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Authors can make links from MathML subexpressions by using the
<element-name ... href="..."> simple link
construction, as described in Section 7.1.4.
However, links are only one of many ways which specific renderers
might provide authors for making math notation active. For example,
in lengthy mathematical expressions, the ability to "fold" expressions
might be provided, i.e. a renderer might allow a reader to toggle
between an ellipsis and a much longer expression which it represents.
To provide a mechanism for binding actions to expressions, MathML provides the <maction> element. This element accepts any number of subexpressions as arguments, and the following attributes:
|actiontype||(described below)||(required attribute, no default value)|
By default, renderers which do not recognize the specified actiontype should render the selected subexpression as defined below. If no selected subexpression exists, it is a MathML error; the appropriate rendering in that case is as described in Section 7.2.2 on the treatment of MathML errors.
Since a MathML-compliant renderer is not required to recognize any particular actiontypes, a renderer can be fully MathML compliant just by implementing the above-described default behavior.
The selection attribute is provided for those actiontypes which permit someone viewing a document to select one of several subexpressions for viewing. Its value should be a positive integer which indicates one of the subexpressions of the <maction> element, numbered from 1 to the number of children of the element. When this is the case, the subexpression so indicated is defined to be the "selected subexpression" of the <maction> element; otherwise the "selected subexpression" does not exist, which is an error. When the selection attribute is not specified (including for actiontypes for which it makes no sense), its default value is 1, so the selected subexpression will be the first subexpression.
Note that the selection attribute of an <maction> element affects not only the rendering of the element itself, but potentially the rendering of elements containing it; this can be due either to the reference to an <maction>'s selected subexpression in the definition of embellished operator in Section 3.2.4, or to alignment elements contained in the selected subexpression (Section 3.5.4). This means that a change of the selection by the viewer may require a rerendering of the surroundings, not just their repositioning due to a change in size of the <maction> element itself.
Furthermore, as described in Chapter 7, if a MathML renderer responds to a user command to copy the currently selected MathML subexpression to the environment's "clipboard", any <maction> elements present in what is copied should be given selection attributes which correspond to their selection state in the MathML rendering at the time of the copy command.
A suggested list of actiontypes and their associated actions is given below. Keep in mind, however, that this list is mainly for illustration, and recognized values and behaviors will vary from renderer to renderer. (Note that hyperlinks, which are part of the actual MathML specification, are not included here, but are described separately in Chapter 7.)
For this action type, a renderer would alternately display the given expressions, cycling through them when a reader clicked on the active expression, starting with the selected expression and updating the selection attribute value as described above. Typical uses would be for exercises in education, ellipses in long computer algebra output, or to illustrate alternate notations. Note that the expressions may be of significantly different size, so that size negotiation with the browser may be desirable. If size negotiation is not available, scrolling, elision, panning, or some other method may be necessary to allow full viewing.
In this case, the renderer would display the expression in context on the screen. When a reader clicked on the expression or moved the mouse over it, the renderer would send a rendering of the message to the browser statusline. Since most browsers in the forseeable future are likely to be limited to displaying text on their statusline, authors would presumably use plain text in an <mtext> element for the message in most circumstances. For non-<mtext> messages, renderers might provide a natural language translation of the markup, but this is not required.
Here the renderer would also display the expression in context on the screen. When the mouse pauses over the expression for a long enough delay time, the renderer displays a rendering of the message in a pop-up "tooltip" box near the expression. These message boxes are also sometimes called "balloon help" boxes. Presumably authors would use plain text in an <mtext> element for the message in most circumstances. For non-<mtext> messages, renderers may provide a natural language translation of the markup if full MathML rendering is not practical, but this is not required.
In this case, a renderer might highlight the enclosed expression on a "mouse-over" event. In the example given above, use is being made of the "other" attribute to pass non-standard attributes to renderers which support them, without violating the MathML DTD (see 7.2.3). The "color" attribute changes the color of the characters in the presentation, while the "background" attribute changes the color of the background behind the characters.
This action type instructs a renderer to provide a pop up menu. This allows a one-to-many linking capability. Note that the menu items may be other <maction actiontype="menu">...</maction> expressions, thereby allowing nested menus.