Model View Controller

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Model View Controller

• Rick Mercer

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Outline

• Model View Controller
• Observer as part of MVC
• An example of the Observer Design Pattern

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Model View Controller (MVC)

• The intent of MVC is to keep neatly separate
  objects into one of tree categories
• Model
• The data, the business logic, rules, strategies,
  and so on
• View
• Displays the model and usually has components
  that allows user to edit change the model
• Controller
• Allows data to flow between the view and the
  model
• The controller mediates between the view and model

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Model

• The Model's responsibilities
• Provide access to the state of the system
• Provide access to the system's functionality
• Can notify the view(s) that its state has changed

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View

• The view's responsibilities
• Display the state of the model to the user
• At some point, the model (a.k.a. the observable)
  must registers the views (a.k.a. observers) so
  the model can notify the observers that its state
  has changed

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Controller

• The controller's responsibilities
• Accept user input
• Button clicks, key presses, mouse movements,
  slider bar changes
• Send messages to the model, which may in turn
  notify it observers
• Send appropriate messages to the view
• In Java, listeners are controllers

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from http//www.enode.com/x/markup/tutorial/mvc.ht
ml)
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MVC Misunderstood

• MVC is understood by different people in
  different ways
• It is often misunderstood, but most software
  developers will say it is important powerful
• Lets start it right MVC is a few patterns put
  together
• Consider the Observer Pattern

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Compound Patterns

• The observer pattern is part of MVC
• In Java, you can use Observer support
• The model extends Observable
• view implements Observer (one method update)
• You need to addObservers to the model
• The model sends these two messages when it thinks
  the view should be updated
• setChanged()
• notifyObservers()

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Model

• import java.awt.Color
• import java.util.Observable
• import java.util.Random
• public class Model extends Observable
• private Color currentColor
• private static Random generator // Or set up
  any color
• public Model()
• generator new Random()
• currentColor Color.PINK
• generator.nextInt(256) 25,
  generator.nextInt(256)39)
• public void changeColor()
• // Make the colors change seemingly randomly
• int red (currentColor.getRed()
  generator.nextInt(256)) 256
• int green (currentColor.getGreen()
  generator.nextInt(256)) 256
• int blue (currentColor.getRed()
  generator.nextInt(256)) 256

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One View

• import java.awt.Color
• import java.util.Observable
• import java.util.Observer
• import javax.swing.JLabel
• import javax.swing.JPanel
• public class TextViewOfColor extends JPanel
  implements Observer
• private Color color
• private JLabel label
• public TextViewOfColor(Color color)
• this.color color
• label new JLabel()
• label.setText("A text 'view' of color "
  color.toString())
• this.setBackground(Color.CYAN)
• this.add(label)

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Controller

• import java.awt.
• import javax.swing.
• public class Controller extends JFrame
• public static void main(String args)
• Controller controller new Controller()
• controller.setVisible(true)
• private Model model
• private OvalViewOfColor view
• private TextViewOfColor view2
• private JButton startButton new
  JButton("Change Color")
• public Controller()
• setSize(450, 250)
• setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)
  
• Container cp getContentPane()

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The Model

• public void setUpModelAndObservers()
• model new Model()
• model.changeColor()
• // Register the observers (views) to be
  notified of state changes
• // when the observed object (model) changes
  its state and notifies observers
• view2 new TextViewOfColor(Color.PINK)
• add(view2, BorderLayout.NORTH)
• model.addObserver(view2)
• view new OvalViewOfColor(Color.PINK)
• getContentPane().add(view, BorderLayout.CENTER
  )
• model.addObserver(view)
• private class ButtonListener implements
  ActionListener
• public void actionPerformed(ActionEvent arg0)
  
• model.changeColor()

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Another View

• import java.awt.
• import java.util.Observable
• import java.util.Observer
• import javax.swing.JPanel
• public class OvalViewOfColor extends JPanel
  implements Observer
• Color color
• public OvalViewOfColor(Color color)
• this.color color
• _at_Override
• public void paintComponent(Graphics g)
• Graphics2D g2 (Graphics2D) g
• super.paintComponent(g2)
• g2.setColor(color)