Java SWT - Canvas: why delete images ?

[zildjian]

Hello,
I have a problem.
I use SWT to make GUI.
In the main window I have drawed some images (canvas, gc on canvas) and pressing a button will appear a new window. When I I move the new window over the main window the images will disappear. Why ?

Sorry for my english :)
I hope that you will understand.

Thank you
Carmine

Ps: if you want i can send an image

[r035198x]

Hello,
I have a problem.
I use SWT to make GUI.
In the main window I have drawed some images (canvas, gc on canvas) and pressing a button will appear a new window. When I I move the new window over the main window the images will disappear. Why ?

Sorry for my english :)
I hope that you will understand.

Thank you
Carmine

Ps: if you want i can send an image

Let's see the code.

[zildjian]

public class grafica extends javax.swing.JFrame {
Shell shell;
Text text;
//GC gc;
public grafica() throws IOException, Exception {
Menu menuBar, fileMenu, helpMenu;
MenuItem fileMenuHeader, aboutMenuHeader;
MenuItem fileExitItem, aboutAboutItem;

display = new Display();
shell = new Shell(display);
shell.setText("Composer");

menuBar = new Menu(shell, SWT.BAR);
fileMenuHeader = new MenuItem(menuBar, SWT.CASCADE);
fileMenuHeader.setText("&File");
fileMenu = new Menu(shell, SWT.DROP_DOWN);
fileMenuHeader.setMenu(fileMenu);

MenuItem expertItem = new MenuItem(fileMenu, SWT.CASCADE);
expertItem.setText("&Expert");
Menu submenu = new Menu(shell, SWT.DROP_DOWN);
expertItem.setMenu(submenu);
MenuItem expertParseInItem = new MenuItem(submenu, SWT.PUSH);
expertParseInItem.setText("Parse Input\tCTRL+P");
expertParseInItem.setAccelerator(SWT.CTRL + 'P');
MenuItem dialogItem = new MenuItem(submenu, SWT.PUSH);
dialogItem.setText("&Parse Output\tCTRL+O");
dialogItem.setAccelerator(SWT.CTRL + 'O');
MenuItem separator = new MenuItem(fileMenu, SWT.SEPARATOR);

fileExitItem = new MenuItem(fileMenu, SWT.PUSH);
fileExitItem.setText("E&xit\tCTRL+C");
fileExitItem.setAccelerator(SWT.CTRL + 'C');

aboutMenuHeader = new MenuItem(menuBar, SWT.CASCADE);
aboutMenuHeader.setText("Info");
helpMenu = new Menu(shell,SWT.DROP_DOWN);
aboutMenuHeader.setMenu(helpMenu);
aboutAboutItem = new MenuItem(helpMenu, SWT.PUSH);
aboutAboutItem.setText("Info");

fileExitItem.addSelectionListener(new MenuItemListener());
aboutAboutItem.addSelectionListener(new MenuItemListener());
expertParseInItem.addSelectionListener(new MenuItemListener());

Rectangle screen = display.getMonitors()[0].getBounds();

shell.setMenuBar(menuBar);
// Creazione ToolBar
Image image = new Image(display,"img/Parse.jpg");
Image image2 = new Image(display,"img/Help.jpg");
//GC gc = new GC(image);
//GC gc2 = new GC(image2);
//gc.dispose();
//gc2.dispose();
ToolBar toolBar = new ToolBar(shell, SWT.NO);

ToolItem itemParse = new ToolItem(toolBar, SWT.PUSH);
itemParse.setImage(image);
itemParse.setText("Parse In");
ToolItem itemHelp = new ToolItem(toolBar, SWT.PUSH);
itemHelp.setImage(image2);
itemHelp.setText("Help");
//itemHelp.addSelectionListener(new ToolBarButtonListener());


itemParse.addSelectionListener(new SelectionAdapter() {
public void widgetSelected(SelectionEvent e2) {
System.out.println("E' stato premuto il tasto ParseIn");
RWmidi midi = null;
try {
midi = new RWmidi("midi.mid");
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Parser parser = new Parser(midi);
String eventi = parser.getParsingTraccia(1);
Shell dialog = new Shell(shell);
dialog.setText("Dialog");
Text text4 = new Text(dialog, SWT.BORDER | SWT.H_SCROLL | SWT.V_SCROLL);
text4.setBounds(0,0,800,500);
text4.setEditable(false);
text4.setText(eventi);
dialog.setSize(800, 500);
dialog.open();
}
});



