Command Design Pattern

The Command pattern is a behavioural design pattern, in which an abstraction exists between an object that invokes a command, and the object that performs it.

E.g., a button will call the Invoker, that will call a pre-registered Command, that the Receiver will perform.

A Concrete Class will delegate a request to a command object, instead of implementing the request directly.

Using a command design pattern allows you to separate concerns and to solve problems of the concerns independently of each other.

E.g., logging the execution of a command and its outcome.

The command pattern is a good solution for implementing UNDO/REDO functionality into your application.

Uses:

• GUI Buttons, menus
• Macro recording
• Multi-level undo/redo
• Networking — send whole command objects across a network, even as a batch
• Parallel processing or thread pools
• Transactional behaviour
• Wizards

Terminology

• Receiver: The object that will receive and execute the command.
• Invoker: The object that sends the command to the receiver. E.g., A button.
• Command Object: Itself, an object, that implements an execute, or action method, and contains all required information to execute it.