Chapter 9 Defining Application Frames

9.1 Conceptual Overview of CLIM Application Frames

Application frames (or simply frames) are the central abstraction defined by CLIM for presenting an application's user interface. Many of the other features and facilities provided by CLIM (for example, the generic command loop, gadgets, look-and-feel independence) can be conveniently accessed through the frame facility. Frames can be displayed as either top-level windows or regions embedded within the space of the user interfaces of other applications. In addition to controlling the screen real estate managed by an application, a frame keeps track of the Lisp state variables that contain the state of the application.

The contents of a frame is established by defining a hierarchy of panes. CLIM panes are interactive objects that are analogous to the windows, gadgets, or widgets of other toolkits. Application builders can compose their application's user interface from a library of standard panes or by defining and using their own pane types. Application frames can use a number of different types of panes, including layout panes for organizing space, extended stream panes for presenting application-specific information, and gadget panes for displaying data and obtaining user input. Panes are described in greater detail in Chapter 10, "Panes and Gadgets."

Frames are managed by special applications called frame managers. Frame managers control the realization of the look and feel of a frame. The frame manager interprets the specification of the application frame in the context of the available window system facilities, taking into account preferences expressed by the user. In addition, the frame manager takes care of attaching the pane hierarchy of an application frame to an appropriate place in a window hierarchy. The most common type of frame manager is one that allows the user to manipulate the frames of other applications. This type of application is typically called a desktop manager, or in X Windows terminology, a window manager. In many cases, the window manager will be a non-Lisp application. In these cases, the frame manager will act as a mediator between the Lisp application and the host desktop manager.

Some applications may act as frame managers that allow the frames of other applications to be displayed with their own frames. For example, a text editor might allow figures generated by a graphic editor to be edited in place by managing the graphics editor's frame within its own frame.

Application frames provide support for a standard interaction processing loop, like the Lisp "read-eval-print" loop, called a command loop. The application programmer only has to write the code that implements the frame-specific commands and output display functions. A key aspect of the command loop is the separation of the specification of the frame's commands from the specification of the end-user interaction style.

The standard interaction loop consists of reading an input "sentence" (the command and all of its operands), executing the command, and updating the displayed information as appropriate.

To write an application that uses the standard interaction loop provided by CLIM, you need to:

In the case of a simple ECAD program, the programmer would first define the appropriate presentation types, such as wires, input and output signals, gates, resistors, and so forth. She would then define the program's commands in terms of these types. For example, the "Connect" command might take two operands, one of type "component" and the other of type "wire." The programmer may wish to specify the interaction style for invoking each command, for example, direct manipulation via translators, or selection of commands from menus. After defining an application frame that includes a CLIM stream pane, the programmer then writes the frame-specific display routine that displays the circuit layout. Now the application is ready to go. The end-user selects a command (via a menu, command-line, or whatever), the top-level loop takes care of collecting the operands for that command (via a variety of user gestures), and then the application invokes the command. The command may affect the frame's "database" of information, which can in turn affect the output displayed by the redisplay phase.

Note that this definition of the standard interaction loop does not constrain the interaction style to be a command-line interface. The input sentence may be entered via single keystrokes, pointer input, menu selection, dialogs, or by typing full command lines.

CLIM 2.0 User's Guide - OCT 1998

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