Proxy, Observer, Symbolic Links

1. Proxy, Observer, Symbolic Links Rebecca Chernoff

2. Proxy Pattern • Intent • Provide a surrogate or placeholder for another object to control access to it • Other Names • Surrogate

3. Proxy: Motivations & Applicability • Remote Proxy • Virtual Proxy • Protection Proxy • Smart Reference • Local representative of object in a different address space. • Defer cost of creation and initialization. • Protection via access control. • Reference counting • Load a persistent object into memory when first referenced • Object locking

4. Proxy: Participants • RealSubject • Defines the real object that the proxy represents. • Subject • Defines the common interface for RealSubject and Proxy. • Allows for a Proxy to be used wherever a RealSubject is expected.

5. Proxy: Participants • Proxy • Maintains a reference to the real subject. May refer to a Subject if the RealSubject and Subject interfaces are the same. • Identical interface to Subject so the Proxy can be substituted. • Controls access to the real subject. • May be responsible for creating and deleting the real subject.

6. Proxy: Participants • Additional Responsibilities of Proxy • Remote Proxy • Encode and send the request to the real subject in a different address space. • Virtual Proxy • Cache additional information in order to postpone accessing the real subject. • Protection Proxy • Check access permissions.

7. Proxy: Structure

8. Proxy: Consequences • Introduces a level of indirection • Remote Proxy • Hides fact that object resides in a different address space. • Virtual Proxy • Perform optimizations such as creation on demand. • Protection Proxy and Smart References • Allow additional housekeeping tasks when object is accessed. • Copy-On-Write

9. Proxy: Related Patterns • Adapter • Provides a different interface to an object, but since Proxy can refuse to perform a request, the interface is effectively a subset. • Decorator • Similar implementation to proxy, but different purpose. Decorator adds responsibilities whereas proxy controls access.

10. Observer Pattern • Intent • Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically • Also Known As • Dependents • Publish-Subscribe

11. Observer: Motivation • Maintain consistency between related objects. • Application Data vs. Presentation • Avoid tightly coupled classes that reduce reusability.

12. Observer: Applicability • An abstraction has two aspects, one dependent on the other. • A change to one object requires changing others. • An object should be able to notify other objects without making assumptions about the objects.

13. Observer: Participants • Observer • Defines an interface for objects to be notified when a subject changes. • Subject • Defines an interface for attaching and detaching Observer objects. • Unknown number of Observer objects.

14. Observer: Participants • ConcreteObserver • Implements the Observer interface to keep its state consistent with the subject’s. • Maintains a reference to the ConcreteSubject • Stores state that should stay consistent with the subject’s. • ConcreteSubject • Implements the Subject interface to update Observers. • Stores state of interest to ConcreteObserver objects. • Sends a notification to its observers when its state changes.

15. Observer: Structure

16. Observer: Collaborations

17. Observer: Consequences • Vary subjects and observers independently. • Abstract coupling between Subject and Observer. • Support for broadcast communication. • Simple update to Server may cause a cascade of updates to Observer and its dependent objects.

18. Observer: Implementation • Mapping subjects to their observers. • Tradeoff between time and space. • Observing more than one subject. • Observer needs to know which Subject is notifying. • Who triggers the update? • Subject: client doesn’t have to remember to update, but consecutive operations cause multiple updates. • Observer: client can wait to trigger the update, but client must remember to call update. • Dangling references to deleted subjects. • Notify observers right before deletion. Observers of multiple Subjects can’t be deleted when just one Subject is deleted.

19. Observer: Implementation • Making sure the Subject state is self-consistent before notification. • Use TemplateMethod pattern with Notify as last operation. • Avoiding observer-specific update protocols: the push and pull models. • Push Model: Detailed information sent regardless. • Pull Model: Observers ask for details explicitly. • Specifying modifications of interest explicitly. • Observers register for a specific aspect of interest.

20. Observer: Implementation • Encapsulating complex update semantics. • Change-Manager • Takes responsibility of maintaining references to observers away from the Subject. • Defines a particular update strategy. • Updates all dependent observers at the request of a subject. • Combining the Subject and Observer classes. • When multiple inheritance not supported by language, both interfaces may reside in one class.

21. Observer • Related Patterns • Mediator • The ChangeManager encapsulates complex update semantics, thus acting as a mediator between the Subject and its Observers. • Singleton • ChangeManager may be unique and globally accessible.

22. But Where Do Surrogates Fit into This? • File System Example • Symbolic Links / Aliases / Shortcuts • Reference to another node in the file system. • Surrogate for a node, not the node itself. • Own access rights. • Edit and save a file. • Add and remove nodes to a directory.

23. But Where Do Surrogates Fit into This? • How do I find the right design pattern for the task? • Consider how design patterns solve design problems. • Scan the Intent sections for something that sounds right. • Study how patterns interrelate. • Look at patterns whose purpose corresponds to what you’re trying to do. • Examine a relevant cause of redesign and apply the patterns that help you avoid it. • Consider what should be variable in your design.

24. But Where Do Surrogates Fit into This? • Proxy Pattern • Subject => Node • Proxy => Link • Real Subject => ?

25. But Where Do Surrogates Fit Into This?