free code 39 barcode generator c# : Menu Handler in C#

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Example: Menu Handler
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Consider a very small part of a menu, with just Paste and Print commands. Assume there is a public clipboard, where the client can place strings. The pasting and printing is done by a receiver, which maintains the current copy of the document. On top of that, there are Undo and Redo commands that might or might not be appropriate (for example, undoing a Print operation does not make sense). There is only one invoker type, but the Paste and Print command classes will implement the Execute, Undo, and Redo invoker objects differently. Therefore, we place them in an abstract class that the commands can inherit. This ensures uniformity for the commands; it is also used in logging, which we will discuss momentarily. In this example, the commands each have their own delegate objects, so one of them looks like this:
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class Paste { public Invoker Execute, Redo, Undo; public Paste(Document document) { Execute = document.Paste; Redo = document.Paste; Undo = document.Restore; } }
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When the Client calls the commands, the object name is required, as in:
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8: Behavioral Patterns: Chain of Responsibility and Command
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Paste paste = new Paste(document); Print print = new Print(document); clipboard = "Hello, everyone"; paste.Execute( ); print.Execute( ); paste.Undo( );
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The rest of the program including the Receiver that handles the Document mirrors the simple Command pattern example. Because command objects are separate from Clients and Receivers, they can undertake system functions such as logging. A simple logging operation is to count how many times commands are called a function of the Invoker type. To implement the logging, we define an extension method on the Invoker delegate type to provide: A Log method that increments a count A Count method that returns the current count value This InvokerExtensions class is shown at the start of the program in Example 8-5.
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Recall that extension methods: Must be static Must be in static classes at the outer level Can only have static fields Can be applied to delegates Details are in 2.
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At the end of the program, we can call the Count method to print out the number of calls, as in:
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Console.WriteLine("Logged "+paste.Execute.Count( )+" commands");
Because the count field is static, the count applies to all invoker calls (thus the reference to the instance paste). Execute is a way of getting at this total count. Any invoker instance would return the same value (7). More sophisticated logging methods are explored in the upcoming Exercises section. The full menu handler program is shown in Example 8-5.
using System; using System.Collections.Generic; // Command Pattern Judith Bishop // Example: simple Paste and Print system // with Undo and logging delegate void Invoker ( ) ; Jan, July 2007
Command Pattern |
static class InvokerExtensions { static int count; public static int Count (this Invoker invoker) { return count; } public static void Log (this Invoker invoker) { count++; } } class CommandMenu { abstract class ICommand { public Invoker Execute, Redo, Undo; } // Command 1 class Paste : ICommand { public Paste(Document document) { Execute = delegate {Execute.Log( ); document.Paste( );}; Redo = delegate {Redo.Log( ); document.Paste( );}; Undo = delegate {Undo.Log( ); document.Restore( );}; } } // Command 2 - without an Undo method class Print : ICommand { public Print(Document document) { Execute = delegate {Redo.Log( ); document.Print( );}; Redo = delegate {Redo.Log( ); document.Print( );}; Undo = delegate{ Redo.Log( ); Console.WriteLine("Cannot undo a Print ");}; } } // Common state static string clipboard; // Receiver class Document { string name; string oldpage, page; public Document (string name) { this.name = name; } public void Paste( ) { oldpage = page; page += clipboard+"\n"; } public void Restore( ) { page = oldpage; }
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