Java Swing - Listeners
TOPIC 03 – MOUSEMOTIONLISTENER

LESSON NOTE

INTRO

We use a MouseMotionListener object to listen for mouse movement and dragging. It works in the same way as the MouseListener & KeyListener objects.

MOUSEMOTIONLISTENER INTERFACE

All of the following methods must be implemented inside a class that implements the interface:

public void mouseDragged(MouseEvent e)
public void mouseMoved(MouseEvent e)

Each method relates to an obvious event. Note that both events do not occur at the same time. So, if the mouse is being dragged, then that is not a movement event.

WHAT WILL THIS LOOK LIKE?

public class MyPanelName extends JPanel implements MouseMotionListener

{

//datafields go here

public MyPanelName()

{

//initialize datafields here

//...

addMouseMotionListener(this);

}

public void paint(Graphics g)

{
//display info based on datafields

}

public void mouseMoved (MouseEvent e)

{

//react to event here (usually update datafields)

}

public void mouseDragged(MouseEvent e)

{

//react to event here (usually update datafields)

}

}

EXAMPLE 1 – SIMULATED DRAGGING

The following example will show how to drag an item around the screen. There are limitations to this implementation as we do not currently have access to knowing when the mouse is clicked or released. Two issues arise:

· dragging too quickly will see the object get left behind;

· dragging before going over the object, the object will stick to your cursor.

public class App

{

public static void main(String[] args)

{

JFrame jf = new JFrame();

jf.setTitle("MouseMotionListeners");

jf.setSize(800,600);

DragPanel kp = new DragPanel();

jf.add(kp);

jf.setVisible(true);

}

}

import java.awt.Graphics;

import java.awt.event.MouseEvent;

import java.awt.event.MouseMotionListener;

import javax.swing.JPanel;

public class DragPanel extends JPanel implements MouseMotionListener

{

public int sx; //square's x, y, width and height

public int sy;

public int sw;

public int sh;

public DragPanel()

{

sx = 100;

sy = 100;

sw = 20;

sh = 20;

this.addMouseMotionListener(this);

}

public void paint(Graphics g)

{

super.paint(g);

g.drawRect(sx, sy, sw, sh);

}

public void mouseDragged(MouseEvent e)

{

int ex = e.getX();

int ey = e.getY();

//if dragging occurs over square

if (ex >= sx && ex <= sx + sw && ey >= sy && ey <= sy + sh)

{

sx = e.getX() - sw/2;

sy = e.getY() - sh/2;

repaint();

}

}

public void mouseMoved(MouseEvent e)

{

}

}

USING TWO LISTENER TYPES

To get full dragging implementation, we need to use both MouseMotionListener and MouseListener. We simply implement both in a single class.

PROPPER DRAGGING OF A SHAPE

To properly drag a shape, it must move only when it is clicked on at the very start of the drag and should follow the mouse no matter how fast it moves.

We can now keep track of whether an object is clicked on at the initial point that a click occurs and hold on to it until the mouse is released.

EXAMPLE 2 – PROPPER DRAGGING

Here is an example of a object being dragged correctly.

public class App

{

public static void main(String[] args)

{

JFrame jf = new JFrame();

jf.setTitle("MouseMotionListeners");

jf.setSize(800,600);

DragPanel kp = new DragPanel();

jf.add(kp);

jf.setVisible(true);

}

}

package multi;

import java.awt.Graphics;

import java.awt.event.MouseEvent;

import java.awt.event.MouseListener;

import java.awt.event.MouseMotionListener;

import javax.swing.JPanel;

public class DragPanel extends JPanel implements MouseMotionListener,MouseListener

{

public int sx;

public int sy;

public int sw;

public int sh;

public boolean dragged;

public DragPanel()

{

sx = 100;

sy = 100;

sw = 20;

sh = 20;

dragged = false;

this.addMouseMotionListener(this);

this.addMouseListener(this);

}

public void paint(Graphics g)

{

super.paint(g);

g.drawRect(sx, sy, sw, sh);

}

public void mouseDragged(MouseEvent e)

{

if (dragged == true)

{

sx=e.getX();

sy=e.getY();

repaint();

}

}

public void mouseMoved(MouseEvent e)

{

}

public void mouseClicked(MouseEvent arg0)

{

}

public void mouseEntered(MouseEvent arg0)

{

}

public void mouseExited(MouseEvent arg0)

{

}

public void mousePressed(MouseEvent e)

{

int ex = e.getX();

int ey = e.getY();

//if object clicked on...

if (ex >= sx && ex <= sx + sw && ey >= sy && ey <= sy + sh)

{

dragged = true;

sx = e.getX() - sw/2;

sy = e.getY() - sh/2;

repaint();

}

}

public void mouseReleased(MouseEvent e)

{

dragged = false;

}

}