Dr. Dobb's Journal - July 2008 - (Page 26)

Core Technology EVENT-BASED ARCHITECTURES The larger the system is, the better off you are with an EBA Figure 4: The wiring diagram of SystemBrowser. intermediate-level classes and low-level classes. In GUI programs, the top-level class is often the main window. Intermediatelevel classes tend to be those that handle important functionality. Low-level classes generally are domain-specific types used by and exchanged between intermediate-level classes. For example, in a word-processing program, a SpellingChecker class might be an intermediate one and a Paragraph class might be a low-level one. If you diagrammed the instantiation sequence of a traditional OO system, you would generally end up with a single-rooted directed graph whose root is the top-level class; see Figure 5. The graph implicitly shows coupling, because a class that contains code to instantiate another class is coupled to that class. The top-level class is hence directly or indirectly coupled to all the other classes in the system. The intermediate-level classes are directly or indirectly coupled to many other intermediate-level classes. In EBAs, much of this coupling is avoided by using a special type of part called a Builder (not to be confused with the Builder OO design pattern). An EBA Builder is called by the program’s entry point and has one job—to create instances of all the intermediate-level classes. 26 Dr. Dobb’s Journal l www.ddj.com l July 2008 Figure 5: The instantiation graph of a traditional object-oriented system. Intermediate-level classes are no longer responsible for instantiating other intermediate-level classes. Intermediate-level classes communicate with each other exclusively using event notifications. When the notifications use untyped calls, there is no type coupling between the classes. The coupling diagram of this kind of system has a star shape, with all the low-level classes in the middle and all the intermediate-level classes at the periphery; see Figure 6. The intermediate-level classes at the periphery of the diagram are not coupled to each other. For these classes to interact, notification paths between them must exist. These paths are created using another special type of part called a Binder. At system startup time, after the Builder has created all the intermediate parts, the Binder takes over and wires them together. When the Binder is finished, the system is ready to perform its intended functions. Both the Builder and Binder have access to all the intermediate classes of the system and hence they are coupled to those classes. The coupling diagram for the Builder and Binder system is star-based, but this time the Builder and Binder are in the center and the coupling arrows are directed outward, towards the intermediate-level classes. SystemBrowser: An Event-Based System SystemBrowser is a small, event-based program I wrote that works somewhat like Windows Explorer. The program has a GUI with a main window divided into sections: a top toolbar, a bottom status bar, and a middle section with left and right panes. The left pane shows a directory tree, the right the files and subdirectories in the selected directory; see Figure 7. Figure 4 is the complete wiring diagram for SystemBrowser. FormMenuToolbar contains a menu and toolbar, which are created by the Builder, then attached to the main http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - July 2008

Dr. Dobb's Journal - July 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Developer’s Notebook Engineers Without Borders Conversations Patricia Tries Event-Based Architectures Graphs Versus Objects Lock-Free Queues Dr. Dobb’s Architecture & Design World Java and the Nokia N10 Internet Tablet Effective Concurrency The Agile Edge Swaine’s Flames

Dr. Dobb's Journal - July 2008

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