itemHelp.addSelectionListener(new SelectionAdapter() {
public void widgetSelected(SelectionEvent e) {
System.out.println("E' stato premuto il tasto Help");
MessageBox aboutBox = new MessageBox(shell,SWT.ICON_INFORMATION);
aboutBox.setText("Informazioni");
StringBuffer messaggio = new StringBuffer();
messaggio.append("Composer Midi\n");
messaggio.append("Programma sviluppato da Fedullo Carmine\n");
messaggio.append("\n");
messaggio.append("Per ulteriori informazioni\n");
messaggio.append("[e-mail]\n");
aboutBox.setMessage(messaggio.toString());
int buttonOK = aboutBox.open();
//shell.pack();
shell.open();
}
});
toolBar.pack();

// Creazione Separatore
Label separatore = new Label(shell, SWT.SEPARATOR | SWT.HORIZONTAL);
separatore.setBounds(0,83,1000,20);

// Creazione Cavas
Canvas canvas = new Canvas(shell, SWT.BORDER);
canvas.setSize(950, 500);
canvas.setLocation(20, 120);

Point size = shell.computeSize(-1,-1);
shell.setBounds(0, 0, 0, 0);
//shell.setSize(screen.width,screen.height-30);
shell.setSize(1000,700);
shell.open();
Color black = display.getSystemColor(SWT.COLOR_BLACK);
Color white = display.getSystemColor(SWT.COLOR_WHITE);
//canvas.setBackground(white);
//canvas.setBackgroundMode(2);
gc = new GC(canvas);


//canvas.setBackground(white);

gc.setBackground(white);
gc.setForeground(black);
gc.fillRectangle(0, 0, 1000, 900);

//gc.setLineWidth(2);
// Costruzione Pentagramma
gc.drawLine(30, 50, 930, 50);
//gc.drawLine(30, 60, 930, 60);
//gc.drawLine(30, 70, 930, 70);
//gc.drawLine(30, 80, 930, 80);
//gc.drawLine(30, 90, 930, 90);
costruisciPentagramma(50,0);
costruisciPentagramma(120,0);
costruisciPentagramma(190,0);
costruisciPentagramma(260,0);
costruisciPentagramma(330,0);
costruisciPentagramma(400,0);
gc.dispose();
//canvas.redraw();
// shell.pack();


while(!shell.isDisposed())
if(!display.readAndDispatch())
display.sleep();
display.dispose();

}
/*
class ToolBarButtonListener extends SelectionAdapter {
public void widgetSelected(SelectionEvent event) {
if(event.widget.getData().)
shell.close();
System.exit(0);
}
}
*/

public void costruisciPentagramma(int altezza, int chiave) {
// Chiave di Basso
if(chiave==0) {
Image img = new Image(display, "img/chiavebasso80.jpg");
gc.drawImage(img, 25, altezza-5);
}
gc.drawLine(30, altezza, 30, altezza+40);
gc.drawLine(30, altezza, 930, altezza);
gc.drawLine(30, altezza+10, 930, altezza+10);
gc.drawLine(30, altezza+20, 930, altezza+20);
gc.drawLine(30, altezza+30, 930, altezza+30);
gc.drawLine(30, altezza+40, 930, altezza+40);
gc.drawLine(300, altezza,300 ,altezza+40);
gc.drawLine(510, altezza,510 ,altezza+40);
gc.drawLine(720, altezza,720 ,altezza+40);
gc.drawLine(930, altezza,930 ,altezza+40);

