18.1 Overview of Window Facilities

A central notion in organizing user interfaces is allocating screen regions to particular tasks and recursively subdividing these regions into subregions. The windowing layer of CLIM defines an extensible framework for constructing, using, and managing such hierarchies of interactive regions. This framework allows uniform treatment of the following things:

• Window objects like those in X or NeWS.
• Lightweight gadgets typical of toolkit layers, such as Motif or OpenLook.
• Structured graphics such as output records and an application's presentation objects.
• Objects that act as Lisp handles for windows or gadgets implemented in a different language (such as OpenLook gadgets implemented in C).

From the perspective of most CLIM users, CLIM's windowing layer plays the role of a window system. However, CLIM usually uses the services of a window system platform to provide efficient windowing, input, and output facilities. We will refer to such window system platforms as host window systems or as display servers.

The fundamental window abstraction defined by CLIM is called a sheet. A sheet can participate in a relationship called a windowing relationship. This relationship is one in which one sheet called the parent provides space to a number of other sheets called children. Support for establishing and maintaining this kind of relationship is the essence of what window systems provide. At any point in time, CLIM allows a sheet to be a child in one relationship and a parent in another relationship.

Programmers can manipulate unrooted hierarchies of sheets (those without a connection to any particular display server). However, a sheet hierarchy must be attached to a display server to make it visible. Ports and grafts provide the functionality for managing this capability. A port is an abstract connection to a display service that is responsible for managing host display server resources and for processing input events received from the host display server. A graft is a special kind of sheet that represents a host window, typically a root window (that is, a screen-level window). A sheet is attached to a display by making it a child of a graft, which represents an appropriate host window. The sheet will then appear to be a child of that host window. In other words, a sheet is put onto a particular screen by making it a child of an appropriate graft and enabling it. Ports and grafts are described in detail in 19 Ports, Grafts, and Mirrored Sheets.

As has been discussed previously, CLIM users will typically be dealing with panes, rather than with sheets, ports, grafts, or mediums, as a call to make-application-frame automatically results in a port specification, a graft instantiation, and the allocation of a medium, to which output directed to the pane will be forwarded.

18.1.1 Properties of Sheets

Sheets have the following properties:

• A coordinate system—Provides the ability to refer to locations in a sheet's abstract plane.
• A region—Defines an area within a sheet's coordinate system that indicates the area of interest within the plane, that is, a clipping region for output and input. This typically corresponds to the visible region of the sheet on the display.
• A parent—A sheet that is the parent in a windowing relationship in which this sheet is a child.
• Children—An ordered set of sheets that are each children in a windowing relationship in which this sheet is a parent. The ordering of the set corresponds to the stacking order of the sheets. Not all sheets have children.
• A transformation—Determines how points in this sheet's coordinate system are mapped into points in its parents' coordinate system.
• An enabled flag—Indicates whether the sheet is currently actively participating in the windowing relationship with its parent and siblings.
• An event handler—A procedure invoked when the display server wishes to inform CLIM of external events.
• Output state—A set of values used when CLIM causes graphical or textual output to appear on the display. This state is often represented by a medium.

18.1.2 Sheet Protocols

A sheet is a participant in a number of protocols. Every sheet must provide methods for the generic functions that make up these protocols. These protocols are:

• The windowing protocol—Describes the relationships between the sheet and its parent and children (and, by extension, all of its ancestors and descendants).
• The input protocol—Provides the event handler for a sheet. Events may be handled synchronously, asynchronously, or not at all.
• The output protocol—Provides graphical and textual output, and manages descriptive output state such as color, transformation, and clipping.
• The repaint protocol—Invoked by the event handler and by user programs to ensure that the output appearing on the display device appears as the program expects it to appear.
• The notification protocol—Invoked by the event handler and user programs to ensure that CLIM's representation of window system information is equivalent to the display server's.

These protocols may be handled directly by a sheet, queued for later processing by some other agent, or passed on to a delegate sheet for further processing.