Scrolling Canvas in eclipse plugin doesnt work (blurrs)

Question

I am developing a plugin for eclipse in which i want to draw some images into a custom view. Creating the view and my layout works fine, but the scrolling in my canvas seems to be broken. If I try to scroll over an image it looks like that: http://prntscr.com/xswwvw It is just repeating small parts of the picture again instead of real scrolling. I tried the same outside a plugin, i didnt get the Problem there.

I tried to search for a solution alot, but couldnt find one. Hopefully someone has an idea or some hints on how to solve this. Here is my relevant Code:

public class FaultTreeView {
private Canvas canvas;
private ArrayList<FTShape> shapeList = new ArrayList<>();
private Point origin = new Point(0, 0);
private Image image;

@PostConstruct
public void createPartControl(Composite parent) {
    Composite container = new Composite(parent, SWT.NONE);
    container.setBackground(getSystemColor(SWT.COLOR_WIDGET_BACKGROUND));
    container.setLayout(new GridLayout(1, false));

    Composite comp = new Composite(container, SWT.NONE);
    addToolBar(comp);


    createCanvas(container);

    AndGate ag = new AndGate(canvas, 0, 0);
    this.image = ag.getImage();

    handleScrollBars();
    paintElementsInCanvas();
    

}

And here the methods:

public void addToolBar(Composite composite) {
    ToolBar toolbar = new ToolBar(composite, SWT.BORDER | SWT.VERTICAL);
    addDropdownMenu(toolbar);
}

public void createCanvas(Composite container) {
    this.canvas = new Canvas(container, SWT.V_SCROLL | SWT.H_SCROLL);
    this.canvas.setLayout(new GridLayout(1, false));
    this.canvas.setLayoutData(new GridData(SWT.FILL, SWT.FILL, true, true, 1, 1));
}

For the ScrollBars i used some Code i found, but i added the *hBar.getPageIncrement() and *vBar.getPageIncrement() because otherwise it didnt let me scroll at all.

    private void handleScrollBars() {
     ScrollBar hBar = canvas.getHorizontalBar();
        hBar.addListener(SWT.Selection, new Listener() {
          public void handleEvent(Event e) {
            int hSelection = hBar.getSelection()*hBar.getPageIncrement();
            int destX = -hSelection - origin.x;
            Rectangle rect = image.getBounds();
            canvas.scroll(destX, 0, 0, 0, rect.width, rect.height, false);
            origin.x = -hSelection;
          }
        });
        ScrollBar vBar = canvas.getVerticalBar();
        vBar.addListener(SWT.Selection, new Listener() {
          public void handleEvent(Event e) {
            int vSelection = vBar.getSelection() * vBar.getPageIncrement();
            int destY = -vSelection - origin.y;
            Rectangle rect = image.getBounds();
            canvas.scroll(0, destY, 0, 0, rect.width, rect.height, false);
            origin.y = -vSelection;
          }
        });
}

public void paintElementsInCanvas() {
    canvas.addPaintListener(new PaintListener() {
        public void paintControl(PaintEvent e) {
            e.gc.drawImage(image, 0, 0);
        }
    });
}

It is usually best to use a ScrolledComposite for the scrolling with a body just containing a Label with the image set in that. — greg-449
Hi Greg, I am planning on adding alot of smaller pictures to the canvas, kind of a diagram containing multiple elements with a picture for each element with relative positions (Point(x,y)). I also want the user to be able to edit the diagram by hand, so i think i need a canvas, right? — Sven

greg-449 greg-449 · Accepted Answer · 2021-01-29T12:05:04

The drawImage call must use the origin to place the image correctly:

e.gc.drawImage(image, origin.x, origin.y);

Also look at SWT Snippet 48 scrolled canvas for how to handle resize and scroll bar sizing correctly.

Scrolling Canvas in eclipse plugin doesnt work (blurrs)

1 Answers