Command routing in C#

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10.5 Command routing
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Arguably, the most interesting and powerful concept of the WPF Command implementation is the command routing process. Command routing enables at least two important things:
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The UI element that triggers a process is completely separated from the objects that perform the actions. This enables much more loosely coupled designs. Elements and classes can enlist in command processing without explicitly knowing how or whom to pass commands on to. UI controls can respond to commands visually to give additional cues to the user about what happens. For example, a window could flash after a snapshot command is executed to indicate to the user that a capture took place. Used sparingly, these kinds of cues can greatly enhance understanding of what the application is doing for the user.
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Microsoft has obviously spent a lot of time thinking through their approach to command handling, and it s incredibly flexible. Although, as always, we have some complaints (which we ll discuss later), overall we re pretty happy with the approach. The next section goes into the implementation in some detail. The following three major types represent commands in WPF:
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ICommand RoutedCommand RoutedUICommand
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ICommand is the base interface for all commands. Technically, ICommand is generic enough that it can be used regardless of whether you re using WPF or not, and is the interface you d extend to support your own non-UI related back-end commanding systems (for example, if you wanted your web services to use commands as well). The ICommand interface follows the traditional GoF pattern closely with an Execute method, a CanExecute method, and also a CanExecuteChanged event.
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Commands
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Notably absent is the Undo method. A bit disappointing To have commands without building in Undo is like getting one of those RC cars that only turns in reverse. Anyhow, this almost immediately calls for an IUndoableCommand interface, or an Undo command. We suppose it wouldn t be any fun if Microsoft solved everything for us. Interestingly, some of the controls support undo internally, but don t expose any standard mechanism for dealing with it. The RoutedCommand class could be called the WPF implementation of ICommand. It s called RoutedCommand because this class is designed to support the WPF framework RoutedEvents, as well as the tunneling and bubbling of commands throughout the visual tree. Anything that has CommandBindings (which happens to be every WPF class derived from UIElement and ContentElement) is a potential direct or indirect command receiver of these types of commands. Most of the time, when we re talking about commands, we re referring to RoutedCommands. RoutedUICommand is derived from RoutedCommand, and adds a Text property that s useful for presenting localized text for display in a UI. Every command defined in WPF has its Text property set, so the built-in commands even come with their own labels. Obviously from a localization standpoint, this is a real time-saver and just kinda nice to have. As far as structure goes, WPF commands aren t quite singletons. By definition, a singleton prevents more than a single instance of a particular class from being instantiated. WPF commands are all instances of RoutedUICommand,5 and we can create instances of those classes willy-nilly. The approach for ensuring a single Copy command, for example, is to create a well-known static instance library class (ApplicationCommands, in this case) and put a property on it for a Copy command instance. As long as everyone agrees to call ApplicationCommands.Copy, they ll all be sure to be dealing with the same command. Likewise, with our About command, all code that wants to use it must refer to the instance in the WikiWindow class. This isn t ideal (in fact, it s downright ugly). In our next example, we ll provide a much cleaner implementation of a custom command.
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