如何在鼠标移动时重绘Java SWT应用程序？

Question

I am trying to write a simple app that create a transparent overlay over the full screen and draw a line at my current mouse position. This line should then follow my mouse movements until mouse press when the app is to exit.

I currently have an issue with redraw in the app that will not work and I guess that I have missplaced and/or missunderstood how the redraw should be used.

How to redraw an application like this correctly and resource efficient?

Sample code:

public class TAGuideLine {

    static Display display = new Display();
    static Shell shell = new Shell(display, SWT.NO_TRIM | SWT.ON_TOP);

    public static void main(String[] args) {

        shell.setBackground(display.getSystemColor(SWT.COLOR_RED));
        shell.setMaximized(true);
        shell.setFullScreen(true);
        shell.setLayoutData(0);
        shell.layout(true, true);
        shell.setAlpha(50);
        shell.addListener(SWT.MouseDown, new Listener() {
            public void handleEvent(Event e) {
                System.exit(0);
            }
        });
        shell.addListener(SWT.MouseMove, new Listener() {
            public void handleEvent(Event e) {
                drawMyLine(MouseInfo.getPointerInfo().getLocation().x,
                        MouseInfo.getPointerInfo().getLocation().y);
            }
        });
        shell.open();
        while (!shell.isDisposed()) {
            if (!display.readAndDispatch())
                display.sleep();
        }
        shell.redraw();
        shell.layout();
        display.dispose();
    }

    public static void drawMyLine(int x, int y) {
        final GC gc = new GC(shell);
        gc.setForeground(display.getSystemColor(SWT.COLOR_GREEN));
        gc.setLineWidth(8);
        gc.drawLine(x - 250, y - 250, x + 250, y + 250);
        gc.dispose();
        shell.open();
    }
}

Answer 1

To draw on a Shell you usually add a paint listener that does the actual drawing. The mouse event listener would just store the coordinates to draw and then trigger a redraw on the shell.

Below is a sketch of what the code could look like:

List<Point> points = new ArrayList<>();

shell.addListener( SWT.Paint, new Listener() {
  public void handleEvent( Event event ) {
    event.gc.setForeground( display.getSystemColor( SWT.COLOR_GREEN ) );
    event.gc.setLineWidth( 8 );
    for( Point point : points ) {
      event.gc.drawLine( point.x - 250, point.y - 250, point.x + 250, point.y + 250 );
  }
} );

shell.addListener( SWT.MouseMove, new Listener() {
  public void handleEvent( Event event ) {
    points.add( MouseInfo.getPointerInfo().getLocation() );
    shell.redraw();
  }
} );

Class GC also has a drawPolyLine() method that might be more suitable for this use case.

Unrelated to your question but still: a more graceful way to exit the application is to dispose of the Shell with shell.dispose() instead of calling System.exit() .