Styles

A style, an instance of the style<%> interface, parameterizes high-level display information that is common to all snip classes. This includes the font, color, and alignment for drawing the item. A single style is attached to each snip.

Styles are hierarchical: each style is defined in terms of another style. There is a single root style, named "Basic", from which all other styles in an editor are derived. The difference between a base style and each of its derived style is encoded in a style delta (or simply delta). A delta encodes changes such as

• change the font family to X;
• enlarge the font by adding Y to the point size;
• toggle the boldness of the font; or
• change everything to match the style description Z.

Style objects are never created separately; rather, they are always be created through a style list, an instance of the style-list% class. A style list manages the styles, servicing external requests to find a particular style, as well as the hierarchical relationship between the styles. A global style list is available (the-style-list), but new style lists can be created for managing separate style hierarchies. For example, each editor will typically have its own style list.

Each new style is defined in one of two ways:

• A derived style is defined in terms of a base style and a delta. Every style (except for the root style) has a base style, even if it does not depend on the base style in any way (i.e., the delta describes a fixed style rather than extensions to an existing style).

• A join style is defined in terms of two other styles: a base style and a shift style. The meaning of a join style is determined by reinterpreting the shift style; in the reinterpretation, the base style is used as the root style for the shift style.

Usually, when text is inserted into a buffer, a it inherits the style of the preceeding snip. If text is inserted into an empty editor, the new snip is usually assigned a style called "Standard". By default, the "Standard" style is unmodified from the root style.

The exception to the above is when change-style in text% is called with the current selection position (when the selection is a position and not a range). In that case, the style is remembered, and if the next buffer-modifying action is a text insertion, the inserted text gets the remembered style.