Event Handling

The problem: connecting user events to code execution.

User events: mouse click, mouse down, mouse up, mouse drag, mouse within a component (focus), key pressed, particular key pressed, etc.

Solution 1: have controls listen to all events, check whether they should respond, and then respond. Roughly this was the JDK 1.0 model.

Solution 2: Tell controls what events they can respond to and have them direct messages to appropriate actions (methods within object) . This is the "delegation" model of JDK 1.1 and is more efficient.

Model:
user action --> source object (control) ----> target object (listener)

What we need to do is:

identify (turn on) controls that will respond to events
For these controls, connect them to appropriate listeners. This is called registration. Note multiple actions can take place for each event, and these are not synchronized. Also any object can be a listener if it implements the appropriate interface.

So what do we need to do?

To the control c, execute c.addSomeListener(some obj).
This makes c respond to the event by sending a message to the object.
Make obj an event listener, i.e. by
1. declaring that it implement the appropriate Listener Interface
2. defining the methods of the Interface.

Abstract Example: Making object c of Foo respond to Button b;

      Foo c = new Foo();
      Button b = new Button("clear");
      b.addActionListener(c);       //.. buttons generate action events
     ...
      // ActionListener has only 1 method, actionPerformed
     ..
      class Foo implements ActionListener

   {
    ...
        public void  actionPerformed()
     {
        whatever foo should do when button is pressed
     }
    ...
   }

Four Designs

This program allows the user to draw lines, clear the screen, and change colors.

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

// Design 1:  Everyone talks to applet and gets messages from applet.
//   Applet acts as god class.  This is usually the simpliest design.


class MyCanvas extends Canvas
{
  Color linecolor= Color.black;  

  public void paint(Graphics g)  
  {
      Dimension d = getSize();
      g.drawRect(2,2,d.width-4,d.height-4);
  }

  public void clear()
 {
   Graphics g = getGraphics();
   Dimension d = getSize();
   Color c = getBackground();
   g.setColor(c);
   g.fillRect(0,0,d.width,d.height);
   repaint();
   //.. repaint() schedules update(). update() (typically) calls paint().
 }

  public void changeColor()
 {
   Color[] colors ={Color.red,Color.black,Color.blue,Color.magenta};
   linecolor = colors[random(colors.length)]; 
 }
 
 public int random(int i)
  {
     return (int) Math.floor(Math.random()*i);
  }

  public void drawLine(Point p, Point q)
  {
   Graphics g = getGraphics();
   g.setColor(linecolor);
   g.drawLine(p.x,p.y,q.x,q.y);
   g.drawOval(q.x,q.y,2,2);
  }
 public void drawOval(Point p)
  {
    Graphics g = getGraphics();
    g.drawOval(p.x,p.y,2,2);
  }

}

public class MouseTest extends Applet
  implements ActionListener, MouseListener
{
  Panel top;
  Button clear;
  Button change;
  MyCanvas mycanvas;
  Point begin,end;

  public void init()
 {
   top = new Panel();
   clear = new Button("Clear");
   clear.addActionListener(this);
   change = new Button("Change Color");
   change.addActionListener(this);
   top.add(new Label("Mouse click to draw lines"));
   top.add(clear);
   top.add(change);
   setLayout(new BorderLayout());
   add(top,"North");
   mycanvas = new MyCanvas();
   mycanvas.addMouseListener(this);
   add(mycanvas,"Center");
 }

  public void actionPerformed(ActionEvent actevt)
 {
   Object arg = actevt.getSource();
   if (arg == clear ) mycanvas.clear();
    else if (arg == change) mycanvas.changeColor();
 }

   public void mousePressed(MouseEvent me)
 {
    begin = me.getPoint();
    mycanvas.drawOval(begin);
 }
    public void mouseReleased(MouseEvent me )
  {
     end = me.getPoint();
     mycanvas.drawLine(begin,end);
   }

   public void mouseClicked(MouseEvent me)
  {}

   public void mouseEntered(MouseEvent me)
  {}

  public void mouseExited(MouseEvent me)
 {}
}

In the next design we group related activities more finely. The code is longer, but may allow easier changes. Code is simpliest when its structure reflects the organization of the computation.

import java.applet.*;
import java.awt.*;
import java.awt.event.*;


class MyCanvas extends Canvas
{
  Color linecolor= Color.black;  


  public void paint(Graphics g)  // why doesn't this work?
  {
      Dimension d = getSize();
      g.drawRect(2,2,d.width-4,d.height-4);
  }


  public void clear()
 {
   Graphics g = getGraphics();
   Dimension d = getSize();
   Color c = getBackground();
   g.setColor(c);
   g.fillRect(0,0,d.width,d.height);
   repaint();
 }

  public void changeColor()
 {
   Color[] colors ={Color.red,Color.black,Color.blue,Color.magenta};
   linecolor = colors[random(colors.length)]; 
 }
 
 public int random(int i)
  {
     return (int) Math.floor(Math.random()*i);
  }

  public void drawLine(Point p, Point q)
  {
   Graphics g = getGraphics();
   g.setColor(linecolor);
   g.drawLine(p.x,p.y,q.x,q.y);
   g.drawOval(q.x,q.y,2,2);
  }
 public void drawOval(Point p)
  {
    Graphics g = getGraphics();
    g.drawOval(p.x,p.y,2,2);
  }

