Subpages: 1. JScrollPane
2. Grab-and-drag scrolling
3. Scrolling programmatically
7.2 Grab-and-drag scrolling
Many paint programs and document readers (such as Adobe Acrobat) support grab-and-drag scrolling. This is the ability to click on an image and drag it in any direction with the mouse. It is fairly simple to implement, however, we must take care to make the operation smooth without allowing the view to be scrolled past its extremities. JViewport takes care of the negative direction for us, as it does not allow the view position coordinates to be less than 0. But it will allow us to change the view position to very large values, which can result in the viewport displaying a portion of the view smaller than the viewport itself.
Note: As of JDK1.2.2 we are allowed to specify negative view position coordinates.
We've modified the simple example from the beginning of this chapter to support grab-and-drag scrolling.
The Code: GrabAndDragDemo.java
see \Chapter7\4
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
public class GrabAndDragDemo extends JFrame
{
public GrabAndDragDemo() {
super("Grab-and-drag Demo");
ImageIcon ii = new ImageIcon("earth.jpg");
JScrollPane jsp = new JScrollPane(new GrabAndScrollLabel(ii));
getContentPane().add(jsp);
setSize(300,250);
setVisible(true);
WindowListener l = new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
};
addWindowListener(l);
}
public static void main(String[] args) {
new GrabAndDragDemo();
}
}
class GrabAndScrollLabel extends JLabel
{
public GrabAndScrollLabel(ImageIcon i){
super(i);
MouseInputAdapter mia = new MouseInputAdapter() {
int m_XDifference, m_YDifference;
Container c;
public void mouseDragged(MouseEvent e) {
c = GrabAndScrollLabel.this.getParent();
if (c instanceof JViewport) {
JViewport jv = (JViewport) c;
Point p = jv.getViewPosition();
int newX = p.x - (e.getX()-m_XDifference);
int newY = p.y - (e.getY()-m_YDifference);
int maxX = GrabAndScrollLabel.this.getWidth()
- jv.getWidth();
int maxY = GrabAndScrollLabel.this.getHeight()
- jv.getHeight();
if (newX < 0)
newX = 0;
if (newX > maxX)
newX = maxX;
if (newY < 0)
newY = 0;
if (newY > maxY)
newY = maxY;
jv.setViewPosition(new Point(newX, newY));
}
}
public void mousePressed(MouseEvent e) {
setCursor(Cursor.getPredefinedCursor(
Cursor.MOVE_CURSOR));
m_XDifference = e.getX();
m_YDifference = e.getY();
}
public void mouseReleased(MouseEvent e) {
setCursor(Cursor.getPredefinedCursor(
Cursor.DEFAULT_CURSOR));
}
};
addMouseMotionListener(mia);
addMouseListener(mia);
}
}
Understanding the Code:
Class GrabAndScrollLabel
This class extends JLabel and overrides the JLabel(Imageicon ii) constructor. The GrabAndScrollLabel constructor starts by calling the super class version and then proceeds to set up a MouseInputAdapter. This adapter is the heart of the GrabAndScrollLabel class.
The adapter uses three variables:
int m_XDifference: x coordinate saved on a mouse press event and used for dragging horizontally
int m_YDifference: y coordinate saved on a mouse press event and used for dragging vertically
Container c: used to hold a local reference to the parent container in the mouseDragged() method.
The mousePressed() method changes the cursor to MOVE_CURSOR, and stores the event coordinates in variables m_XDifference and m_YDifference to be use in mouseDragged().
The mouseDragged() method first grabs a reference to the parent and checks if it is a JViewport. If it isn't we do nothing. If it is we store the current view position and calculate the new view position the drag will bring us into:
Point p = jv.getViewPosition();
int newX = p.x - (e.getX()-m_XDifference);
int newY = p.y - (e.getY()-m_YDifference);
When dragging components, normally this would be enough (as we will see in future chapters), however we must make sure that we do not move this label in such a way that the viewport is not filled by it. So we calculate the maximum allowable x and y coordinates by subtracting the viewport dimensions from the size of this label (since the view position coordinates are upper-left hand corner):
int maxX = GrabAndScrollLabel.this.getWidth()
- jv.getWidth();
int maxY = GrabAndScrollLabel.this.getHeight()
- jv.getHeight();
The remainder of this method compares the newX and newY values with the maxX and maxY values, and adjusts the view position accordingly. If newX or newY is ever greater than the maxX or maxY values respectively, we use the max values instead. If newX or newY is ever less than 0 (which can happen only with JDK1.2.2), we use 0 instead. This is necessary to allow smooth scrolling in all situations.
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