Observer Design Pattern

1
Observer Design Pattern

• By Eric Perret
• Pages 372-380 in Applying UML and Patterns

2
What we are covering.

• Definition
• Example
• Example using code
• Why it is a good idea to use the pattern.
• Summary

3
What is an Observer?

• The Observer design pattern, or more accurately
  some implementation of this pattern, is used to
  assure consistency between different objects.
• AKA Publish-Subscribe

4
What does this really mean.

• When an object changes state, all of the objects
  that need to know about it are notified and
  updated automatically.

5
Example

• Let the state of an object "B" depend on the
  state of an object "A". Whenever the state of
  object "A" changes, "B" has to recompute its
  state in order to remain consistent with "A".
• The observer design pattern supports such
  dependencies and at the same time tries to reduce
  the coupling between the object that changes (the
  publisher) and the object that needs a change
  notification (the subscriber).

6
Here's an example of the interfaces in java-like
code

• public abstract interface Subject public
  abstract void addObserver(Observer
  observer) public abstract void
  removeObserver(Observer observer) public
  abstract void notify()public abstract class
  Observer public abstract void update()

• The subject calls its "notify" operation whenever
  it changes. "Notify" calls the "update" operation
  on each observer. The observers then call back to
  the subject and update themselves accordingly.

7
Why Use the Pattern?

• State dependencies between objects are
  unfavorable for several reasons.
• If we could model our problem domain as a set of
  independent classes, we would get a software
  system without state dependencies.
• Coding and especially maintaining such a system
  would be much easier. The programmer could focus
  on single classes only.

8
Summary

• Loosely couple objects in terms of communication
• Publishers know about subscribers only through an
  interface and subscribers can register or
  de-register dynamically with the publisher

9
Question, comments, theories, or conundrums?

• Slides can be found here http//www.rpi.edu/perr
  ee/sdd

10
References

• Applying UML and Patterns by Craig Larman,
  352-380
• On Using the Observer Design Pattern by
  Constantin Szallies, http//www.wohnklo.de/pattern
  s/observer.html
• Observer Design Pattern by Stephen Lam,
  http//sern.ucalgary.ca/courses/SENG/609.04/W98/la
  msh/observerLib.html