}
public void newWindow(String titolo, String messaggio) {
Shell dialog = new Shell(shell);
dialog.setText("Dialog");
Text text4 = new Text(dialog, SWT.BORDER | SWT.H_SCROLL | SWT.V_SCROLL);
text4.setBounds(0,0,800,500);
text4.setEditable(false);
text4.setText("ciao provola affumicata");
dialog.setSize(800, 500);
dialog.open();
}
class MenuItemListener extends SelectionAdapter {
public void widgetSelected(SelectionEvent event) {
if (((MenuItem) event.widget).getText().equals("E&xit\tCTRL+C")) {
shell.close();
System.exit(0);
}
if (((MenuItem) event.widget).getText().equals("Info")) {
MessageBox aboutBox = new MessageBox(shell,SWT.ICON_INFORMATION);
aboutBox.setText("Informazioni");
StringBuffer messaggio = new StringBuffer();
messaggio.append("Composer Midi\n");
messaggio.append("Programma sviluppato da Fedullo Carmine\n");
messaggio.append("\n");
messaggio.append("Per ulteriori informazioni\n");
messaggio.append("[e-mail]\n");
aboutBox.setMessage(messaggio.toString());
int buttonOK = aboutBox.open();
//shell.pack();
shell.open();
System.out.println("premuto help");
}
if (((MenuItem) event.widget).getText().equals("Parse Input\tCTRL+P")) {
RWmidi midi = null;
try {
midi = new RWmidi("midi.mid");
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Parser parser = new Parser(midi);
String eventi = parser.getParsingTraccia(1);
Shell dialog = new Shell(shell);
dialog.setText("Dialog");
Text text4 = new Text(dialog, SWT.BORDER | SWT.H_SCROLL | SWT.V_SCROLL);
text4.setBounds(0,0,800,500);
text4.setEditable(false);
text4.setText(eventi);
dialog.setSize(800, 500);
dialog.open();


}
}
}
private GC gc;
private Display display;
}

[zildjian]

This occur even when I move another application window over images as you can see by attached picture.

[JosAH]

Either use Swing components or SWT components but not both.

kind regards,

Jos

[zildjian]

Either use Swing components or SWT components but not both.

kind regards,

Jos

Dear Jos,
I use only SWT in my code.
Where do you see Swing components ?

Thank you,
Carmine

[r035198x]

Dear Jos,
I use only SWT in my code.
Where do you see Swing components ?

Thank you,
Carmine

What does your class extend?

[zildjian]

What does your class extend?

mmm ... sorry I am distract :)

and now I must change all code ?
What is the class to extend about SWT ?

I have deleted "extends javax.swing.JFrame" but it is the same about my problem.

[r035198x]

mmm ... sorry I am distract :)

and now I must change all code ?
What is the class to extend about SWT ?

I have deleted "extends javax.swing.JFrame" but it is the same about my problem.

If you want to use AWT then you use Frame instead of JFrame. If you want to use swing then there's lots of things you want to change in your code. You have read something about AWT vs swing haven't you? You have to use the right tool for the right job.

[JosAH]

If you want to use AWT then you use Frame instead of JFrame. If you want to use swing then there's lots of things you want to change in your code. You have read something about AWT vs swing haven't you? You have to use the right tool for the right job.

It's a bit more complicated here: the OP wants to use SWT, not AWT; I guess
that some of the imports in the source are incorrect; we can't know because we
haven't seen the imports.

kind regards,

Jos

[zildjian]

It's a bit more complicated here: the OP wants to use SWT, not AWT; I guess
that some of the imports in the source are incorrect; we can't know because we
haven't seen the imports.

kind regards,

Jos

Sorry if my question is stupid but I am learning SWT only since yesterday.

My imports are:

import java.io.IOException;
import org.eclipse.swt.events.SelectionAdapter;
import org.eclipse.swt.events.SelectionEvent;
import org.eclipse.swt.graphics.Color;
import org.eclipse.swt.graphics.GC;
import org.eclipse.swt.graphics.Image;
import org.eclipse.swt.graphics.Point;
import org.eclipse.swt.graphics.Rectangle;
import org.eclipse.swt.widgets.*;
import org.eclipse.swt.*;

I have deleted "extends java.awt.Frame" and I have the same previous fault.

Thank you
Carmine

[r035198x]

It's a bit more complicated here: the OP wants to use SWT, not AWT; I guess
that some of the imports in the source are incorrect; we can't know because we
haven't seen the imports.

kind regards,

Jos

Yep, I realized after posting that it's SWT not AWT that they want.

[zildjian]

Dear Sirs,
please can you confirm that I am using only SWT ?

[r035198x]

Dear Sirs,
please can you confirm that I am using only SWT ?

javax.swing.JFrame is swing.

