Command

Intent

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

Structure

Participants

• Command - declares an interface for executing an operation.
• ConcreteCommand - implements the Command interface, binding an action to a receiver.
• Client - creates a ConcreteCommand and sets its receiver.
• Invoker - invokes the Command interface.
• Receiver - provides operations supporting the implementation of a ConcreteCommand.

Notes

One of the interesting aspects of the Command design pattern is that it serves several purposes. This results in higher-level design patterns - patterns where the Command design pattern is combined in predictable ways with other design patterns.

Commands as User Actions

When the Command design pattern is used to support user actions in a graphical user interface, some of the Command participants also are participants in the Observer design pattern.

• Command:Invoker is an Observer:ConcreteSubject - a menu item, toolbar button, or other user interface object which can register listeners.
• Command:Command is an Observer:Observer.
• Command:ConcreteCommand is an Observer:ConcreteObserver.

Complex Commands

A complex command can be broken down into a sequence of simpler commands. The cleanest way of supporting this decomposition is to use the Composite design pattern.

• Command:Command is a Composite:Component.
• Command:ConcreteCommand is a Composite:Leaf for simple commands or a Composite:Composite for complex commands.

Undoable Operations

A command may be undoable. This often involves the Memento design pattern.

• Command:ConcreteCommand is a Memento:Caretaker.
• Command:Receiver is a Memento:Originator.

Examples

The Java javax.swing and javax.swing.text packages contain several classes that jointly illustrate the Command design pattern.

• Command - javax.swing.text.TextAction (also an Observer:Observer)
• ConcreteCommand - javax.swing.text.DefaultEditorKit.PasteAction (also an Observer:ConcreteObserver)
• Client - javax.swing.text.EditorKit and its subclasses javax.swing.text.DefaultEditorKit and javax.swing.text.StyledEditorKit
• Invoker - The TextAction class implements the ActionListener interface. This allows buttons and menu items to act as Invokers. A JEditorPane also implements a mechanism for triggering a TextAction in response to user keyboard actions. This supports keyboard shortcuts. These are all also Observer:ConcreteObservers.
• Receiver -
  • javax.swing.text.Document is the target of editing changes.
  • javax.swing.JEditorPane is used to locate the cursor within a Document.