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Mediator

Mediator. Matt G. Ellis. Overview. Intent Motivation Mediators in GUI applications Mediators and Relational Integrity Conclusion Questions. Intent.

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Mediator

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  1. Mediator Matt G. Ellis

  2. Overview • Intent • Motivation • Mediators in GUI applications • Mediators and Relational Integrity • Conclusion • Questions

  3. Intent • Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you very their interaction independently

  4. Motivation • Object Oriented design encourages distribution of behavior among objects • However, Good design is thwarted by every object referencing every other object • Changing systems behavior becomes difficult • Helps to prevent classes from becoming “thick”

  5. Mediator versus Façade • Façade pattern help refractor FlightPanel from Oozinoz • Refactoring can only go so far, complex applications still might need complex code even after applying Façade pattern

  6. Mediators at Oozinoz • Chemicals for fireworks kept in tubs • Robots move most of the tubs from machine to machine • However, humans can override the system

  7. FlightPanel_1 • Many methods exist to lazy-initialize variables • Rest control event handling logic

  8. Challenge 1 • Refactor PlaceATub_1 into two classes, introducing a new PlaceATubMediator that receives the events of the GUI

  9. Challenge 1 • Refactor PlaceATub_1 into two classes, introducing a new PlaceATubMediator that receives the events of the GUI

  10. Relational Integrity • If Object A points to Object B then… • Object B points to Object A • A more rigorous definition can be found in Metsker, page 108

  11. Relational Integrity and Java • Two Major Problems • Objects forget previous values • No built in support for Relational Integrity

  12. Model

  13. Challenge 2 • Suppose we have this code: //tell tub about machine, and machine about tub t.setMachine(m); m.addTub(t); • What happens when t is tub T308 and m is Fuser-2101?

  14. Challenge 2

  15. Challenge 2

  16. Challenge 2

  17. Challenge 2 • Really Bad Things… • Two machines think they have tub T308 in them • This can’t happen in the real world, why should it happen at Oozinoz? • Mediators can help

  18. Mediators for Relational Integrity • Pull all relational information into a mediator outside both classes • Have both tubs and machines have a reference to this mediator • Use a Map to store these key/value pairs

  19. Mediators for Relational Integrity • getMachine is simple, since t is the key of the map, HashMap makes it easy to get the value.

  20. Mediators for Relational Integrity • Somewhat more complex, but the intent is the same.

  21. Mediators for Relational Integrity • The most trivial method of all. Relational Integrity is maintained by the internal structure of the Map

  22. Challenge 3 • Write the code for the Tub methods: getMachine() and setMachine()

  23. Conclusions • Mediators provide loose coupling creating a “pluggable” system • Changing a mediator can change how applications deal with events • Mediators often found in GUIs • Swing’s event framework nudges the use of mediators, but they can be in the same class • Mediators also help to provide relational integrity between objects

  24. Questions

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