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Observer, Strategy, Decorator, Prototype,

Observer, Strategy, Decorator, Prototype,. Observer Pattern. Dependence mechanism / publish-subscribe / event handler / constraints / broadcast Let objects propagate information without depending on each other much.

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Observer, Strategy, Decorator, Prototype,

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  1. Observer, Strategy, Decorator, Prototype,

  2. Observer Pattern • Dependence mechanism / publish-subscribe / event handler / constraints / broadcast • Let objects propagate information without depending on each other much. • Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

  3. Observer Pattern observer/ dependent Subject addDependent: removeDependent: changed: Observer update: model ValueHolder value: value TextView update:

  4. Observer Pattern: • Registration • subject addDependent: observer • Notification • self changed. self changed: #value • Update • define update: aSymbol

  5. Notification • Object notifies dependents when information changes by sending itself a #changed message. • changed: anArg • self dependents • do: [:each | each update: anArg]

  6. Problem • A man and dog are in the room. When the dog wants to go out, he barks. When the man hears the dog barking, he opens the door. When the dog wants to go out and the door is open, he leaves.

  7. Basic classes • Dog • bark / move • Person • Door • open / close / isOpen • Object • addDependent: • changed: • update:

  8. Collaborations Dog bark watch listen Door openclose Person

  9. Dynamic Model Record order of events, interaction between objects. Sequence diagram Dog Person Door register register bark notify open notify go thru door unregister notify close

  10. A Script • | person door dog | • door := Door new close. • dog := Dog new. • dog door: door. • person := Person new. • person door: door; dog: dog. • dog bark.

  11. Door • opened <Boolean> • open • opened := true. • self changed: #open

  12. Door • close • opened := false. • self changed: #close • isOpen • ^opened

  13. Dog • currentState :: #sleeping, #waiting, #outside • bark • currentState := #waiting. • self changed: #bark • door: aDoor • door := aDoor. • door addDependent: self

  14. Dog • goOut • currentState := #outside. • door removeDependent: self. • self changed: #move

  15. Dog • update: aSymbol • (currentState == #waiting) & (aSymbol == #open) • ifTrue: [self goOut ]

  16. Person • dog: aDog • dog := aDog. • dog addDependent: self

  17. Person • update: aSymbol • aSymbol == #bark • ifTrue: [door open]. • aSymbol == #move • ifTrue: [door close]

  18. Watcher • instance variable: name <String> • update: aSymbol • Transcript show: subjectName; show: ' '; show: aSymbol; cr • name: aString • name := aString

  19. | person door dog | • door := Door new close. • door addDependent: • (Watcher new name: 'door'). • dog := Dog new. • door addDependent: dog. • dog addDependent: • (Watcher new name: 'Fido'). • person := Person new. • person door: door; dog: dog. • dog bark.

  20. Improvements • Creating method • (have class method return initialized object) • Compose method • (Watcher on: door name: 'door') • (door watcherNamed: 'door')

  21. Model and Memory Management • Dog allInstances • reports a lot of dogs! Garbage collection doesn't help. • Object uses a global dictionary to store dependents. Subject must "release" its observers/dependents.

  22. Make Dog a subclass of Model • Model uses an instance variable to store dependents. It does not have to release its observers. • Subclasses of Model cause less problems with garbage collection. • Class that has dependents should be subclass of Model.

  23. If you are not using Model then after the script says • dog bark. • add the messages • dog release. • door release. • person release

  24. Advantage of Observer Pattern • Easy to add new observers. • Coupling between observer and subject is abstract.

  25. Disadvantage of Observer Pattern • Often hard to understand relationships between objects in system. • Sometimes inefficient.

  26. Adding New Observer • Suppose room also has a bird, which is usually in a cage. • If bird is not in cage and door opens, bird flies out.

  27. Bird • update: aSymbol • (aSymbol == #open) & • (self isCaged not) • ifTrue: [self flyOut] • Script must now create a bird and make it depend on the door.

  28. Summary • Observer is an important pattern • Used in a UI to make reusable components • Reduces coupling by increasing abstraction • Abstraction makes programs easier to change, harder to understand • Often requires a “script” to set up dependencies

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