Command

Command Pattern

Encapsulate a request as a stand-alone object, which can be used to

• parameterize clients with different requests
• queue or log requests
• support undoable operations

Problem

• Coupling the invoker of a request to a particular request should be avoided.

Solution

• Define separate Command objects that encapsulate a request.
• A class delegates a request to a command object instead of implementing a particular request directly.

Common Structure

Source: Refactoring.Guru

• Command
  • declares an interface for executing an operation
• ConcreteCommand
  • defines a binding between a Receiver object and an action.
  • implements Execute by invoking the corresponding operation(s) on Receiver.
• Client
  • creates a ConcreteCommand object and sets its receiver and params.
• Invoker
  • asks the Command to execute the request.
• Receiver
  • knows how to perform the operations. Any class may serve as Receiver.

Collaboration

Source: GoF Design Patterns

• When commands are undoable, ConcreteCommand stores state for undoing the command prior to invoking Execute.

Benefits

• Decouples classes that invoke operations from the one that knows how to perform it.
• Extensibility: Commands are first-class objects. They can be manipulated and extended. It is easy to add new commands.
• Allows assembling simple commands into larger ones --> composite command

Drawbacks

• Increases overall code complexity by creating multiple additional classes.

Example

Definition

Usage

Comparison with other patterns

• Memento can keep state the command requires to undo its effect.

• Composite can be used to implement MacroCommands.

• Prototype can help when you need to save copy of the Command into history.

• Chain of Responsibility passes a request sequentially along a dynamic chain of receivers. Command establishes a one-directional connection from senders to receivers.

• Mediator has senders and receivers reference each other indirectly. Command holds the reference of a receiver only.

• Observer passes a request to all receivers at the same time but allows them to subscribe and unsubscribe from receiving further requests dynamically.