Observer and Decorator Pattern
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This is Class 5 on a 6 week course I taught on Software Design Patterns. This course discusses the Observer and Decorator patterns. Class based on "Head First Design Patterns."
![Design Patterns 05/28/10 Week 5: Observer and Decorator Jonathan Simon [email_address]](https://image.slidesharecdn.com/observeranddecorator-ppt-100527200307-phpapp02/85/Observer-and-Decorator-Pattern-1-320.jpg)
![Weather Monitoring Application ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],05/28/10](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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Observer and Decorator Pattern
- 1. Design Patterns 05/28/10 Week 5: Observer and Decorator Jonathan Simon [email_address]
- 5. Interfaces 05/28/10 Contains signatures for registering, removing, and notifying observers. Contains Update method so that information can be sent from Subject to Observer.
- 6. WeatherData (Subject) Questions: Which field is used for storing the list of Observers? How does an Observer add itself to this list? The fields for humidity, pressure, and temperature represent the state of the Subject.
- 9. Current Conditions Display (Observer) Note that the individual Observer is composed of the Subject. So the Subjects has a reference to all Observers…and each Observer has a reference to a Subject. Questions: What method on this Observer is called by the Subject when data is changed?
- 21. So far… 05/28/10 NotifyObserver will loop through each Observer and call Update() and pass a reference of itself Observer registers itself with the Subject
- 27. The Big Picture 05/28/10
- 33. Subject (Current Implementation) 05/28/10 Our Subject class contains the one notification event. Our ConcreteSubject gets the notification event by inheritance. We want our ConcreteSubject to contain multiple events. Does Inheritance make sense??? Would could we use instead?
- 34. Subject/SubjectEvent 05/28/10 This class represents a specific event of the Subject.
- 35. SubjectEvent 05/28/10 Now the SubjectEvent, not Subject, contains a reference to all of the Observers who are interested in being notified.
- 36. Subject/SubjectEvent 05/28/10 The constructor of WeatherData takes a SubjectEvent as parameter.
- 39. SubjectEvent 05/28/10 Added name parameter to the constructor and storing it in eventName Field.
Editor's Notes
- 13 November 2008
- Interface breaks. By value. The Observer needs to have it’s own version of the state data. Problem is that the Subject sends a lot more information that needed. We can resolve by sending a null value for data that has not changed. 13 November 2008
- 1. No. We would need to cast the subject to a specific subject so that we can get the properties that we want. This ties our Observer to a very specific Subject. Note with the Push model, the Observer did not need to know about the Subject data. 13 November 2008
- 1. reigsterObserver removeObserver notifyObservers 2. ArrayList observers 3. Initialize the ArrayList of observers 13 November 2008
- Temperature, humidity, and pressure aren’t part of a generic subject. 13 November 2008
- The Subject passed on a reference to itself. The SubjectEvent needs to pass the Subject as well. The SubjectEvent needs to tell the Observer the name of the event. 13 November 2008
- Observer = Search Screen. Just one observer Multiple Subjects 13 November 2008
- Observers = Each Child Node Subject = Parent Node 13 November 2008