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Week 4, Day 3 Observer, continued

Week 4, Day 3 Observer, continued. Return Quiz Coupling & Cohesion Observers Class structure for observers Sequence diagram for observers How it improves coupling How it improves cohesion Downsides Exam next week (Week 5), Tuesday. Coupling vs. Cohesion.

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Week 4, Day 3 Observer, continued

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  1. Week 4, Day 3Observer, continued • Return Quiz • Coupling & Cohesion • Observers • Class structure for observers • Sequence diagram for observers • How it improves coupling • How it improves cohesion • Downsides • Exam next week (Week 5), Tuesday SE-2811 Slide design: Dr. Mark L. Hornick Content: Dr. Hornick Errors: Dr. Yoder

  2. Coupling vs. Cohesion • Module 4 shows what we are trying to avoid: low cohesion and high coupling (more lines==lower) (more lines==higher) SE-2811 Dr. Mark L. Hornick http://www.jesusgilhernandez.com/2012/11/28/cohesion-and-coupling/

  3. Basic class relationships Subject ------------------------------ attach():voiddetach():voidnotifyObservers():void Observer---------------- update(???):void -observers 1..* SubjectClass ObserverClass1 ObserverClass2

  4. Collaborations between objects in the Observer pattern s:SubjectClass o1:ObserverClass1 o2:ObserverClass2 attach(???) attach(???) notifyObservers() update(???) getContextSpecificInfo() update(???) getContextSpecificInfo()

  5. Consequences (positive) Coupling between Subject and Observers: • Subject knows it has a list of Observers, but not specific classes • Each Observer conforms to the simple interface of the abstract Observer Interface. • Hence, coupling is • Minimal • Abstract

  6. Consequences (positive) Cohesion is increased from single-class implementation • State management and display/response are separated • E.g. GUI innards separated from “your code” • E.g. Web access separated from display

  7. Consequences (negative) • Broadcast communication • Notification is broadcast to all interested objects. • Observers can be added/removed at any time. • Observer decides when it needs to be notified. • Unexpected updates • Observers have no knowledge • Of each other’s presence. • About the cost of “state change of subject” • Cascade of updates. SE-2811 Dr. Mark L. Hornick

  8. [ADDED SLIDE]Timing of multiple observers • All observers get notified at essentially the same time. • In the lab, all observers should be integrated into the same program and run at the same time. SE-2811 Dr. Mark L. Hornick

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