}

class ClearButton extends Button
{
   ClearButton(MouseTest mt)
  {
    super("Clear");
    addActionListener(mt);
  }

}
class ChangeButton extends Button
{
   ChangeButton(MouseTest mt)
  {
   super("Change Color");
   addActionListener(mt);
  }
}



public class MouseTest extends Applet
  implements ActionListener, MouseListener
{
  Panel top;
  ClearButton clear;
  ChangeButton change;
  MyCanvas mycanvas;
  Point begin,end;

  public void init()
 {
   top = new Panel();
   clear = new ClearButton(this);
   change = new ChangeButton(this);
   top.add(new Label("Mouse click to draw lines"));
   top.add(clear);
   top.add(change);
   setLayout(new BorderLayout());
   add(top,"North");
   mycanvas = new MyCanvas();
   mycanvas.addMouseListener(this);
   add(mycanvas,"Center");
}

  public void actionPerformed(ActionEvent actevt)
 {
   Object arg = actevt.getSource();
   if (arg == clear ) mycanvas.clear();
    else if (arg == change) mycanvas.changeColor();
 }

   public void mousePressed(MouseEvent me)
 {
    begin = me.getPoint();
    mycanvas.drawOval(begin);
 }
    public void mouseReleased(MouseEvent me )
  {
     end = me.getPoint();
     mycanvas.drawLine(begin,end);
   }

   public void mouseClicked(MouseEvent me)
  {}

   public void mouseEntered(MouseEvent me)
  {}

  public void mouseExited(MouseEvent me)
 {}
}

In this design we examine the interactions between the events.

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

/* Design 2: here we analyze the objects and see where message are going
 The  Listener for the message must be able to see the object generating
 the message. 
Message Diagram:
                 clearbutton --> canvas  
                  changebutton --> canvas
                  canvas -->(mouse clicks) -->canvas
So the buttons will still need to be mediated by the applet, while
the canvas can talk to itself.
*/		  


class MyCanvas extends Canvas
     implements  MouseListener
{
  Color linecolor= Color.black;  //ugh
  Point begin,end;



  public void paint(Graphics g)  
  {
      Dimension d = getSize();
      g.drawRect(2,2,d.width-4,d.height-4);
  }


  public void clear()
 {
   Graphics g = getGraphics();
   Dimension d = getSize();
   Color c = getBackground();
   g.setColor(c);
   g.fillRect(0,0,d.width,d.height);
   repaint();
 }

  public void changeColor()
 {
   Color[] colors =
            {Color.red,Color.black,Color.blue,Color.magenta,Color.green};
   linecolor = colors[random(colors.length)]; 
 }
 
 public int random(int i)
  {
     return (int) Math.floor(Math.random()*i);
  }

  public void drawLine(Point p, Point q)
  {
   Graphics g = getGraphics();
   g.setColor(linecolor);
   g.drawLine(p.x,p.y,q.x,q.y);
   g.drawOval(q.x,q.y,2,2);
  }

 public void drawOval(Point p)
  {
    Graphics g = getGraphics();
    g.drawOval(p.x,p.y,2,2);
  }
   public void mousePressed(MouseEvent me)
 {
    begin = me.getPoint();
    drawOval(begin);
 }
    public void mouseReleased(MouseEvent me )
  {
     end = me.getPoint();
    drawLine(begin,end);
   }

   public void mouseClicked(MouseEvent me)
  {}

   public void mouseEntered(MouseEvent me)
  {}

  public void mouseExited(MouseEvent me)
 {}

}


public class MouseTest extends Applet
  implements ActionListener
{
  Panel top;
  Button clear;
  Button change;
  MyCanvas mycanvas;


  public void init()
 {
   mycanvas = new MyCanvas();  
   top = new Panel();
   clear = new Button("Clear");
   clear.addActionListener(this);
   change = new Button("Change Color");
   change.addActionListener(this);
   top.add(new Label("Mouse click to draw lines"));
   top.add(clear);
   top.add(change);
   setLayout(new BorderLayout());
   add(top,"North");
   mycanvas.addMouseListener(mycanvas);
   add(mycanvas,"Center");
 }

  public void actionPerformed(ActionEvent actevt)
 {
   Object arg = actevt.getSource();
   if (arg == clear) mycanvas.clear();
    else if (arg == change) mycanvas.changeColor();
 }
}

Example redone with Inner Classes

Inner classes are a recent addition to Java.
Each inner class is local to the class but has access to all the members of the class.
Inner classes seem to violate the principle of information hiding.
Inner classes are not the same as nested classes in C++.

