1064 SOME O-O TECHNIQUES FOR GRAPHICAL INTERACTIVE APPLICATIONS $32.1 More progress remains necessary in this fast-moving area.Object technology can help tremendously,and in fact the fields denoted by the two buzzwords,GUI and O-O, have had a closely linked history.Simply stated,the purpose ofthis chapter is to disprove the Interface Conjecture,by showing that to be user-friendly an application does not have to be developer-hostile.Object-oriented techniques will help us concentrate on the proper data abstractions,suggest some of these abstractions,and give us the ability to reuse everything that can be reused. A complete exploration of O-O techniques for building graphical and interactive applications would take a book of its own.The aim of the present chapter is much more modest.It will simply select a few of the less obvious aspects of GUI building,and introduce a few fundamental techniques that you should find widely applicable if your work involves designing graphical systems. 32.1 NEEDED TOOLS What tools do we need for building useful and pleasant interactive applications? End users,application developers and tool developers First,a point of terminology to avoid any confusion.The word "user"(one of the most abused terms in the computer field)is potentially misleading here.Certain people,called application developers,will produce interactive applications to be used by other people, to be called end users;a typical end user would be a dentist's assistant,using a system built by some application developer for recording and accessing patient history.The application developers themselves will rely,for their graphical needs,on tools built by the third group,tool developers.The presence of three categories is the reason why "user" without further qualification is ambiguous:the end users are the application developers users;but the application developers themselves are the tool developers'users. An application is an interactive system produced by a developer.An end user who uses an application will do so by starting a session,exercising the application's various facilities by providing the input ofhis choice.Sessions are to applications what objects are to classes:individual instances of a general pattern. This chapter analyzes the requirements of developers who want to provide their end users with useful applications offering graphical interfaces. Graphical systems,window systems,toolkits Many computing platforms offer some tools for building graphical interactive applications.For the graphical part,libraries are available to implement designs such as GKS and PHIGS.For the user interface part,basic window systems(such as the Windows Application Programming Interface,the Xlib API under Unix and the Presentation Manager API under OS/2)are too low-level to make direct use convenient for application developers,but they are complemented by "toolkits",such as those based on the Motif user interface protocol.1064 SOME O-O TECHNIQUES FOR GRAPHICAL INTERACTIVE APPLICATIONS §32.1 More progress remains necessary in this fast-moving area. Object technology can help tremendously, and in fact the fields denoted by the two buzzwords, GUI and O-O, have had a closely linked history. Simply stated, the purpose of this chapter is to disprove the Interface Conjecture, by showing that to be user-friendly an application does not have to be developer-hostile. Object-oriented techniques will help us concentrate on the proper data abstractions, suggest some of these abstractions, and give us the ability to reuse everything that can be reused. A complete exploration of O-O techniques for building graphical and interactive applications would take a book of its own. The aim of the present chapter is much more modest. It will simply select a few of the less obvious aspects of GUI building, and introduce a few fundamental techniques that you should find widely applicable if your work involves designing graphical systems. 32.1 NEEDED TOOLS What tools do we need for building useful and pleasant interactive applications? End users, application developers and tool developers First, a point of terminology to avoid any confusion. The word “user” (one of the most abused terms in the computer field) is potentially misleading here. Certain people, called application developers, will produce interactive applications to be used by other people, to be called end users; a typical end user would be a dentist’s assistant, using a system built by some application developer for recording and accessing patient history. The application developers themselves will rely, for their graphical needs, on tools built by the third group, tool developers. The presence of three categories is the reason why “user” without further qualification is ambiguous: the end users are the application developers’ users; but the application developers themselves are the tool developers’ users. An application is an interactive system produced by a developer. An end user who uses an application will do so by starting a session, exercising the application’s various facilities by providing the input of his choice. Sessions are to applications what objects are to classes: individual instances of a general pattern. This chapter analyzes the requirements of developers who want to provide their end users with useful applications offering graphical interfaces. Graphical systems, window systems, toolkits Many computing platforms offer some tools for building graphical interactive applications. For the graphical part, libraries are available to implement designs such as GKS and PHIGS. For the user interface part, basic window systems (such as the Windows Application Programming Interface, the Xlib API under Unix and the Presentation Manager API under OS/2) are too low-level to make direct use convenient for application developers, but they are complemented by “toolkits”, such as those based on the Motif user interface protocol