Transcript 08.MVC-Swing - Calvin College

MVC and Swing
Joel Adams and Jeremy Frens
Calvin College
(1/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Review: The Java Class Hierarchy
We’ve seen that Java provides a unified class hierarchy…
Object
Component
Container
Number
Double Integer
JComponent
(2/15)
Long
Exception
Error
IO
Runtime
Exception Exception
AbstractButton
JButton
Throwable
Almost everything in Java is ultimately
derived from the Object class…
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Java User Interface Widgets
Original Java had the awt package of user-interface widgets:
Object
Component
Button
Canvas Checkbox Container … Scrollbar
JComponent
AbstractButton
JButton
(3/15)
…
Panel
ScrollPane Window
JFile JLabel JPanel JScrollPane … JTabbedPane
Chooser
Java 1.2 added the swing
package of lighter-weight widgets
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Designing User Interfaces
Java strongly encourages UI designers to use the model-viewcontroller (MVC) design pattern, in which:
•The application’s core functionality JFrame Listener Object
is isolated in a model class:
•The application’s user-interface is
isolated in a view class:
View
•The application’s control is
isolated in a controller class:
Viewi
Model
Controller
Controlleri
This pattern lets you modify the UI (or add additional ones)
without touching the app’s core functionality.
(4/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Example
Let’s build an app with a background-color-changing button.
JFrame
SampleController
To change background
Push Me
Basic
Design:
Sample
View
Object
Action
Listener
Sample
Controller
Sample
Model
SampleView provides the UI, and a method to
modify its background
SampleModel provides the new background color
SampleController acts as mediator between them
(5/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Example: SampleModel
SampleModel is the simplest of our three classes:
import java.awt.Color;
public class SampleModel {
private Color [] myColors = {Color.white, Color.red,
Color.green, Color.blue};
private int myIndex = 0;
public SampleModel() { } // nothing to do
public Color getNextColor() {
myIndex = (myIndex+1) % myColors.length;
return myColors[myIndex];
}
}
A model “knows” nothing about the classes that use it…
(6/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Example: SampleView
… defines instance variables for the GUI’s widgets:
import java.awt.Color;
import java.awt.event.ActionListener;
import javax.swing.*;
// JLabel, JButton, ...
public class SampleView extends JFrame {
private JPanel myPane;
private JButton myButton;
...
We could also define a JLabel instance variable for our “To
change background” label, but we’ll avoid that for now
(7/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Example: SampleView (ii)
The constructor actually builds the GUI:
…
public SampleView() {
myPane = new JPanel();
myPane.add(new JLabel("To change background color"));
myButton = new JButton("Push me");
myPane.add(myButton);
this.setContentPane(myPane);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
...
Our class only needs handles to GUI widgets if it needs to
access them after they have been created…
(8/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Example: SampleView (iii)
Pressing a JButton triggers an ActionEvent object, that
is usually handled by an ActionListener object…
What to do in response is the responsibility of our controller,
so our view needs to provide a method to let its controller
register itself as the ActionListener for our JButton:
...
public void setButtonActionListener(ActionListener al) {
myButton.addActionListener(al);
}
...
The API tells us the messages we can send to a JButton …
(9/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Example: SampleView (iv)
Since our GUI has to let its controller change its background
color, we need a method to do this…
...
public void setBackgroundColor(Color c) {
myPane.setBackground(c);
}
} // end of class SampleView
The API tells us the messages we can send to a JPanel …
Given a Model and View, we just need a Controller…
(10/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Example: SampleController
To handle a JButton’s ActionEvent, a controller
must implement the ActionListener interface:
import java.awt.Color;
import java.awt.event.ActionListener;
public
class SampleController implements ActionListener {
private SampleModel myModel;
private SampleView myView;
...
In our design, our controller has-a model and has-a view…
(11/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Example: SampleController (ii)
Its constructor initializes its two components:
…
public SampleController() {
myModel = new SampleModel();
myView = new SampleView();
myView.setButtonActionListener(this);
myView.pack(); // or myView.setSize(w, h);
myView.setVisible(true);
}
...
Note that our controller uses our view’s method to register
itself as the ActionListener for the view’s JButton.
(12/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Example: SampleController (iii)
To implement the ActionListener interface, our
controller must define method actionPerformed()
…
public void actionPerformed(ActionEvent ae) {
Color c = myModel.getNextColor();
myView.setBackgroundColor(c);
}
...
Each press of the JButton in our view, the JRE sends the
actionPerformed() message to that button’s
ActionListener (i.e., our controller), causing this
method to be executed.
(13/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Example: SampleController (iv)
Finally, our controller must define the main()
method so that execution begins with itself:
…
public static void main(String [] args) {
SampleController sc = new SampleController();
}
} // end of class SampleController
Now, when we run our app: java SampleController
our main() method creates an instance of the controller,
which creates instances of the model and view, and then
begins handling user-events (clicks on the JButton).
(14/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College
Exercise
Go to today’s exercise and complete
parts I, II, and III
(15/15)
2003 Joel C. Adams. All Rights Reserved.
Dept of Computer Science
Calvin College