Scrolling Logic in AWT

[Albert Kam]

Hi all,

I'm currently engaged with developing an AWT Table, which can contain editable cells, and i'm stuck technically in how to enable scrolling.
In horizontal scrolling scenario, i just want to synchronize scrolling the cells data area and scrolls the header area. In vertical scrolling scenario, i want the header row area to stay on top, like the spreadsheet header.

I assume the correct way to enable scrolling in AWT is to do the following :
1 create one panel for header at the top
2 create another panel for the data cells
3 add an invisible horizontal scrollbar for the header panel
4 add visible horizontal and vertical scrollbar for the data panel
5 when scrolled vertically, the header panel is unaffected
6 when scrolled horizontally, the header panel also scrolls, which updates his own graphics

And i have no idea how i'm gonna implement step 6, about how to synchronize the horizontal scrolling. Anyone got a simple example ? Or perhaps there are easier ways to do this ?

Thanks,
Albert Kam

[Craig Wood]

import java.awt.*;
import java.awt.event.*;
import java.awt.font.*;
import java.awt.geom.*;
 
public class TableScroll extends Panel implements AdjustmentListener {
    Object[][] data = new Object[20][10];
    ScrollPane headerScrollPane;
 
    public void adjustmentValueChanged(AdjustmentEvent e) {
        headerScrollPane.setScrollPosition(e.getValue(), 0);
        headerScrollPane.repaint();
    }
 
    public void paint(Graphics g) {
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        int rows = data.length;
        int cols = data[0].length;
        Dimension d = getPreferredSize();
        float cellWidth  = (float)(d.width-1)/cols;
        float cellHeight = (float)(d.height-1)/rows;
        // Draw horizontal lines.
        for(int j = 0; j <= rows; j++) {
            double y = j*cellHeight;
            g2.draw(new Line2D.Double(0, y, d.width-1, y));
        }
        for(int j = 0; j <= cols; j++) {
            double x = j*cellWidth;
            g2.draw(new Line2D.Double(x, 0, x, d.height-1));
        }
        // Draw data.
        Font font = g2.getFont().deriveFont(16f);
        g2.setFont(font);
        FontRenderContext frc = g2.getFontRenderContext();
        LineMetrics lm = font.getLineMetrics("0", frc);
        float height = lm.getAscent() + lm.getDescent();
        for(int j = 0; j < rows; j++) {
            float h = j*cellHeight;
            float y = h + (cellHeight + height)/2 - lm.getDescent();
            for(int k = 0; k < cols; k++) {
                String s = String.valueOf(j*cols + k+1);
                float width = (float)font.getStringBounds(s, frc).getWidth();
                float x = k*cellWidth + (cellWidth - width)/2;
                g2.drawString(s, x, y);
            }
        }
    }
 
    public Dimension getPreferredSize() {
        return new Dimension(500,500);
    }
 
    private Panel getContent() {
        final Dimension d = getPreferredSize();
        ScrollPane scrollPane = new ScrollPane() {
            public Dimension getPreferredSize() {
                Insets i = getInsets();
                int u = i.left + i.right;
                int v = i.top + i.bottom;
                return new Dimension(d.width+u,d.height+v);
            }
        };
        scrollPane.getHAdjustable().addAdjustmentListener(this);
        scrollPane.add(this);
        Panel panel = new Panel(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.gridwidth = GridBagConstraints.REMAINDER;
        gbc.weightx = 1.0;
        gbc.fill = GridBagConstraints.HORIZONTAL;
        panel.add(getHeader(), gbc);
        gbc.weighty = 1.0;
        gbc.fill = GridBagConstraints.BOTH;
        panel.add(scrollPane, gbc);
        return panel;
    }
 
    private ScrollPane getHeader() {
        final int w = getPreferredSize().width;
        final int h = 25;
        Panel panel = new Panel() {
            public void paint(Graphics g) {
                Graphics2D g2 = (Graphics2D)g;
                g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                                    RenderingHints.VALUE_ANTIALIAS_ON);
                int cols = data[0].length;
                float cellWidth = (float)(w-1)/cols;
                g2.drawLine(0,0,w-1,0);
                g2.drawLine(0,h-1,w-1,h-1);
                for(int j = 0; j <= cols; j++) {
                    double x = j*cellWidth;
                    g2.draw(new Line2D.Double(x, 0, x, h-1));
                }
                String ids = "abcdefghijklmnopqrstuvwxyz";
                Font font = g2.getFont().deriveFont(16f);
                g2.setFont(font);
                FontRenderContext frc = g2.getFontRenderContext();
                LineMetrics lm = font.getLineMetrics("0", frc);
                float height = lm.getAscent() + lm.getDescent();
                for(int j = 0; j < cols; j++) {
                    String s = String.valueOf(ids.charAt(j));
                    float width = (float)font.getStringBounds(s, frc).getWidth();
                    float x = j*cellWidth + (cellWidth - width)/2;
                    float y = (h + height)/2 - lm.getDescent();
                    g2.drawString(s, x, y);
                }
            }
 
            public Dimension getPreferredSize() {
                return new Dimension(w,h);
            }
        };
        headerScrollPane = new ScrollPane(ScrollPane.SCROLLBARS_NEVER) {
            public Dimension getPreferredSize() {
                Insets i = getInsets();
                int v = i.top + i.bottom;
                int width = 350;
                return new Dimension(width,h+v);
            }
 
            public Dimension getMinimumSize() {
                return getPreferredSize();
            }
 
            public Dimension getViewportSize() {
                return getPreferredSize();
            }
        };
        headerScrollPane.add(panel);
        return headerScrollPane;
    }
 
    public static void main(String[] args) {
        TableScroll panel = new TableScroll();
        Frame f = new Frame("TableScroll");
        f.addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });
        f.add(panel.getContent());
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}

[Albert Kam]

Thank you very much ! It runs cool !
I'm gonna study the code to get it's logic