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Command Pattern

This very simple little pattern can be used in everything from websites implementing complicated features like undo, to implementing very simple features like command line parsing in a console application.

Intent and Applicability

• Represent an action as an object

• Decouple clients that execute the command from the details and dependencies of the command logic

• Enables delayed execution

  • Can queue commands for later execution
  • If command objects are also persistent, can delay across process restarts

The intent of this pattern is really simple, we want to represent a piece of business logic or just a piece of logic in general, some action, as an object that implements an interface.

Also Knows As

• Action, Transaction You may have heard this command pattern referred to as Action pattern or Transactions those are also very common names for these things.

Consequences

• commands must be completely self contained
  • The client doesn't pass in any arguments
• Easy to add new commands
  • Just add a new class (open/closed principal)

Related Patterns

• Factory Pattern
  • Factories are often useful to construct command objects
• Null Object
  • Often times returning a "null command" can be useful instead of returning null
• Composite
  • A composite command can be useful
  • Construct it with several "child" commands
  • Execute() on the composite will call Execute on the child commands