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8: Behavioral Patterns: Chain of Responsibility and Command
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public void Print( ) { Console.WriteLine( "File "+name+" at "+DateTime.Now+"\n"+page); } } static void Main( ) { Document document = new Document("Greetings"); Paste paste = new Paste(document); Print print = new Print(document); clipboard = "Hello, everyone"; paste.Execute( ); print.Execute( ); paste.Undo( ); clipboard = "Bonjour, mes amis"; paste.Execute( ); clipboard = "Guten morgen, meine Freunde"; paste.Redo( ); print.Execute( ); print.Undo( ); Console.WriteLine("Logged "+paste.Execute.Count( )+" commands"); } }
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/* Output File Greetings at 2007/09/21 01:40:52 AM Hello, everyone File Greetings at 2007/09/21 01:40:52 AM Bonjour, mes amis Guten morgen, meine Freunde Cannot undo a Print Logged 7 commands */
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The Command pattern can be applied wherever domain-specific commands are required and the tool has system-specific operations. Once the separation has been made between the two, extra functionality can be added at the command level.
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Use the Command pattern when You have: Commands that different receivers can handle in different ways A high-level set of commands that are implemented by primitive operations You want to: Specify, queue, and execute commands at different times Support an Undo function for commands Support auditing and logging of all changes via commands
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1. In the simple theory code in Example 8-3, introduce a second delegate type for an Invoker that handles commands with a string parameter. Integrate this into the Command class and observe what other changes are required to make the program run using suitable test calls from Main. 2. In Example 8-4, all of the void Invoker objects are set up by one Command object and go to one Receiver, while all of the string Invoker objects are set up by a second Command object and go to a different Receiver. Investigate how to get a mix of Invoker objects to be handled by each Command object and to go to different Receivers. Report on any limitations you encounter. 3. The Photo Library example for the Composite pattern in 3, as extended with the Prototype pattern in 5 for the Photo Archive application, has a fairly complex set of commands. Consider an auxiliary program called UpLoad that just takes a set of photos out of a file and creates it as a set in the library. Implement this as a command that calls other commands. Also include an Undo function, and add a logging function that will report on how many photos were uploaded.
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8: Behavioral Patterns: Chain of Responsibility and Command
Pattern Comparison
A similarity between the Chain of Responsibility and the Command patterns is that they decouple senders and receivers, thus improving the layering and reusability of a system. The Chain of Responsibility pattern supports decoupling by passing a request between potential receivers, whereas the Command pattern supports using a command object to encapsulate a request. The implementation issues are described in Table 8-1.
Table 8-1. Comparison of the Chain of Responsibility and Command patterns Chain of Responsibility Client creates Different kinds of Client can work with Client calls Work is done in Decisions based on Deals with unimplementable requests Handler objects Handler classes at different levels Multiple handlers Handler objects HandleRequest, in a handler Limits in handlers Yes Command Command objects Command classes and Receiver classes Different receivers Receiver objects Action, in a receiver Routing in commands Yes
Pattern Comparison |
9 9 CHAPTER
Behavioral Patterns: Iterator, Mediator, and Observer
The three behavioral patterns we will study in this chapter support communication between objects while letting them keep their independence and, in some cases, their anonymity. The Iterator pattern is a fundamental pattern that has wide application in many situations and is substantially supported by C# 3.0 s LINQ or Linq (Language INtegrated Query) extensions. The Mediator and Observer patterns mirror the mailbox and publish/subscribe methodologies that are well-known in distributed systems.
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