[JosAH]

javax.swing.JFrame is swing.

Have a look at reply #11; all the Swing stuff is gone now; I gotta run now; I'll look
at this thread later.

kind regards,

Jos

[zildjian]

javax.swing.JFrame is swing.

Now this is the code and I think that there is only SWT

import java.io.IOException;

import org.eclipse.swt.events.SelectionAdapter;
import org.eclipse.swt.events.SelectionEvent;
import org.eclipse.swt.graphics.Color;
import org.eclipse.swt.graphics.GC;
import org.eclipse.swt.graphics.Image;
import org.eclipse.swt.graphics.Point;
import org.eclipse.swt.graphics.Rectangle;
import org.eclipse.swt.widgets.*;
import org.eclipse.swt.*;

public class grafica {
Shell shell;
Text text;
//GC gc;
public grafica() throws IOException, Exception {
Menu menuBar, fileMenu, helpMenu;
MenuItem fileMenuHeader, aboutMenuHeader;
MenuItem fileExitItem, aboutAboutItem;

display = new Display();
shell = new Shell(display);
shell.setText("Composer");

menuBar = new Menu(shell, SWT.BAR);
fileMenuHeader = new MenuItem(menuBar, SWT.CASCADE);
fileMenuHeader.setText("&File");
fileMenu = new Menu(shell, SWT.DROP_DOWN);
fileMenuHeader.setMenu(fileMenu);

MenuItem expertItem = new MenuItem(fileMenu, SWT.CASCADE);
expertItem.setText("&Expert");
Menu submenu = new Menu(shell, SWT.DROP_DOWN);
expertItem.setMenu(submenu);
MenuItem expertParseInItem = new MenuItem(submenu, SWT.PUSH);
expertParseInItem.setText("Parse Input\tCTRL+P");
expertParseInItem.setAccelerator(SWT.CTRL + 'P');
MenuItem dialogItem = new MenuItem(submenu, SWT.PUSH);
dialogItem.setText("&Parse Output\tCTRL+O");
dialogItem.setAccelerator(SWT.CTRL + 'O');
MenuItem separator = new MenuItem(fileMenu, SWT.SEPARATOR);

fileExitItem = new MenuItem(fileMenu, SWT.PUSH);
fileExitItem.setText("E&xit\tCTRL+C");
fileExitItem.setAccelerator(SWT.CTRL + 'C');

aboutMenuHeader = new MenuItem(menuBar, SWT.CASCADE);
aboutMenuHeader.setText("Info");
helpMenu = new Menu(shell,SWT.DROP_DOWN);
aboutMenuHeader.setMenu(helpMenu);
aboutAboutItem = new MenuItem(helpMenu, SWT.PUSH);
aboutAboutItem.setText("Info");

fileExitItem.addSelectionListener(new MenuItemListener());
aboutAboutItem.addSelectionListener(new MenuItemListener());
expertParseInItem.addSelectionListener(new MenuItemListener());
/*
Text text1 = new Text(shell, SWT.BORDER);
text1.setText("Scrivi");
text1.setBounds(50,100,300,20);
text1.setTextLimit(30);
Text text2 = new Text(shell, SWT.NONE);
text2.setEchoChar('*');
text2.setBounds(50,150,300,20);
text2.setText("Password");
Text text3 = new Text(shell, SWT.BORDER | SWT.H_SCROLL | SWT.V_SCROLL);
text3.setBounds(50,190,300,100);
text3.setEditable(false);
text3.setText("Qui non puoi scrivere");

*/
Rectangle screen = display.getMonitors()[0].getBounds();

shell.setMenuBar(menuBar);
// Creazione ToolBar
Image image = new Image(display,"img/Parse.jpg");
Image image2 = new Image(display,"img/Help.jpg");
//GC gc = new GC(image);
//GC gc2 = new GC(image2);
//gc.dispose();
//gc2.dispose();
ToolBar toolBar = new ToolBar(shell, SWT.NO);

