Chapter 18 Integrating user interface and model: the Model-View-Controller pattern

An Introduction to Programming and Object

Oriented Design using Java2nd Edition. May 2004

Jaime NiñoFrederick Hosch

Chapter 18Integrating user interface and model:

the Model-View-Controller pattern

2May 2004 NH-Chapter 18

Objectives

After studying this chapter you should understand the following: the Model-View-Controller pattern: components and their

roles;

the observer pattern, as realized by the class Observable and interface Observer, and its use in the MVC pattern.

Also, you should be able to: write a graphical user interface for a simple application

using the MVC architecture.


Model-View-Controller

An Application structure model components –objects that model and solve problem;

view components –objects that display model;

controller components –objects that handle user input.


Model-View-Controller advantages

Processing of input and output is separate;

Controllers can be interchanged, allowing different user interaction modes;

Allows multiple views of model.


M-V-C architecture

Model

Controller control1

Controller control2

View view1

View view2

View view3


Observer/Observable relationships

Observer

SomeClient

«interface»Observable

notifies

registers with

java.util provides class Observable, interface Observer. Observer is client and Observable is server. Observer registers with the Observable, Observable informs Observer when it changes state.


M-V-C example

Model: a right triangle. base and height can be modified.

Three different views of triangle. View one displays lengths of sides. View two displays triangle graphically. View three logs changes in triangle state to file.

A controller for view one, to modify triangle.

No controller for other views.


public class RightTriangle { private int base; private int height; private int hypotenuse;

public RightTriangle (int base, int height) { this.base = base; this.height = height; setHypotenuse(); }

public int base () { return this.base; }

public int height () { return this.height; }

public int hypotenuse () { return this.hypotenuse; }

public void setBase (int newBase) { this.base = newBase; setHypotenuse(); }

public void setHeight (int newHeight) { this.height = newHeight; setHypotenuse(); }

private void setHypotenuse () { this.hypotenuse = (int) Math.round( Math.sqrt(base*base + height*height)); }}//end RightTriangle.

RightTriangle class


Observable methods

public void addObserver (Observer o);

protected void setChanged ();

public void notifyObservers ();

public void notifyObservers (Object arg);

java.util.Observable class specification provides several methods; among those:


When RightTriangle changes state, to notify all registered observers of the event, modifies commands to add:

Structuring an Observer/ObservableRightTriangle extends Observable:

public class RightTriangle extends Observable …

All views are Observer’s of RightTriangle instance rt

Client Observer registers to rt invoking addObserver.

rt.addObserver(this);

setChanged();

notifyObservers();


Implementing an Observable

RightTriangle changes state in setBase or setHeight :

public void setBase (int newBase) {

this.base = newBase;

setHypotenuse();

setChanged();

notifyObservers();

}

public void setHeight (int newHeight) {

this.height = newHeight;

setHypotenuse();

setChanged();

notifyObservers();

}


Interface Observer

interface Observer {

void update (Observable o, Object arg);

}


Observer structure

class RTObserver implements Observer {private RightTriangle target;

// Create an observer of RightTriangle rt.public RTObserver (RightTriangle rt) {

target = rt;target.addObserver(this);…

}

public void update((Observable o, Object arg){ do something about o’s state

change.}…

}

observer registers with model

Observer must know target object. Observer registers itself with the target. When target notifies observer, observer executes update.


A simple view and controller for RightTriangle

Build a view that shows the three components of the triangle in text fields.

Controller will capture input from text fields labeled Base and Height, and modify state of the RightTriangle appropriately.


The View View extends JPanel and implements Observer. RightTriangle to display is provided as constructor argument.

class TextView extends JPanel implements Observer {private final static int FIELD_SIZE = 16;private JTextField base;private JTextField height;private JTextField hypotenuse;

public TextView (RightTriangle model) {super();…base = new JTextField(FIELD_SIZE);base.setActionCommand("Base");…height = new JTextField(FIELD_SIZE);height.setActionCommand("Height");…hypotenuse = new JTextField(FIELD_SIZE);hypotenuse.setEditable(false);…

}


The View View extends JPanel and implements Observer. RightTriangle to display is provided as constructor argument.


The View When model changes state, notifies view to update text fields. View’s update queries model for new state information,

and writes it into text fields:

public void update (Observable model, Object arg) {int side;RightTriangle rt = (RightTriangle)model;side = rt.base();base.setText(String.valueOf(side));side = rt.height();height.setText(String.valueOf(side));side = rt.hypotenuse();hypotenuse.setText(String.valueOf(side));

}


Controller structure

Captures user input from base and height text fields must have references to the text fields, must respond to ActionEvents generated by text fields, must be an ActionListener, must be added as listener to text fields.

Updates model. This is response to ActionEvent text fields generate. Must have reference to RightTriangle.


