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pattern

Command

command

/patterns/command/

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Behavioral

Java

Gang Of Four

Difficulty-Intermediate

Functional

Also known as

Action, Transaction

Intent

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

Applicability

Use the Command pattern when you want to

parameterize objects by an action to perform. You can express such parameterization in a procedural language with a callback function, that is, a function that's registered somewhere to be called at a later point. Commands are an object-oriented replacement for callbacks.
specify, queue, and execute requests at different times. A Command object can have a lifetime independent of the original request. If the receiver of a request can be represented in an address space-independent way, then you can transfer a command object for the request to a different process and fulfill the request there
support undo. The Command's execute operation can store state for reversing its effects in the command itself. The Command interface must have an added Unexecute operation that reverses the effects of a previous call to execute. Executed commands are stored in a history list. Unlimited-level undo and redo is achieved by traversing this list backwards and forwards calling unexecute and execute, respectively
support logging changes so that they can be reapplied in case of a system crash. By augmenting the Command interface with load and store operations, you can keep a persistent log of changes. Recovering from a crash involves reloading logged commands from disk and re-executing them with the execute operation
structure a system around high-level operations build on primitive operations. Such a structure is common in information systems that support transactions. A transaction encapsulates a set of changes to data. The Command pattern offers a way to model transactions. Commands have a common interface, letting you invoke all transactions the same way. The pattern also makes it easy to extend the system with new transactions

Typical Use Case

to keep a history of requests
implement callback functionality
implement the undo functionality

Real world examples

Credits

Design Patterns: Elements of Reusable Object-Oriented Software