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Mediator Pattern and Multiuser Protection

Mediator Pattern and Multiuser Protection. Billy Bennett June 8 th , 2009. Mediator. Applicability. A set of objects communicates in well-defined, but complex ways. Often with unstructured dependencies.

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Mediator Pattern and Multiuser Protection

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  1. Mediator Pattern and Multiuser Protection Billy Bennett June 8th, 2009

  2. Mediator

  3. Applicability • A set of objects communicates in well-defined, but complex ways. Often with unstructured dependencies. • Reusing objects is difficult because it refers to and communicates with many other objects. • A behavior that’s distributed between several classes should be customizable without a lot of subclassing.

  4. Participants • Mediator • ConcreteMediator • Colleague • ConcreteColleague

  5. Structure

  6. Structure

  7. Consequences • Limits subclassing • Localizes behavior that would be otherwise distributed among many objects • Changes in behavior require changing only the Mediator class

  8. Consequences • Decouples colleagues • Colleagues become more reusable. • You can have multiple types of interactions between colleagues, and you don’t need to subclass or otherwise change the colleague class to do that.

  9. Consequences • Simplifies object protocols • Many-to-many interactions replaced with one-to-many interactions • More intuitive • More extensible • Easier to maintain

  10. Consequences • Abstracts object cooperation • Mediation becomes an object itself • Interaction and individual behaviors are separate concepts that are encapsulated in separate objects

  11. Consequences • Centralizes control • Mediator can become very complex • With more complex interactions, extensibility and maintenance may become more difficult

  12. Sample Code Void Widget::Changed () { _director->WidgetChanged(this); } class DialogDirector { public: virtual ~DialogDirector (); virtual void ShowDialog (); virtual void WidgetChanged (Widget*) = 0; protected: DialogDirector (); virtual void CreateWidgets () = 0; }; class Widget { public: Widget (DialogDirector*); virtual void Changed (); virtual void HandleMouse (MouseEvent& event); //. . . private: DialogDirector* _director; };

  13. Implementation Issues • Omitting the abstract Mediator class – possible when only one mediator exists • Strategies for Colleague-Mediator communication • Observer class • Pointer / other identifier to “self” passed from colleague to mediator, who pass it to the other colleague(s)

  14. Related Patterns • Façade • Unidirectional rather than cooperative interactions between object and subsystem • Observer • May be used as a means of communication between Colleagues and the Mediator

  15. Other thoughts • Which strategy (or alternative pattern) should we use when the Mediator becomes too monolithic?

  16. Multiuser Protection

  17. Example from UNIX • Each node in the file system has a “user” • Everyone else is “other” • Different actions can be assigned to “user” or to “user and other” (read, write, etc.) • User defined by a unique login / password

  18. Concrete • Need security • Use encapsulation • Data should be inaccessible to the client • Actually everything must be inaccessible to the client

  19. Which Pattern? • We’re creating an object, so probably a creational pattern • Abstract factory? • No – we don’t need families of objects and we’re fine creating an abstract class • Builder? • No – not an overly complex object • Factory method? • No – similar to Abstract Factory

  20. Which Pattern? • Prototype? • No – addresses what, not how • Singleton? • Ahhhhhhh – pattern designed to return a single instance of an object. • NOTE: Here, we’re not worried about one instance of object total, but rather per user • So, we can just change Instance (); static const User* User::logIn ( const string& loginName, const string& password);

  21. Next Problem • What about groups (a la UNIX)? • Composite? • No – groups are not actually a hierarchy (no groups of groups and users may belong > 1 group) • Mediator? • Yes – good for centralizing, objects don’t need to know about each other (no wasted resources), good security

  22. Wrapping Up

  23. Visitor • Composite:Component • Proxy: Subject • TemplateMethod:AbstractClass • Visitor:Element ConcreteVisitor • Leaf • Proxy • TemplateMethod:ConcreteClass • Composite • Proxy:RealSubject • TemplateMethod:ConcreteClass • Leaf • Proxy:RealSubject • TemplateMethod:ConcreteClass • Mediator:ConcreteColleague • Singleton (variant) • ConcreteMediator • Singleton • Mediator:ConcreteColleague

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