Text canvas vertical scroll

[Dmitry Danilov]

Hello,

I have an application that draws strings of text on a canvas doing word wrapping and breaking whenever necessary. Canvas' height is initially zero and grows as strings are drawn onto it. The final canvas' height is calculated as a last step of the paint() method. Vertical text scrolling has to be implemented as well, for which purpose I'm using ScrollPane. I'm experiencing one problem here: it looks like I cannot dynamically set the canvas' height by the time all strings have been drawn on it. The getPreferredSize() method is called by the layout manager when the canvas class is instantiated and overriding the method doesn't help much.
The only way to make scrolling work is to resize the application frame as soon as it's created at least one pixel in any direction. Then apparently the getPreferredSize() method is called again and the updated canvas' size is set.
Using setSize() explicitly doesn't help for some reason...
Resizing the frame seems to fire some hidden underlying mechanism which sets the updated size of the canvas and let the scrolling work perfectly...
Another way to make it work is to set preferred size to a constant value, eg.

public Dimension getPreferredSize() {
 return new Dimension(200, 828);
}

Both workarounds are obviously very weird and cannot be used seriously.

I'm totally stuck here, tried hundreds of different tricks, but still can't help it...

Anybody ever experienced something like this?

P.S. Main app frame class extends Frame, text canvas class extends Canvas. Default layout manager (BorderLayout) is used to layout the ScrollPane with text canvas.

[Craig Wood]

import java.awt.*;
import java.awt.event.*;
import java.awt.font.*;
import java.awt.geom.*;
 
public class Sizing
{
    public static void main(String[] args)
    {
        GraphicSizedPanel sizedPanel = new GraphicSizedPanel();
        ScrollPane scrollPane = new ScrollPane();
        scrollPane.add(sizedPanel);
        Frame f = new Frame();
        f.addWindowListener(new WindowAdapter()
        {
            public void windowClosing(WindowEvent e)
            {
                System.exit(0);
            }
        });
        f.add(sizedPanel.getUIPanel(), "North");
        f.add(scrollPane);
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}
 
class GraphicSizedPanel extends Canvas
{
    String[] lines = {
        "A container class which implements automatic ",
        "horizontal and/or vertical scrolling for a ",
        "single child component. ",
        "The display policy for the scrollbars can be set to:"
    };
    Font font;
    int PAD, reps;
    float maxWidth, y;
 
    public GraphicSizedPanel()
    {
        font = new Font("lucida bright regular", Font.PLAIN, 18);
        PAD = 20;
        reps = 0;
    }
 
    public void paint(Graphics g)
    {
        super.paint(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setFont(font);
        FontRenderContext frc = g2.getFontRenderContext();
        float x = PAD;
        maxWidth = getWidth();
        y = 0;
        for(int j = 0; j < reps * lines.length; j++)
        {
            String s = lines[j % lines.length];
            float w = (float)font.getStringBounds(s, frc).getWidth() + 2*PAD;
            if(w > maxWidth)
                maxWidth = w;
            LineMetrics lm = font.getLineMetrics(s, frc);
            y += lm.getHeight();
            g2.drawString(s, x, y);
        }
    }
 
    public void update(Graphics g)
    {
        super.update(g);
        invalidate();
        getScrollPane().validate();
    }
 
    public Dimension getPreferredSize()
    {
        Insets insets = getScrollPane().getInsets();
        int width = (int)maxWidth;
        int height = (int)y + insets.top + insets.bottom;
        return new Dimension(width, height);
    }
 
    private ScrollPane getScrollPane()
    {
        ScrollPane scrollPane = null;
        Component c = this;
        while((c = c.getParent()) != null)
            if(c instanceof ScrollPane)
                scrollPane = (ScrollPane)c;
        return scrollPane;
    }
 
    public Panel getUIPanel()
    {
        Button addLines = new Button("add lines");
        addLines.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent e)
            {
                reps++;
                repaint();
            }
        });
        Panel panel = new Panel();
        panel.add(addLines);
        return panel;
    }
}   

[Dmitry Danilov]

Wow!..
This has to be digested...
Thanks a lot for the reply, Craig!