Chapter 8 Presentation Translators in CLIM

8.1 Conceptual Overview of Presentation Translators

CLIM provides a mechanism for "translating" between presentation types. In other words, within an input context for type A, the translator mechanism allows a programmer to have presentations of some other type B treated as though they were objects of type A.

You can define presentation translators to make the user interface of your application more flexible. For example, suppose the input context is expecting a command. In this input context, all displayed commands are sensitive, so the user can point to one to execute it. However, suppose the user points to another kind of displayed object, such as a student. In the absence of a presentation translator, the student is not sensitive because only commands can be entered to this input context.

If you used a presentation translator that translates from students to commands, however, both students and commands would be sensitive. When the student is highlighted, the middle pointer button might execute the command show-transcript for that student.

A presentation translator defines how to translate from one presentation type to another. In the previous scenario, the input context is command. A user-defined presentation translator states how to translate from the student presentation type to the command presentation type.

The concept of translating from an arbitrary presentation type to a command is so useful that CLIM provides the special define-presentation-to-command-translator macro for this purpose. You can think of these presentation-to-command translators as a convenience for the users; users can select the command and give the argument at the same time.

Note that presentation-to-command translators make it easier to write applications that give a "direct manipulation" feel to the user.

A presentation that appears on the screen can be sensitive. This means that the presentation can be operated on directly by using the pointer. A sensitive presentation will be highlighted when the pointer is over it. (In rare cases, the highlighting of some sensitive presentations is turned off.)

Sensitivity is controlled by three factors:

Presentation translators link these three factors.

A presentation translator specifies the conditions under which it is applicable, a description to be displayed, and what to do when it is invoked by clicking the pointer.

A presentation is sensitive (and highlighted) if there is at least one applicable translator that could be invoked by clicking a button with the pointer at its current location and the modifier keys in their current state. If there is no applicable translator, there is no sensitivity and no highlighting.

Each presentation translator has two associated presentation types, its from-presentation-type and to-presentation-type, which are the primary factors in its applicability. Since a presentation translator translates an output presentation into an input presentation, a presentation translator is applicable if the type of the presentation at the pointer "matches" the from-presentation-type and the input context type "matches" the to-presentation-type. (We define what "match" means in the next section.) Each presentation translator is attached to a particular pointer gesture, which is a combination of a pointer button and a set of modifier keys. Clicking the pointer button while holding down the modifier keys invokes the translator.

Note that a translator produces an input presentation consisting of an object and a presentation type to satisfy the program accepting input. The result of a translator might be returned from accept, or it might be absorbed by a parser and provide only part of the input. An input presentation is not actually represented as an object. Instead, a translator's body returns two values. The object is the first value. The presentation type is the second value; it defaults to the to-presentation-type if the body returns only one value.

CLIM 2.0 User's Guide - OCT 1998

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