Observer Pattern: Update Notification [60]

$\bullet$ Problem: How to maintain consistency between multiple observers of an observable object's state information?

$\bullet$ Solution: Observer Pattern

$\bullet$ Outline: Consider an Observable Supplier object with Number, Name, Status, City. Two Observer Frames both display the information about this particular Supplier:

$\bullet$ A user modifies two fields in the first Frame, and pushes SUBMIT

$\bullet$ The goal is to have the other Frame(s) automatically refresh their own displays:

Observer Pattern: UML [61]

$\bullet$ Supplier inherits from Java's Observable class

$\bullet$ ObsFrame implements Java's Observer interface

$\bullet$ Many ObsFrames observe the one Supplier

$\bullet$ Each ObsFrame registers itself with the Supplier via addObserver(this)

$\bullet$ addObserver() puts the Observer into the Vector of all Observers

$\bullet$ If a ObsFrame does a SUBMIT, then it mutates the Supplier

$\bullet$ Supplier calls notifyObservers()

$\bullet$ notifyObservers() iterates through the Vector, and calls each Observers's update()

$\bullet$ update() is the automatic update (callback) which allows the ObsFrame to refresh the display

$\bullet$ Refresh can be performed by using the Supplier's accessor methods

$\bullet$ SEE CASE STUDY