学步园

Definition

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

UML class diagram

Participants

The classes and/or objects participating in this pattern are: Command  (Command) declares an interface for executing an operation ConcreteCommand  (CalculatorCommand) defines a binding between a Receiver object and an action implements Execute by invoking the corresponding operation(s) on Receiver Client  (CommandApp) creates a ConcreteCommand object and sets its receiver Invoker  (User) asks the command to carry out the request Receiver  (Calculator) knows how to perform the operations associated with carrying out the request. Sample code in C# This structural code demonstrates the Command pattern which stores requests as objects allowing clients to execute or playback the requests. // Command pattern -- Structural example using System; // "Command" abstract class Command {   // Fields   protected Receiver receiver;   // Constructors   public Command( Receiver receiver )   {     this.receiver = receiver;   }   // Methods   abstract public void Execute(); } // "ConcreteCommand" class ConcreteCommand : Command {   // Constructors   public ConcreteCommand( Receiver receiver ) :                               base ( receiver ) {}   // Methods   public override void Execute()   {     receiver.Action();   } } // "Receiver" class Receiver {   // Methods   public void Action()   {     Console.WriteLine("Called Receiver.Action()");   } } // "Invoker" class Invoker {   // Fields   private Command command;   // Methods   public void SetCommand( Command command )   {     this.command = command;   }   public void ExecuteCommand()   {     command.Execute();   } } /// <summary> ///  Client test /// </summary> public class Client {   public static void Main( string[] args )   {     // Create receiver, command, and invoker     Receiver r = new Receiver();     Command c = new ConcreteCommand( r );     Invoker i = new Invoker();     // Set and execute command     i.SetCommand(c);     i.ExecuteCommand();   } } This real-world code demonstrates the Command pattern used in a simple calculator with unlimited number of undo's and redo's. Note that in C#  the word 'operator' is a keyword. Prefixing it with '@' allows using it as an identifier. // Command pattern -- Real World example using System; using System.Collections; // "Command" abstract class Command {   // Methods   abstract public void Execute();   abstract public void UnExecute(); } // "ConcreteCommand" class CalculatorCommand : Command {   // Fields   char @operator;   int operand;   Calculator calculator;   // Constructor   public CalculatorCommand( Calculator calculator,                         char @operator, int operand )   {     this.calculator = calculator;     this.@operator = @operator;     this.operand = operand;   }   // Properties   public char Operator   {     set{ @operator = value; }   }   public int Operand   {     set{ operand = value; }   }   // Methods   override public void Execute()   {     calculator.Operation( @operator, operand );   }     override public void UnExecute()   {      calculator.Operation( Undo( @operator ), operand );   }   // Private helper function   private char Undo( char @operator )   {     char undo = ' ';     switch( @operator )     {       case '+': undo = '-'; break;       case '-': undo = '+'; break;       case '*': undo = '/'; break;       case '/': undo = '*'; break;     }     return undo;   } } // "Receiver" class Calculator {   // Fields   private int total = 0;   // Methods   public void Operation( char @operator, int operand )   {     switch( @operator )     {       case '+': total += operand; break;       case '-': total -= operand; break;       case '*': total *= operand; break;       case '/': total /= operand; break;     }     Console.WriteLine( "Total = {0} (following {1} {2})",                     total, @operator, operand );   } } // "Invoker" class User {   // Fields   private Calculator calculator = new Calculator();   private ArrayList commands = new ArrayList();   private int current = 0;   // Methods   public void Redo( int levels )   {     Console.WriteLine( "---- Redo {0} levels ", levels );     // Perform redo operations     for( int i = 0; i < levels; i++ )       if( current < commands.Count - 1 )         ((Command)commands[ current++ ]).Execute();   }   public void Undo( int levels )   {     Console.WriteLine( "---- Undo {0} levels ", levels );     // Perform undo operations     for( int i = 0; i < levels; i++ )       if( current > 0 )         ((Command)commands[ --current ]).UnExecute();   }   public void Compute( char @operator, int operand )   {     // Create command operation and execute it     Command command = new CalculatorCommand(                 calculator, @operator, operand );     command.Execute();     // Add command to undo list     commands.Add( command );     current++;   } } /// <summary> /// CommandApp test /// </summary> public class CommandApp {   public static void Main( string[] args )   {     // Create user and let her compute     User user = new User();     user.Compute( '+', 100 );     user.Compute( '-', 50 );     user.Compute( '*', 10 );     user.Compute( '/', 2 );     // Undo and then redo some commands     user.Undo( 4 );     user.Redo( 3 );   } }