Observer Pattern

1
Observer Pattern

• Only inform subscriber before the publishers
  value is retrieved. The publisher may be set
  several times but the subscribers are only
  informed before the first get after a change.
• To demonstrate sharing, we want the list of
  subscribers to be on a per-publisher basis.
• The flag that reports change is shared by all
  subscribers.

2

• per attachment basis?

3
Collaborations

• encapsulate several roles into a behavioral unit
  of reuse
• is one aspectual scope?
• an aspectual group

4
Aspectual Methods

• Aspectual methods may be attached to several
  methods. Aspectual group.
• changeOp is played by set
• in-out methods.

5
Attachment

• attach methods
• attach classes (roles to classes)
• adapter PubSubConn1
• Publisher is played by Point
• changeOp is played by set

6
Johans Example
collaboration ObsAspect role Watched
boolean changed true in Collection v
out addWatcher(Watcher w) v.addElement(w)
out remWatcher(Watcher w) v.removeElement(w)
in-out ResultA set(Method e) ResultA r
e.invoke() changed true return r
in-out ResultB get(Method e) ResultB r
e.invoke() if (changed) for
(Iterator iv.iterator() i.hasNext())
(Watcher) i.next().gotten(this) changed
false return r
7
Johans Example (continued)
collaboration ObsAspect role Watcher void
gotten(Watched) if (w.changed) inform(w)
// needed because too many observers will be
informed, // as the list of observers is
shared betw. attachments in void
inform(Watched w)
8
Small collaboration

• collaboration ObsShared
• role CollectionHolder
• Collection vh new Vector()

9
Bind host class
adapter AddObserver runnable host,
observe scope host.Vars
observe.Observed hostadd_a_Printer(Print
er) provided by observeaddWatcher
10
Now combine the two.Is this the task of an
adapter?

• The semantic purpose of an adapter is to
  reconcile the interface a collaboration requires
  with the behavior provided by the host.
• In addition to specifying how the host fills in
  the holes of the collaborations roles, the
  adapter also specifies the exportation back to
  the host application.

11
What is the host?

• adapter ObsCombined
• ObsAspect, ObsShared
• combine two collaborations to get an adapter?
  Overloading the adapter concept?

12
Combining collaborations

• adapter ObsCombined
• ObsAspect, ObsShared
• Observed ObsAspect.Watched,
  ObsShared.VectorHolder
• ObsAspectv provided by ObsSharedvh
• export out addWatcher, out remWatcher,
• in-out get, in-out set
• Observer ObsAspect.Watcher
• export in inform

13
Example 2 The Publisher-Subscriber Protocol

• Have two collaborating participants

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Publisher

• collaboration PublisherSubscriberProtocol
• participant Publisher
• protected Vector subscribers new
  Vector()
• out void attach(Subscriber subsc)
• subscribers.addElement(subsc)
• out void detach(Subscriber subsc)
• subscribers.removeElement(subsc)
• in-out void changeOp()
• imported()
• for (int i 0 i lt subscribers.size()
  i)
• ((Subscriber)subscribers.elementAt(i)
  ).
• newUpdate(this)

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Subscriber

• participant Subscriber
• in void subUpdate(Publisher publ)
• protected Publisher publ
• out void newUpdate(Publisher aPubl)
• publ aPubl
• subUpdate(publ)

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Classes for deployment

• Class Point
• void set()
• class InterestedInPointChange
• void public notifyChange()
• System.out.println("CHANGE ...")

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Deployment 1

• adapter PubSubConn1
• Publisher is played by Point
• changeOp is played by set
• Subscriber is played by InterestedInPointChange
  
• void subUpdate(Publisher publ)
• notifyChange()
• System.out.println(on Point object "
  
• ((Point) publ).toString())

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Deployment 1
Red expected replaced
Blue from adapter
Point
Publisher
changeOp calls newUpdate attach(Subscriber) deta
ch(Subscriber)
set
InterestedInPointChange
Subscriber
subUpdate(Publisher) newUpdate(Publisher)
subUpdate(Publisher) notifyChange()
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Deployment 2

• adapter PubSubConn2
• Publisher is played by TicTacToe
• changeOp is played by startGame, newPlayer,
  putMark, endGame
• Subscriber is played by BoardDisplay,
  StatusDisplay
• void subUpdate(Publisher publ)
• setGame((Game) publ)
• repaint()