Controller structureprivate class TVController implements ActionListener {

private RightTriangle model;

/** * Create a new controller for this TextView of the * specified RightTriangle. */public TVController (RightTriangle model) {

this.model = model; TextView.this.base.addActionListener(this);

TextView.this.height.addActionListener(this);}


Controller structure/** * Update the model in response to user input. */public void actionPerformed (ActionEvent e) {

JTextField tf = (JTextField)e.getSource();try {

int i = Integer.parseInt(tf.getText());if (i < 0) throw new NumberFormatException();String which = e.getActionCommand();if (which.equals("Base"))

model.setBase(i);else

model.setHeight(i);} catch (NumberFormatException ex) {

TextView.this.update(model, null);}

}


Model, View and Controller

JTextField RightTriangle

hasTextView

TVController

listens-to

observes

modifies

2

3


Nim Game : Model

Player

«interface»

IndependentPlayerInteractivePlayer

AbstractPlayer

Game Pile

directs

provides

2

us

es


Nim Game : TUI

NimGame

creates

NimTUI

Gamecreates

creates

InteractiveController

TUIController

«interface»InteractivePlayer

notifies

provides moves

Player

«interface»


Nim Game : TUI v.s. GUI design

TUI design: TUIController

registers with InteractivePlayer, prompts and reads user’s move, forwards it to InteractivePlayer.

The play loop is in the interface.


Nim Game : GUI design

Observer

NimInterface

«interface»Observable

observes

Game


Nim Game : TUI v.s. GUI design

GUI design No explicit loop coded. User repeats some event.

NimController When user presses an input button it invokes setNumberToTake on InteractivePlayer.

Invokes Game’s play, once for InteractivePlayer, once for IndependentPlayer.


View

Display composed of four panels: a panel to display number of sticks remaining, two panels each with a text field to report player’s moves, a panel containing buttons for user to make a move.


NimController

provides moves

Game

InteractivePlayer

NimInterface.NimController

directs play

JButtonlistens to 3


View

A NimInterface instance: builds display, observes the Game.


Model’s Game

When Game completes a play, it notifies its Observers.

public void play () {

if (!gameOver()) {

nextPlayer.takeTurn(pile,MAX_ON_A_TURN);

previousPlayer = nextPlayer;

nextPlayer = otherPlayer(nextPlayer);

setChanged();

notifyObservers();

}

}


GUI structure User selects “New Game” from main menu.

Menu item listener invokes NimController’s initializeGame: displays a JDialog to get initialization data from user, creates a Game.


NimInterface

When notified, queries Game, updates display.

NimInterface responsible for displaying the number of sticks remaining in the game; reporting the winner when the game is over.


Sub-views

NimInterface defines an inner class PlayerView with the responsibility to report

player’s moves.

A PlayerView observes a Player, and updates the text field when the Player takes a turn.

NimInterface.PlayerViewobserves

Player

«interface»


Delaying IdependentPlayer’s move

NimController invokes Game’s play, once for InteractivePlayer, once for IndependentPlayer.

Need a delay between the two play invocations.



Attempt to delay the two plays fails. Application will pause, but moves appear to take place at the

same time.

public void actionPerformed (ActionEvent e) {

…

user.setNumberToTake(number);

game.play();

nhUtilities.utilities.Control.sleep(2); //delay

game.play();

…

}



user presses button

thread sleeps for two seconds

display updates resulting form

display is updated

display updates resulting fromInteractivePlayer move are scheduled

IndependentPlayer move are scheduled

button-press event handler starts

button-press event handler completes

Event dispatch thread


Scheduling a move to delay the action

NimController uses javax.swing.Timer to schedule IndependentPlayer’s play seconds after play of InteractivePlayer.

User’s Button-press event handling is complete.

IndependentPlayer’s move occurs after and when scheduled.


MVC and Swing components

Swing components are structured using MVC pattern.

Each Swing JComponent has an associated model object responsible for maintaining component’s state. JButton or JCheckBox has a ButtonModel, JTextArea or JTextField has a Document.

A Swing component delegates view and control responsibilities to its UI delegate.

The package javax.swing.plaf contains an abstract delegate class for each Swing component.


Summary

Overview basic format of Model-View-Controller pattern.

MVC commonly used to structure event-driven graphical user interfaces.

With MVC, interface responsibilities are partitioned into two segments: view has display responsibilities, controller modifies model in accordance with user input.


Summary

Developed three views of a right triangle.

Noted Java Observer interface and Observable class for implementing the observes relationship.

Fundamental relationship between view and model is observes, implemented by making model object an extension of the class Observable, view implements interface Observer.

Added a graphical user interface to the nim game, using the MVC pattern.


Summary

Swing components are structured along the lines of MVC.

Each Swing component has an associated model object responsible for maintaining

component’s state. a UI delegate, to which view and control responsibilities

are delegated.

This structure allows to change application look-and-feel in a system-independent way.

Documents

Chapter 18 Integrating user interface and model: the Model-View-Controller pattern