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The design of the Command pattern is shown in Figure 8-6. The Client has a certain way of saying what is required, usually in high-level and domain-specific terms. It thinks in terms of commands such as Cut, Redo, Open, and so on. The Receivers and there may be several know how to carry out these requests. Referring to the menu example, a Cut command for text would go to one part of a system and a Cut command for an image would be handled elsewhere.
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8: Behavioral Patterns: Chain of Responsibility and Command
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Invoker +Execute( )
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ICommand +Execute( )
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Receiver +Action( )
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Calls receiver.Action( )
The Command class forms the interface between the Client and the Receivers. In a Command object, the Client s requests are declared and associated with viable corresponding operations in a Receiver. There might also be a state that the Client wants to pass on to the Receiver, and this must be allowed for. The Invoker is there to distance the Client from the Receiver. Put simply, the Client issues a call to Execute in the Invoker, and the request goes to the Command and then to Action in the Receiver. In a program, there could be many requests of different types being routed to different Receivers. The ICommand interface ensures that they all conform to a standard form. The players in this pattern are:
Client
Creates and executes Commands
ICommand
An interface that specifies the Execute operation
Invoker
Asks the Command to carry out the Action
Command
A class that implements the Execute operation by invoking operations on the
Receiver Receiver
Any class that can perform the required Action
Action
The operation that needs to be performed The Command pattern does seem to have many players, but some of them fall away when delegates are used, as shown in the next section. Other aspects of the design are: Commands can be assembled into composite commands in the Command class. New commands can be added without disturbing existing ones.
Command Pattern |
QUIZ
Match the Command Pattern Players with the Cut Request Illustration
To test whether you understand the Command pattern, cover the lefthand column of the table below and see if you can identify its players among the items from the illustrative example (Figure 8-5), as shown in the righthand column. Then check your answers against the lefthand column.
Client ICommand Command Receiver Invoker Action
User of the editor, who selects a Cut command on a menu Menu Holder of a Cut request An object that can perform a Cut Process the selection of a Cut option Cut
Implementation
The theory code for implementing a simple version of the Command pattern is shown in Example 8-3.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 using System; class CommandPattern { // Command Pattern Judith Bishop June 2007 // Uses a single delegate for the single type of commands that // the client invokes delegate void Invoker ( ); static Invoker Execute, Undo, Redo; class Command { public Command(Receiver receiver) { Execute = receiver.Action; Redo = receiver.Action; Undo = receiver.Reverse; } } public class Receiver { string build, oldbuild; string s = "some string ";
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8: Behavioral Patterns: Chain of Responsibility and Command
24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 public void Action( ) { oldbuild = build; build +=s; Console.WriteLine("Receiver is adding "+build); } public void Reverse( ) { build = oldbuild; Console.WriteLine("Receiver is reverting to "+build); } } static void Main( ) { new Command (new Receiver( )); Execute( ); Redo( ); Undo( ); Execute( ); } } /* Output Receiver is Receiver is Receiver is Receiver is */
adding some string adding some string some string reverting to some string adding some string some string
The Invoker and ICommand interface are implemented together as a delegate type (line 9) with its instantiated delegate objects (line 10). The names of the delegate objects (the Invokers) are based on what the client wants; their names are Execute, Redo, and Undo (line 10). The Command associates the delegate command objects with the two methods inside the Receiver, Action, and Reverse (lines 14 16). Execute and Redo both go to Action, and Undo goes to Reverse. The Receiver keeps track of the state and is responsible for output. This arrangement is unlike previous patterns, where the receivers returned values from their fields for the client to display or not. To adopt the same mechanism, we would make the delegate type return a string, as in:
delegate string CommandStr ( );
This implementation is vastly shorter than many of the standard ones because it uses a delegate type (line 9) and instantiates it for three delegate objects in one line (line 10). The object-based solution would have three separate single-method classes for the three invokers.
The actual Undo/Redo implementations depend on actual operations to be undone or redone. In the Command pattern, Undo and Redo serve only as abstract placeholders.
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