Scrolling an extension of JLabel doesn't work :s

[Olivier Legat]

Howdy folks,

I'm having a lot of trouble getting a custom JComponent to scroll inside a JScollPane. I can only scroll if the mouse pointer is over the scroll bars, when I move the pointer into the JViewport (or whatever it's called) I can no longer scroll.

I've launched the 1st demo on the How to Use Scroll Panes pages and obviously it does what you'd expect. I noticed that this example uses an extension of JLabel (called ScrollablePicture). I've also noticed that this example still works if the ScrollablePicture class doesn't implement Scrollable and MouseMotionListener.

So I decided to do something similar to that but my program persistently refuses to work

@SuppressWarnings("serial")
public class MyCanvas extends JLabel
{

	/** Root node of the drawing */
	protected CanvasNode canvasNode;
	/** UndoManager for this specific Canvas */
	protected UndoManager undoManager;
	
	/** Dimension of the Canvas */
	private Dimension dim;
	
	// Grid / snap control vars:
	/** An object used for displaying the grid and snapping */
	private CanvasGrid grid;
	/** True if the grid should be drawn. False otherwise */
	private boolean drawGrid;
	/** True if the grid should be drawn on top of the other objects	 */
	private boolean drawGridOnTop;
	
	public MyCanvas(Dimension dim) {
		this.canvasNode = TreeManager.get().addCanvas(this);
		this.undoManager = new UndoManager();
		this.dim = dim;
		this.grid = new CanvasGrid(dim);
		this.drawGrid = true;
		this.drawGridOnTop = false;
		this.setOpaque(true);
		this.setBackground(Color.WHITE);
		this.setPreferredSize(this.dim);
	}
	
	/* **********************************************************************
	 * 							UI specific methods							*
	 ************************************************************************/
	
	public void paintComponent(Graphics g) {
		super.paintComponent(g);
		Graphics2D g2 = (Graphics2D)g;
		
		if(drawGrid && !drawGridOnTop)  grid.paintComponent(g2);
		
		@SuppressWarnings("unchecked")
		Enumeration<DrawableNode> i = canvasNode.children();
		while(i.hasMoreElements()) {
			i.nextElement().draw(g2);
		}
		
		if(drawGrid && drawGridOnTop)  grid.paintComponent(g2);
	}
	
	@Override
	public Dimension getPreferredSize() {
		return this.dim;
	}
	
	/* **********************************************************************
	 * 						MyCanvas specific methods						*
	 ************************************************************************/
	public CanvasNode getCanvasNode() {return this.canvasNode;}
    
	public boolean getDrawGrid() {return drawGrid;}
	public void setDrawGrid(boolean drawOn) {
		if (drawOn == drawGrid) return;
		else {
			drawGrid = drawOn;
			repaint();
		}
	}
	
	public boolean getDrawGridOnTop() {return drawGridOnTop;}
	public void setDrawGridOnTop(boolean drawOnTop) {
		if (drawOnTop == drawGridOnTop) return;
		else {
			drawGridOnTop = drawOnTop;
			repaint();
		}
	}
	
	public CanvasGrid getCanvasGrid() {return this.grid;}
	public void setCanvasGrid(CanvasGrid grid) {
		this.grid = grid;
		if(drawGrid) {
			repaint();
		}
	}
	
	public RealPoint snap(RealPoint pt) {
		return grid.snap(pt);
	}
	public RealPoint snap(int x, int y) {
		return grid.snap(new RealPoint(x,y));
	}
}

@SuppressWarnings("serial")
public class CanvasPanel extends JPanel {

	private JScrollPane scrollPane;
	protected MyCanvas canvas;
	
	public CanvasPanel(Dimension dim) {
		super(new BorderLayout());
		this.canvas = new MyCanvas(dim);
		this.scrollPane = new JScrollPane(canvas);
		
		scrollPane.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
		scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
		this.add(scrollPane, "Center");
	}
}

Any pointers would be much appreciated.

Thank-you

[Olivier Legat]

I found a work-around (with a little bit of help). It's not particularly tidy but it works:

@SuppressWarnings("serial")
public class MyCanvas extends JLabel implements MouseWheelListener
{
	...

	public MyCanvas(Dimension dim) {
		this.canvasNode = TreeManager.get().addCanvas(this);
		this.undoManager = new UndoManager();
		this.dim = dim;
		this.grid = new CanvasGrid(dim);
		this.drawGrid = true;
		this.drawGridOnTop = false;
		this.setOpaque(true);
		this.setBackground(Color.WHITE);
		this.setPreferredSize(this.dim);
	}
	
	...

	@Override
	public void mouseWheelMoved(MouseWheelEvent e) {
		JViewport jvp = null;
		try {
			jvp = (JViewport) this.getParent();
		}
		catch(ClassCastException exc) {
			System.err.print("this MyCanvas hasn't been added to a JViewport! :O");
			exc.printStackTrace();
			return;
		}
		
		int dy = e.getScrollAmount();
		int r = e.getWheelRotation();
		dy *= r;
		
		Point p = jvp.getViewPosition();
		int x = p.x;
		int y = p.y + dy;
		
		int maxX = this.getWidth() - jvp.getWidth();
		int maxY = this.getHeight() - jvp.getHeight();
		
			 if(x < 0)	  x = 0;
		else if(x > maxX) x = maxX;
			 if(y < 0) 	  y = 0;
		else if(y > maxY) y = maxY;
		
		jvp.setViewPosition(new Point(x,y));
	}
}