1. Computer Science 313 – Advanced Programming Topics

2. Strategy Pattern  Define interface for family of algorithms  Encapsulate algorithm in class of its own  Algorithm instance held in field of Context class  Delegate responsibility for action to this object  Set field to change algorithm dynamically

3. Limits To Strategy  Several limitations to strategy pattern  Details of Strategy must be known by Context  Context must execute actual Strategy  Strategy should not maintain state  Cannot determine past Strategy s

4. Limits To Strategy  Several limitations to strategy pattern  Details of Strategy must be known by Context  Context must execute actual Strategy  Strategy should not maintain state  Cannot determine past Strategy s

5. Why Is This A Problem?  Unable to log steps taken during program  But a Strategy is not a tangible object  Cannot undo a Strategy once executed  To enable “undo” would need log of actions  But Strategy lacks state to save actual results  Strategy independent of Context  Cannot use external objects when executed  Specific context cannot host a Strategy

6. Command Pattern  Encapsulates single action as an object  Each instance can be used one or more times  Define range of actions in abstract Command  But pattern has one way to execute Command  Adjust the state of a Command using fields  Modify how it executes and what it does  Use to specify which instance Command uses  More than parameters to control exposed method

7. Command Pattern Intent  Encapsulate single request within an instance  Client code can then pass requests as parameter  Pattern treats Command as normal object  Queue Command s to execute in certain order  Once executed, record Command s in log

8. Command Pattern Usage  Command s support undo & redo options  Additional method in Command for undo support  Use Command ’s state to record what it did  Stack holds Command s once executed  Actions then are un-done by popping Command s

9. Command Pattern Intent  Split execution from performance  Pattern creates one class executes Command s  Command can select actor who performs it

10. Decoupling  Command pattern decouples (splits) What we want to do Who does it When it is done

11. Decoupling  Command pattern decouples (splits) What we want to do Who does it When it is done Wish I knew about the Command Pattern

12. For Next Class