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Display Items and Display Styles 

The appearance of a display item is controlled by a set of attributes. For example, the text attribute controls the text string displayed on the item and the font attribute specifies what font should be used.

Usually, each of the attributes falls into one of two categroies: ``individual'' or ``collective''. For example, each of the items inside a TixTList widget may display a different text string; therefore we call the text string an individual attribute. However, in most cases, the items share the same color, font and spacing and we call these collective attributes.

One question concerns where we keep the collective attribute for the display items. Certainly, we can keep a font attribute for each item, but this is not really an efficient solution. In fact, if all the items have the same font, we would be keeping a duplicated copy of the same font for each of the items we create. Since a host widget may have many thousands of items, keeping thousands of dupilcated copys of the same font, or any other collective attributes, would be very wasteful.

To avoid the unnecessary duplication of resources, Tix stores the collective attributes in special objects called display styles. The relationship between display items and their styles is depicted in figure 3-4. Each item holds its own copy of the individual attributes, such as text. However, the collective attributes are stored in the style objects. Each item has a special style attribute that tells it which style it should use. In figure 3-4 , since items a and b are assigned the same style, therefore, they share the same font and color. Item c is assigned a different style, thus, it uses a different font than a and b.