Command

In object-oriented programming, the command pattern is a behavioral design pattern Chain of responsibility, Command, Interpreter, Iterator, Mediator, Memento, Observer, State, Strategy, Template method, Visitor in which an object is used to encapsulate all information needed to perform an action or trigger an event at a later time.

Four terms always associated with the command pattern are command, receiver, invoker and client. A command object knows about the receiver and invokes a method of the receiver. Values for parameters of the receiver method are stored in the command. The receiver object to execute these methods is also stored in the command object by aggregation. The receiver then does the work when the execute() method in command is called. An invoker object knows how to execute a command, and optionally does bookkeeping about the command execution. The invoker does not know anything about a concrete command, it knows only about the command interface. Invoker object(s), command objects and receiver objects are held by a client object, the client decides which receiver objects it assigns to the command objects, and which commands it assigns to the invoker. The client decides which commands to execute at which points. To execute a command, it passes the command object to the invoker object.

Overview

Using the command design pattern can solve these problems:
1. Coupling the invoker of a request to a particular request should be avoided. That is, hard-wired requests should be avoided.
2. It should be possible to configure an object (that invokes a request) with a request.

Implementing (hard-wiring) a request directly into a class is inflexible because it couples the class to a particular request at compile-time, which makes it impossible to specify a request at run-time.

Using the command design pattern describes the following 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.

This enables one to configure a class with a command object that is used to perform a request. The class is no longer coupled to a particular request and has no knowledge of how the request is carried out.