Appendix A

User Interface System Reference Model (UISRM)

Developed by the National Institute of Standards and Technology (NIST), the User Interface System Reference Model (UISRM) describes components of a window system in terms of six layers (see figure 2-2). Systems based on this reference model will have one or more layers represented in the model. In particular, many applications are built directly on the Toolkit layer, with no Dialogue/Presentation layer support from a user interface management system (UIMS). In some systems, the Toolkit, Subroutine Foundation, and/or Data Stream Interface layers may be combined.

The layers are arranged from bottom to top in ascending complexity. The bottom layers contain primitive constructs on which the upper layer functions are built. Each layer is described below:

• Layer 0: Data Stream Encoding defines the format and sequence of byte streams passed between client processes that require services and server processes on the same or separate platform that provide the services required. The Data Stream Encoding is the "wire" or "network" protocol. As a specification of message formats, the Data Stream Encoding is independent of operating system, programming language, or network communication.

• Layer 1: The Data Stream Interface specifies a function call interface to build the messages defined in the Data Stream Encoding layer. In programs, this interface is a function library. The Data Stream Interface converts parameters passed from a program into the bit stream that is transmitted over the network, and converts messages from the server into values passed to the program. The Data Stream Interface provides access to basic graphic functions from Layer 0, and may support system functions such as error handling and synchronization.

• Layer 2: The Subroutine Foundation uses features of the Data Stream Interface to provide the means to build components of window interfaces, such as scroll bars. Functions often provided by the Subroutine Foundation include initialization and destruction of objects, management of events and object hierarchy, and the saving and restoration of the interface state. The Subroutine Foundation can be thought of as a toolkit with which to build toolkits.

• Layer 3: Toolkit components such as menus, pushbuttons, scroll bars, or help boxes can be used to build an application interface. These "prefabricated" components make up the Toolkit. The components of Toolkits vary with vendors, but they typically contain most of the common user interface elements.

• Layer 4: The Presentation layer determines the appearance of the user interface, including aspects such as size, style, and color. It specifies how the components in the Toolkit should be composed to create windows. The appearance may be specified using a User Interface Definition Language (UIDL) and may be enforced by a window manager, which controls the size and location of window, and decorates windows in the style specified by the user.

• Layer 5: The Dialogue layer coordinates the interaction between the computer system and the user. It can be thought of as a mediator between the user and the application program. Communication between user and application program is through the Dialogue layer, which may be implemented by a User Interface Management System (UIMS). The user/application interaction is specified by a "dialogue" that associates user actions, such as clicking on a menu item, with application actions. Some UIMS tools can accept a dialogue and a presentation style from which to generate an instance of the UIMS that controls the interaction between user and application.

• Layer 6: The application program implements the functions required by the user. Its interaction with the user is through the Dialogue layer. The application may call routines at the Toolkit, Subroutine Foundation, or Data Stream Interface levels as well, but portability may be reduced.

The services provided in these layers are spread across the user interface services portion of the DODIIS Profile in the API and EEI, depending on specific implementation.