MSDN Magazine - June 2008 - (Page 39)

www.radmin.com public OrderProcessingModule() { _handlers = new IOrderHandler[] { new InternationalOrderHandler(), new SmallDomesticOrderHandler(), new LargeDomesticOrderHandler(), new GovernmentOrderHandler(), }; } By following the Open Closed Principle with the order processing rules, I’ve made it much easier to add new types of order handling logic to the system. I was able to add the government order rules with much less risk of destabilizing the other types of orders than I would have faced if a single class had implemented all of the various types of order handling. Double Dispatch What if the steps become more complicated in the future? What if pure polymorphism just isn’t enough to allow for all the possible variations coming up in the future? We can use a pattern called Double Dispatch to pull out the variation into the subclasses in such a way that we don’t break existing interface contracts. For example, let’s say that I am building a composite desktop application that displays one screen at a time in some sort of main panel. Every time I open a new screen in the application I need to a number of things. I might need to change the available menus, check the state of the screens already open, do any number of things to customize the display of the whole screen and, oh yeah, display the new screen in some way. I typically use some variety of the Model View Presenter (MVP) pattern for my desktop client architectures, and I generally use the Application Controller pattern to coordinate the various MVP triads in the application. Given the usage of MVP with an Application Controller (for more information on MVP, see Jean-Paul Boodhoo’s MSDN Magazine Design Patterns column on MVP, available at msdn2.microsoft.com/magazine/cc188690), the screen activation might involve these three basic parts: 1. A Presenter for a single screen. Each Presenter knows everything there is to know about a single specific screen. 2. An ApplicationShell for the main form of the application. The ApplicationShell is responsible for displaying individual views within a Panel or TabControl of some kind in itself. The ApplicationShell will also include all the menus. 3. An ApplicationController to act as the traffic cop of the application. The ApplicationController knows about the ApplicationShell and every Presenter that passes through the application. The ApplicationController governs the screen activation and deactivation lifecycles. If all I need to do is simply show the View in the ApplicationShell on activation the code might look like Figure 3. This is perfectly workable for a simple application, but what if the application becomes more complicated? What if, in the second release, I have new requirements to add menu items to the main shell when some screens are active? And what if I want to start showing additional controls in a new pane along the left edge of the main screen for some views, but not all? I still want the architecture to be pluggable so that I can add new msdnmagazine.com RADMIN 3 remote control software Radmin is a Windows-based program, which allows you to remotely control your computer in a LAN or over the Internet. You can view and control the desktop and applications of the remote computer, and use its keyboard and mouse just as if you were sitting in front of it. Radmin is the fastest and the most secure remote control program due to a broad spectrum of advanced technologies like DirectScreenTransfer™, providing unprecedented speed of work with minimum CPU ® Windows Vista™ compatibility Support for 64-bit Windows OS Remote screen viewing and mouse/keyboard control File transfer with resume support Multiuser text and voice chat Any-screen resolution support Any-colour depth values support Support for multiple connections Wine compatibility (remote control from Linux machines) Mouse wheel support Special keystrokes handling and optional full keyboard control Unlimited phonebook for Radmin connections with explorer-like interface © 2008 Famatech International Corporation. Radmin®, Remote Administrator, DirectScreenTransfer™ are registered trademarks of Famatech International Corporation. June 2008 39 http://www.radmin.com http://msdn2.microsoft.com/magazine/cc188690 http://www.radmin.com http://msdnmagazine.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - June 2008

MSDN Magazine - June 2008 Contents Toolbox CLR Inside Out Cutting Edge Patterns In Practice SAAS Concurrency Robotics Form Filler GUI Library Service Station Foundations Windows With C++ Concurrent Affairs Going Places { End Bracket }

MSDN Magazine - June 2008

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