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

// Version using inner classes. Arguable bad style.

public class MouseTest extends Applet
 
{
  Panel top;
  Button clear;
  Button change;
  MyCanvas mycanvas;

  public void init()
 {
   mycanvas = new MyCanvas();  
   top = new Panel();
   clear = new Button("clear");
   clear.addActionListener(mycanvas);
   change = new Button("Change Color");
   change.addActionListener(mycanvas);
   top.add(new Label("Mouse click to draw lines"));
   top.add(clear);
   top.add(change);
   setLayout(new BorderLayout());
   add(top,"North");
   mycanvas.addMouseListener(mycanvas);
   add(mycanvas,"Center");
 }


class MyCanvas extends Canvas
     implements  MouseListener, ActionListener //.. note: inner class
{
  Color linecolor= Color.black; 
  Point begin,end;


  public void paint(Graphics g)
  {
      Dimension d = getSize();
      g.drawRect(2,2,d.width-4,d.height-4);
  }


  public void clear()
 {
   Graphics g = getGraphics();
   Dimension d = getSize();
   Color c = getBackground();
   g.setColor(c);
   g.fillRect(0,0,d.width,d.height);
   repaint();
 }

  public void changeColor()
 {
   Color[] colors =
            {Color.red,Color.black,Color.blue,Color.magenta,Color.green};
   linecolor = colors[random(colors.length)]; 
 }
 
 public int random(int i)
  {
     return (int) Math.floor(Math.random()*i);
  }

  public void drawLine(Point p, Point q)
  {
   Graphics g = getGraphics();
   g.setColor(linecolor);
   g.drawLine(p.x,p.y,q.x,q.y);
   g.drawOval(q.x,q.y,2,2);
  }

 public void drawOval(Point p)
  {
    Graphics g = getGraphics();
    g.drawOval(p.x,p.y,2,2);
  }
   public void mousePressed(MouseEvent me)
 {
    begin = me.getPoint();
    drawOval(begin);
 }
    public void mouseReleased(MouseEvent me )
  {
     end = me.getPoint();
    drawLine(begin,end);
   }

   public void mouseClicked(MouseEvent me)
  {}

   public void mouseEntered(MouseEvent me)
  {}

  public void mouseExited(MouseEvent me)
 {}

  public void actionPerformed(ActionEvent actevt)
 {
   Object arg = actevt.getSource();
   if (arg == clear) clear();
    else if (arg == change) changeColor();
 }
}
}

Event processing, in review

Event sources (controls) are objects that respond to user actions Examples are buttons, scrollbars etc.
An event source sends a message to an event listener.
Any object can be an event listener, if it implements the appropriate interface.
Several objects may respond to the same event, but there is no stipulation about which will respond first.
Currently there are 11 types of event listeners.
Event sources registers its event listeners, via the method add...Listener(..), e.g. addActionListener(this).

Recipe for making object2 responds to events in object1

add to object1 (source) add...Listener(object2). The ... corresponds to the type of event object1 can respond to.
make object2 a Listener, i.e. say
- implements ...Listener
- add the methods designated by ...Listener
- Or, extend an Adapter for the Listener
Some of these complications occur because Java does not support full multiple inheritance.

Controls, Listeners, and interface method

Control	Registers Interest	Receives Event
Button	addActionListener(..)	actionPerformed
List
MenuItem
TextField
Checkbox	addItemListener	itemStateChanged
Choice
List
CheckBoxMenuItem
Dialog Frame	addWindowListener	windowClosing
		windowOpening
		windowIconified
		windowDeiconified
		windowClosed
		windowActivated
		windowDeactivated
Dialog Frame	addComponentListener	componentMoved
		componentHidden
		componentResized
		componentShown
ScrollBar	addAdjustmentListener	adjustmentValueChanged
Canvas	addMouseListener	mousePressed
Dialog		mouseReleased
Frame		mouseEntered
Panel		mouseExited
Window		mouseClicked
Canvas	addMouseMotionListener	mouseDragged
Dialog		mouseMoved
Frame
Panel
Window
Component	addKeyListener	keyPressed
		keyReleased
		keyTyped
Component	addFocusListener	focusGained
Component	addFocusListener	focusLost
TextComponent	addTextListener	textValueChanged

Table Use

The first column names the control.
The second column tells how to awaken control on a particular event and who to send the message to when the event takes place.
The third column tells what method(s) needs to implemented by the object which is responded to the message.
Focus is the component that can accept user input.