document representation consistent with what is visible on the screen. If document parts not visible on the screen where given access to via the UAA each AT device had to figure out on its own what parts are visible and shall be exposed to the user and which parts to hold back.
The second reason is the reduction of the amount of data that has to be transmitted from application to AT. This point is mainly relevant when application and AT are not running on the same computer. When exposing the whole document over the UAA the AT would have to probe a lot of objects in order to determine the visible ones.
There is one exception to not represent off-screen objects. Calc tables will give access to all their cells at once regardless of whether they are on- or off-screen. There are again two reasons for this. The first is consistency with existing implementations of the AccessibleTable interface like the javax.swing.JTable.AccessibleJTable class. The second reason has to do with cell indices. When only the visible cells would be represented scrolling the document would result in each cell getting new indices--relative to the upper left visible cell--assigned. This would render using indices virtually useless. Representing all cells leads to indices that remain unchanged by scrolling.
There still remains the issue of partially visible document parts. Some, like text paragraphs, may be split into on- and off-screen parts. If that is not possible partially visible objects are handled exactly like completely visible objects.
Use the VISIBLE and SHOWING states to appropriately mark objects. If they are completely or partially visible on the screen they are VISIBLE and SHOWING. Objects contained in the UAA representation tree, that lie outside the visible area are marked as VISIBLE but not SHOWING.
- Don't represent hidden objects
This is very similar to the previous rule. Objects that have been actively marked as hidden are not visible on the screen and therefore are not represented by the API.
- Do not represent the cursor position explicitly beyond the means of
The cursor position, i.e. the position at which text typed on the keyboard will be inserted into a document, has not to be explicitely exposed by the UAA e.g. by providing special AccessibleAction objects. It is taken care of by sending events with every change of the cursor position, which contain information about the AccessibleText object which contains the cursor. This object can then be queried for the caret position.