Refining ModelViewController

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Refining Model-View-Controller

• Daniel Chait
• Partner
• Lab49, Inc.

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Classic MVC Pattern

• Model
• Domain data objects
• View
• Presentation/display
• Controller
• Map user inputs to View and Model

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Example PowerPoint Object Model
Views
Slide Editing View
Outline View
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Example Contd
Run-Time Characteristics

• One instance of Model
• Many View instances, each should update in
  realtime as Model changes
• Controller? Depends

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Controller Options

• Ignore it
• Traditional job of Controller (Smalltalk-80)
  interpreting mouse/keyboard input has been
  subsumed by the system and is no longer needed
  as such.
• For simple apps, Controller can be considered as
  part of the view Controls can both present data
  and handle user input. DocumentView in MFC
• When to use it
• Aspect-oriented programming need to mediate all
  changes to the model. Examples Undo/Redo,
  Logging, Unit Testing

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Controller Implementing Undo

• Command Pattern to change the model
• Command cmd new ChangeColorCommand(
• App.Selection.Shapes,
• Color.Blue)
• CommandManager.Execute(cmd)
• Requires that View has a Read-Only handle to the
  Model, otherwise it might circumvent the command
  manager.
• Undo is then very straightforward
• if (CommandManager.HasUndo()) CommandManager.Undo(
  )

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Option1 for tying Model to View

• Observer/Observable (most common in C/Java)
• events/delegates
• App.PresentationAdded Presentation.SlideAdded
  Slide.ShapeAdded events
• Each model object hooks events of objects below
  it -- Observes them -- and bubbles them
  upwards, eg

Shape1.ColorBlue
Shape1.ColorChanged()
Slide1.ShapeChanged()
Presentation1.Changed()
View1.TitlePresentation1
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Option2 for tying Model to View

• Pub/Sub
• Message bus where events are published and others
  can express interest
• Makes dependencies explicit and possibly
  declarative
• Allows aspect-oriented intermediation on event
  level. Examples Event Rollup, Logging,
  Record/Replay

Shape1.ColorBlue
Publish event
TargetShape1PropertyColorOldValRedNewValBlu
e
View1.TitlePresentation1
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AbstractView The Missing Abstraction

• Goal is to define the user interface abstractly
  ie. Not in Controls!
• Models Data such as Open Windows, Current
  Document, Selected Shape
• Models Events (State Transitions) such as
  WindowOpened/Closed, DocumentActivated,
  SelectionChanged, etc.
• Allows a very thin UI/Presentation layer to read
  data and watch for events (either via Observer or
  Pub/Sub) with very little logic in it.
• Easy to change UI or build new UI without
  affecting the underlying interaction model
• Variously called ViewModel (read Model of the
  View) AbstractView (read An Abstraction of
  the View) PresentationModel (read A Model of
  the Presentation Layer)

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Further Reading

• John Gossmans blog blogs.msdn.com/johngossman
• Talks about how Sparkle is built using
  Model/View/ModelView and then the UI (XAML) is
  attached to the ModelView using pure databinding.
  Nice!
• Martin Fowler on the Presentation Model
  pattern martinfowler.com/eaaDev/PresentationModel
  .html

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Questions?

• Come work at Lab49!
• Daniel Chait
• Lab49, Inc.
• [phone] x112
• daniel_at_lab49.com
• http//www.lab49.com