design patterns model – view – controller
DESCRIPTION
Design Patterns Model – View – Controller. History. A framework pattern for reusable applications. Depends on the Observer pattern. First developed by Xerox PARC for Smalltalk-80. Used by the Application Kit system in NeXTstep. Used by the Cocoa APIs for Apple’s OS X. - PowerPoint PPT PresentationTRANSCRIPT
Design PatternsDesign PatternsModel – View – Model – View –
ControllerController
Copyright © 2001 DeLorme28 November 2001
HistoryHistory
► A framework pattern for reusable applications.A framework pattern for reusable applications.► Depends on the Observer pattern.Depends on the Observer pattern.
► First developed by Xerox PARC for Smalltalk-80.First developed by Xerox PARC for Smalltalk-80.► Used by the Application Kit system in NeXTstep.Used by the Application Kit system in NeXTstep.► Used by the Cocoa APIs for Apple’s OS X.Used by the Cocoa APIs for Apple’s OS X.► Recommended structural framework pattern in Recommended structural framework pattern in
J2EE.J2EE.
► I have used this pattern for nearly ten years.I have used this pattern for nearly ten years.
Copyright © 2001 DeLorme28 November 2001
Observer PatternObserver Pattern
►Defines a one-to-many dependency Defines a one-to-many dependency between objects so that when one between objects so that when one object changes state, all its dependents object changes state, all its dependents are notified and updated automatically.are notified and updated automatically.
►Used to decouple the subject from the Used to decouple the subject from the observer, since the subject needs little observer, since the subject needs little information to notify the observer.information to notify the observer.
►Can result in excessive notifications.Can result in excessive notifications.
Copyright © 2001 DeLorme28 November 2001
Observer Class DiagramObserver Class Diagram
Observable
+addObserver(Observer)+deleteObserver(Observer)+notifyObservers(Object)
#hasChanged() : boolean#setChanged()
Observer
+update(Observable,Object)
AccountView
+update(Observable,Object)
BankAccount
+widthdraw(double) : long+deposit(double) : long+getBalance() : double
SummaryView
+update(Observable,Object)
Copyright © 2001 DeLorme28 November 2001
Transactions Happen!Transactions Happen!
Controller BankAccount AccountView SummaryViewdeposit()
setChanged()
notifyObservers()
update()
update()
getBalance()
getBalance()
Copyright © 2001 DeLorme28 November 2001
Observer Rocks!Observer Rocks!
►The Observer pattern allowed the The Observer pattern allowed the BankAccount class to notify multiple BankAccount class to notify multiple views without minimal information.views without minimal information.
►Observers can register themselves with Observers can register themselves with their Subjects. No strings attached!their Subjects. No strings attached!
►Transactions would cause this design to Transactions would cause this design to collapse under spurious notifications!collapse under spurious notifications!
Copyright © 2001 DeLorme28 November 2001
Architecture DiagramArchitecture Diagram
ViewView
model representation
ModelModel
business logic
ControllerController
user interaction
UpdatUpdatee
EventEvent
UserUserActionAction
ss
ChangChangee
ViewView
SetSet
StateState
GetGetStateState
Copyright © 2001 DeLorme28 November 2001
AdvantagesAdvantages
►Separation between the data layer and Separation between the data layer and the interface is the key:the interface is the key: The view is easily replaced or expanded.The view is easily replaced or expanded. Model data changes are reflected in all Model data changes are reflected in all
interfaces because all views are Observers.interfaces because all views are Observers. Better scalability since UI and application Better scalability since UI and application
logic are separated.logic are separated. Distribution over a network is greatly Distribution over a network is greatly
simplified.simplified.
Copyright © 2001 DeLorme28 November 2001
ProblemsProblems
►Problems of translation:Problems of translation: Business logic bleeds into the Controller.Business logic bleeds into the Controller. Observer pattern is non-obvious for EJBs.Observer pattern is non-obvious for EJBs.
See See EJBObserverEJBObserver by Greg Comeau. by Greg Comeau.
►Problems of the pattern:Problems of the pattern: Excessive coupling between the Model Excessive coupling between the Model
and View and the Model and Controller.and View and the Model and Controller. Frozen interfaces are hard to manage!Frozen interfaces are hard to manage!