ToolItem itemParse = new ToolItem(toolBar, SWT.PUSH);
itemParse.setImage(image);
itemParse.setText("Parse In");
ToolItem itemHelp = new ToolItem(toolBar, SWT.PUSH);
itemHelp.setImage(image2);
itemHelp.setText("Help");
//itemHelp.addSelectionListener(new ToolBarButtonListener());

itemParse.addSelectionListener(new SelectionAdapter() {
public void widgetSelected(SelectionEvent e2) {
System.out.println("E' stato premuto il tasto ParseIn");
RWmidi midi = null;
try {
midi = new RWmidi("midi.mid");
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Parser parser = new Parser(midi);
String eventi = parser.getParsingTraccia(1);
Shell dialog = new Shell(shell);
dialog.setText("Dialog");
Text text4 = new Text(dialog, SWT.BORDER | SWT.H_SCROLL | SWT.V_SCROLL);
text4.setBounds(0,0,800,500);
text4.setEditable(false);
text4.setText(eventi);
dialog.setSize(800, 500);
dialog.open();
}
});

itemHelp.addSelectionListener(new SelectionAdapter() {
public void widgetSelected(SelectionEvent e) {
System.out.println("E' stato premuto il tasto Help");
MessageBox aboutBox = new MessageBox(shell,SWT.ICON_INFORMATION);
aboutBox.setText("Informazioni");
StringBuffer messaggio = new StringBuffer();
messaggio.append("Composer Midi\n");
messaggio.append("Programma sviluppato da Fedullo Carmine\n");
messaggio.append("\n");
messaggio.append("Per ulteriori informazioni\n");
messaggio.append("[e-mail]\n");
aboutBox.setMessage(messaggio.toString());
int buttonOK = aboutBox.open();
//shell.pack();
shell.open();
}
});
toolBar.pack();

// Creazione Separatore
Label separatore = new Label(shell, SWT.SEPARATOR | SWT.HORIZONTAL);
separatore.setBounds(0,83,1000,20);

// Creazione Cavas
Canvas canvas = new Canvas(shell, SWT.BORDER);
canvas.setSize(950, 500);
canvas.setLocation(20, 120);

Point size = shell.computeSize(-1,-1);
shell.setBounds(0, 0, 0, 0);
//shell.setSize(screen.width,screen.height-30);
shell.setSize(1000,700);
shell.open();
Color black = display.getSystemColor(SWT.COLOR_BLACK);
Color white = display.getSystemColor(SWT.COLOR_WHITE);
//canvas.setBackground(white);
//canvas.setBackgroundMode(2);
gc = new GC(canvas);


//canvas.setBackground(white);

gc.setBackground(white);
gc.setForeground(black);
gc.fillRectangle(0, 0, 1000, 900);

//gc.setLineWidth(2);
// Costruzione Pentagramma
gc.drawLine(30, 50, 930, 50);
//gc.drawLine(30, 60, 930, 60);
//gc.drawLine(30, 70, 930, 70);
//gc.drawLine(30, 80, 930, 80);
//gc.drawLine(30, 90, 930, 90);
costruisciPentagramma(50,0);
costruisciPentagramma(120,0);
costruisciPentagramma(190,0);
costruisciPentagramma(260,0);
costruisciPentagramma(330,0);
costruisciPentagramma(400,0);
gc.dispose();
//canvas.redraw();
// shell.pack();


while(!shell.isDisposed())
if(!display.readAndDispatch())
display.sleep();
display.dispose();

}
/*
class ToolBarButtonListener extends SelectionAdapter {
public void widgetSelected(SelectionEvent event) {
if(event.widget.getData().)
shell.close();
System.exit(0);
}
}
*/

public void costruisciPentagramma(int altezza, int chiave) {
// Chiave di Basso
if(chiave==0) {
Image img = new Image(display, "img/chiavebasso80.jpg");
gc.drawImage(img, 25, altezza-5);
}
gc.drawLine(30, altezza, 30, altezza+40);
gc.drawLine(30, altezza, 930, altezza);
gc.drawLine(30, altezza+10, 930, altezza+10);
gc.drawLine(30, altezza+20, 930, altezza+20);
gc.drawLine(30, altezza+30, 930, altezza+30);
gc.drawLine(30, altezza+40, 930, altezza+40);
gc.drawLine(300, altezza,300 ,altezza+40);
gc.drawLine(510, altezza,510 ,altezza+40);
gc.drawLine(720, altezza,720 ,altezza+40);
gc.drawLine(930, altezza,930 ,altezza+40);

