Editor Structure and Terminology

MrEd supports extensible and nestable editors by decomposing an editor assembly into three functional parts:

• The editor itself stores the state of the text or pasteboard and handles most events and editing operations. The editor<%> interface defines the core editor functionality, but editors are created as instances of text% or pasteboard%.
• A snip is a segment of information within the editor. Each snip can contain a sequence of characters, a picture, or an interactive object (such as an embedded editor). In a text editor, snips are constrained to fit on a single line and generally contain data of a single type. The snip% class implements a basic snip. Other snip classes include string-snip% for managing text, image-snip% for managing pictures, and editor-snip% for managing embedded editors.
• A display presents the editor on the screen. The display lets the user scroll around an editor or change editors. Most displays are instances of the editor-canvas% class, but the editor-snip% class also acts as a display for embedded editors.

These three parts are illustrated by a simple word processor. The editor corresponds to the text document. The editor object receives keyboard and mouse commands for editing the text. The text itself is distributed among snips. Each character could be a separate snip, or multiple characters on a single line could be grouped together into a snip. The display roughly corresponds to the window in which the text is displayed. While the editor manages the arrangement of the text as it is displayed into a window, the display determines which window to draw into and which part of the editor to display.

Each selectable entity in an editor is an item. In a pasteboard, all selection and dragging operations work on snips, so there is a one-to-one correspondence between snips and items. In an editor, one snip contains one or more consecutive items, and every item belongs to some snip. For example, in a simple text editor, each character is an item, but multiple adjacent characters may be grouped into a single snip.

Each place where the insertion point can appear in a text editor is a text position. A text editor with n items contains n+1 text positions: one position before each item, and one position after the last item.

The order of snips within a pasteboard determines each snip's drawing plane. When two snips overlap within the pasteboard, the snip that is earlier in the order is in front of the other snip (i.e., the former is drawn after the latter, such that the former snip may cover part of the latter snip).

When an editor is drawn into a display, each snip and text position has a graphic location. The location of a position or snip is specified in coordinates relative to the top-left corner of the editor. Locations in an editor are only meaningful when the editor is displayed.

• Administrators
• Styles