Command

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

• Behavioral Pattern
• AKA: Action, Transaction

Command: Motivation

Paste Command

Open Command

Macro Command

Command: Applicability

• Use the command pattern when you want to:
• Parameterize objects by an action to perform, as MenuItem objects did above.
• Commands are an object-oriented replacement for callbacks.
• Specify, queue, and execute requests at different times

Command: Applicability

• Support undo
• execute() stores state for reversal
• unexecute() reverses the effects of a previous execute()
• Executed commands are stored in a history list, with unlimited undo and redo
• Support logging changes to reapply in case of a system crash
• Structure a system around high-level operations built on primitive operations and support transactions

Command: Structure

Command: Participants

• Command - Declares an interface for executing an operation.
• ConcreteCommand (e.g., PasteCommand, OpenCommand)
• Defines a binding between a Receiver object and an action
• Implements execute() by invoking the corresponding operation(s) on Receiver

Command: Participants

• Client (e.g., Application) Creates a ConcreteCommand object and sets its receiver
• Invoker (e.g., Menultem) Asks the command to carry out the request
• Receiver (e.g., Document, Application) Knows how to perform the operations associated with carrying out a request

Command: Collaborations

• The client creates a ConcreteCommand object and specifies its receiver
• An Invoker object stores the ConcreteCommand object.
• The invoker issues a request by calling execute() on the command. When commands are undoable, ConcreteCommand stores the state for undoing the command before invoking Execute.
• The ConcreteCommand object invokes operations on its receiver to carry out the request.

Flow of Control

Command: Consequences

• Command decouples the object that invokes the operation from the one that knows how to perform it
• Commands are first-class objects and can be manipulated and extended like any other object.
• Multiple commands can be assembled into a composite command (e.g., MacroCommand) and are instances of the Composite pattern
• It is easy to add new Commands because you don’t have to change existing classes.

Implementation

• How intelligent should a command be?
• Supporting undo and redo
• Avoiding error accumulation in the undo
• Using C++ templates to avoid creating Command subclasses

Related Patterns

• Composite
• Used to implement MacroCommands
• Memento
• Keep state the command requires for an undo
• Prototype
• Command is copied before being placed on a history list and acts as a Prototype