}
public void newWindow(String titolo, String messaggio) {
Shell dialog = new Shell(shell);
dialog.setText("Dialog");
Text text4 = new Text(dialog, SWT.BORDER | SWT.H_SCROLL | SWT.V_SCROLL);
text4.setBounds(0,0,800,500);
text4.setEditable(false);
text4.setText("ciao provola affumicata");
dialog.setSize(800, 500);
dialog.open();
}
class MenuItemListener extends SelectionAdapter {
public void widgetSelected(SelectionEvent event) {
if (((MenuItem) event.widget).getText().equals("E&xit\tCTRL+C")) {
shell.close();
System.exit(0);
}
if (((MenuItem) event.widget).getText().equals("Info")) {
MessageBox aboutBox = new MessageBox(shell,SWT.ICON_INFORMATION);
aboutBox.setText("Informazioni");
StringBuffer messaggio = new StringBuffer();
messaggio.append("Composer Midi\n");
messaggio.append("Programma sviluppato da Fedullo Carmine\n");
messaggio.append("\n");
messaggio.append("Per ulteriori informazioni\n");
messaggio.append("[e-mail]\n");
aboutBox.setMessage(messaggio.toString());
int buttonOK = aboutBox.open();
//shell.pack();
shell.open();
System.out.println("premuto help");
}
if (((MenuItem) event.widget).getText().equals("Parse Input\tCTRL+P")) {
RWmidi midi = null;
try {
midi = new RWmidi("midi.mid");
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
Parser parser = new Parser(midi);
String eventi = parser.getParsingTraccia(1);
Shell dialog = new Shell(shell);
dialog.setText("Dialog");
Text text4 = new Text(dialog, SWT.BORDER | SWT.H_SCROLL | SWT.V_SCROLL);
text4.setBounds(0,0,800,500);
text4.setEditable(false);
text4.setText(eventi);
dialog.setSize(800, 500);
dialog.open();
}
}
}
private GC gc;
private Display display;
}

[r035198x]

1.) Please use code tags everytime for posting code.
2.) Shouldn't your class be extending Frame? What happens when you try it with SWT only?

[zildjian]

1.) Please use code tags everytime for posting code.
2.) Shouldn't your class be extending Frame? What happens when you try it with SWT only?

1) Ok, sorry...I will use Code tag in the future
2) No, I don't extend Frame Class and when I run the program it work normally but with the same problem.

Early I write this program with swing and after I changed to SWT for this reason I forgot to delete the "extending ....." text.
But all code is only with SWT as you can see.
I have read various manual and tutorial about SWT, and it is not necessary to extend any class. You can use SWT widgets only importing the right packages.
This is Hello World Exemple Code:

1 import org.eclipse.swt.*;
2 import org.eclipse.swt.graphics.*;
3 import org.eclipse.swt.widgets.*;
 
4 public class HelloWorld {
5 public static void main(String[] args){
6 Display display = new Display();
7 Shell shell = new Shell(display);
8 shell.setText("Hello World");
9 shell.setSize(200,100);
10 shell.open ();
11 while (!shell.isDisposed()) {
12 if (!display.readAndDispatch())
13 display.sleep ();
14 }
15 display.dispose ();
16 }
17 }

Thank you
Carmine

[JosAH]

Dear Sirs,
please can you confirm that I am using only SWT ?

Yep, you're not using any Swing components at all. I had a look at your code
but my SWT knowledge is minimal. I suggest that you strip your application as
much as possible (eg. leave out the Toolbar and the Menubar etc. etc.) until
you end up with a barebones application that either works fine (i.e. it redraws
that image properly) or it still exhibits the same error when the image needs
to be redrawn.

Now it's too much code to plough through.

kind regards,

Jos

[valce]

I know this thread is oooold but it was the first thread I found on Google when I had the same issue. The OP (and myself) were using the wrong way to draw on a canvas. In order to have stuff on the canvas stick around after a repaint, you should add draw functions to the a paintlistener on the canvas, e.g:

canvas.addPaintListener(new PaintListener() {
public void paintControl(PaintEvent e) {
GC gc = new GC(c);
drawStuff(gc);
gc.dispose();
}
});

Hopefully the next person who runs into this error and stumbles here will find it useful :)

-C