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Command Design Pattern

Command Design Pattern. Source: Design Patterns – Elements of Reusable Object-Oriented Software; Gamma, et. al. Problem. Sometimes a class needs to perform actions without knowing what the actions are. Example: GUI Toolkit. A GUI toolkit provides a full complement of components

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Command Design Pattern

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  1. Command Design Pattern Source: Design Patterns – Elements of Reusable Object-Oriented Software; Gamma, et. al.

  2. Problem • Sometimes a class needs to perform actions without knowing what the actions are

  3. Example: GUI Toolkit • A GUI toolkit provides a full complement of components • Buttons, scroll bars, text boxes, menus, etc. • Toolkit components know how to draw themselves on the screen, but they don't know how to perform application logic • Application developers need a way to associate application logic with GUI components • What should happen when a button is pressed? • What should happen when a menu item is selected? • GUI components need to perform an unknown operation when they are activated by the user

  4. Solution

  5. Solution

  6. Consequences • Completely decouples objects from the actions they execute • Objects can be parameterized with arbitrary actions • Adding new kinds of actions is easy • Just create a new class that implements the Command interface

  7. Known Uses: Task Scheduling Service • Generic task scheduling service allow tasks to be scheduled by • Interval (e.g., every hour) • Date/time (e.g., on February 8th at 1:00pm) • Etc. • Tasks are defined using Command objects • Scheduling service executes Command object at the appropriate time, having no idea what it actually does

  8. Known Uses: Undo/Redo • Store a list of actions performed by the user • Each action has • A “do” method that knows how to perform the action • An “undo” method that knows how to reverse the action • Store a pointer to the most recent action performed by the user • Undo – “undo” the current action and back up the pointer • Redo – move the pointer forward and “redo” the current action

  9. Known Uses: Recording user actions • Macros • Record a sequence of user actions so they can be turned into a macro • Macro can be re-executed on demand by the user

  10. Known Uses: Multi-user network games • Actions performed by a player must be broadcast to all other players and executed on their local machines • How could we use the Command pattern to do this?

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