Design of Web Components --- an example

As an illustration of our approach, we will discuss the design of the Web-related components  [WWW95b] of our framework. The design has been influenced by the following requirements:

• Communication protocols -- The dominant communication protocol of the Web is currently HTTP. New or enhanced communication  [Chatting] and network transfer protocols will be needed.
• Client-side computing -- With the announcement of Java and the realization of the HotJava  [Java] browser, client-side computing by means of high-level scripting languages has been accepted as a powerful and flexible means to enhance the dynamic capabilities of Web-browsers.
• Document formats -- While HTML is well suited as a general purpose mark-up language, more domain specific and more powerful document formats will be needed, especially when more sophisticated multimedia structures have to be encoded.

The Web Components

Figure 1 shows the design of the basic Web-related components of the hush class library. The browser provides the toplevel user interface for all Web components, including a viewer, scrollbars, navigation buttons (back, forward, home, reload) and an entry box to enter URLs. The client, web and viewer components form the conceptual base of our approach of viewing the Web. Each component can be extended in a simple way, due to a systematic application of the idioms discussed previously.

Multiple viewers are accessible via the MIMEviewer which employs the actor pattern to be able to display various types of MIME-encoded data (e.g. HTML, SGML combined with Tcl style sheets, VRML). The viewer contains a dictionary of several viewer actors, that is indexed on the associated MIME-types. In this way, new viewers can be easily added, even dynamically. The viewer implementation classes can make use of existing widgets to display parts of structured documents, which simplifies their implementation. Another advantage of this design is the fact that whenever a viewer for a new MIME type has been added, it is possible to inline this data type in an SGML or extended-HTML document, via recursive usage of a viewer widget in another viewer. In other words, it is possible to inline VRML or video into an HTML document, or even to insert an HTML-page into another HTML-page. The SGML-viewer uses an SGML-compliant  [SGML] validating parser  [SP] and an (experimental) style sheet mechanism based on the Tcl scripting language. This approach allows for experimenting with new (multimedia) document formats. Since the viewer provides no network functionality at all, it generates an event whenever it needs to retrieve data pointed to by a URL. Such events are generated as a response to user interaction (e.g. clicking an anchor) or to fetch inline data during the parsing process. These events (realized by the hidden urlevent class) are typically handled by a web component.

The client component provides an abstraction of network (file) access and transport protocols. An important consideration in the design of this component was the requirement to allow for new network transfer protocols. This requirement has been met by employing the actor pattern. The client component implicitly changes its role, depending on the protocol required for fetching the data. When the client component gets the instruction to retrieve a URL, it inspects the URL, decides what role it needs and switches to the appropriate role.

The web component combines the functionality of the viewer and client components. It is able to follow links by retrieving data via several protocols (e.g. HTTP, files, FTP), a task delegated to its client component. Additionally, the web component adds a history and caching mechanism to the MIMEviewer. The web widget uses the nested component pattern described in section 3. This means that web inherits the interface of the MIMEviewer and delegates its (viewer) functionality to a inner viewer component. The web component's behavior is similar to the standard widgets of the framework, and can be conveniently used as a part of an application's GUI. Because the web widget has both a C++ interface and a Tcl interface, it is easy to create, or extend, applications with Web functionality.