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Command.md

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Command

Warning YOU SHALL NOT PASS!

An object which represents an instruction to perform a particular action. Contains all the information necessary for the action to be taken.

Intent

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

Explanation

The Command design pattern encapsulates a request as an object, thereby letting you parameterize other objects with different requests, queue or log requests, and support undoable operations. It involves three components:

  1. Command: Declares an interface for executing an operation.
  2. Concrete Command: Defines a binding between a Receiver object and an action.
  3. Invoker: Asks the command to carry out the request.

This pattern decouples the sender (Invoker) and receiver (Receiver) of a request, promoting loose coupling and flexibility.

Motivation to use

  • Ordinary C# statements are perishable
    • Cannot undo a field/property assignment
    • Cannot directly serialize a sequence of actions (calls)
  • Want an object that represents an operation
    • X should change its property Y to Z
    • X should do W()
  • Uses: GUI commands, multi-level undo/redo, macro recording and more!

Summary from the course

  • Encapsulate all details of an operation in a separate object
  • Define instruction for applying the command (either in the command itself, or elsewhere)
  • Optionally define instructions for undoing the command
  • Can create composite commands (a.k.a. macros)