• It encapsulates a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.
• The Command design pattern encapsulates an action or a request as an object.
  • This pattern referred to "Action" or "Transaction" as well.
• The classic usage of this pattern is a menu system where each command object represents an action and an associated undo action.
  • Each of which gets mapped to its own command object.
• All Commands implement the same interface, so they can be handled polymorphically.
  • Typically their interface includes methods such as Execute and Undo.

Participants

• Receiver (Light)
  • Knows how to perform the operations associated with carrying out the request.
• Command (ICommand)
  • Declares an interface for executing an operation.
• ConcreteCommand (FlipUpCommand, FlipDownCommand)
  • Defines a binding between a Receiver object and an action.
  • Implements Execute by invoking the corresponding operation(s) on Receiver.
• Invoker (LightSwitch)
  • Asks the Command to carry out the request.

var light = new Light();
ICommand switchUp = new FlipUpCommand(light);
ICommand switchDown = new FlipDownCommand(light);

var lightSwitch = new LightSwitch();
lightSwitch.StoreAndExecute(switchUp);
lightSwitch.StoreAndExecute(switchDown);

public class Light
{
    public void TurnOn() => Console.WriteLine("The light is on");
    public void TurnOff() => Console.WriteLine("The light is off");
}

public interface ICommand
{
    void Execute();
}

public class LightSwitch
{
    private List<ICommand> _Commands = new List<ICommand>();
    public void StoreAndExecute(ICommand command)
    {
        _Commands.Add(command);
        command.Execute();
    }
}    

public class FlipUpCommand : ICommand
{
    private Light _Light;
    public FlipUpCommand(Light light) => _Light = light;
    public void Execute() => _Light.TurnOn();
}

public class FlipDownCommand : ICommand
{
    private Light _Light;
    public FlipDownCommand(Light light) => _Light = light;
    public void Execute() => _Light.TurnOff();
}

Rules of Thumb

• Chain of Responsibility, Command, Mediator, and Observer, address how you can decouple senders and receivers, but with different trade-offs.
  • Command normally specifies a sender-receiver connection with a subclass.
• Chain of Responsibility can use Command to represent requests as objects.
• Command and Memento act as magic tokens to be passed around and invoked at a later time.
  • In Command, the token represents a request.
  • In Memento, it represents the internal state of an object at a particular time.
  • Polymorphism is important to Command, but not to Memento because its interface is so narrow that a memento can only be passed as a value.
• Command can use Memento to maintain the state required for an undo operation.
• MacroCommands can be implemented with Composite.
• A Command that must be copied before being placed on a history list acts as a Prototype.
• Two important aspects of the Command pattern:
  • Interface separation (the invoker is isolated from the receiver).
  • Time separation (stores a ready-to-go processing request that’s to be stated later).

Composite command pattern