SWT：合成内的图像； 获取IllegalArgumentException

Question

I am currently making a stand alone application (not an eclipse plugin) using SWT and Eclipse's WindowBuilder, and I was testing putting an image in a composite to use it as a way to easily plop images into my composites, when I try to draw the image, it throws an IllegalArgumentException. I have no clue what's happening, and I'm looking for an explanation/alternative. What's happening and how would I approach fixing this?

If I comment out the e.gc.drawImage line, and nothing more, it will not throw the exception.

Here's the code that's giving me the error:

import org.eclipse.swt.SWT;
import org.eclipse.swt.layout.FillLayout;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Shell;

public class GUI {
    public static final Display display = Display.getDefault();;
    private final Shell shell;

    public GUI() {
        shell = new Shell(display);
        shell.setLayout(new FillLayout(SWT.HORIZONTAL));
    }

    public static void main(String[] args) {
        GUI window = new GUI();
        window.open();
    }

    public void open() {
        createContents();
        shell.open();
        shell.layout();
        while (!shell.isDisposed()) {
            if (!display.readAndDispatch()) {
                display.sleep();
            }
        }
    }

    private void createContents() {
        shell.setSize(450, 300);

        ImageTest img = new ImageTest(shell, SWT.NONE);
    }
}

import org.eclipse.swt.graphics.Image;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.events.PaintListener;
import org.eclipse.swt.events.PaintEvent;
import org.eclipse.swt.layout.FillLayout;
import org.eclipse.swt.SWT;
import org.eclipse.swt.widgets.Canvas;

public class ImageTest extends Composite {
    public ImageTest(Composite parent, int style) {
        super(parent, style);
        setLayout(new FillLayout(SWT.HORIZONTAL));

        final Image img = new Image(GUI.display, "img.gif");

            // I tried drawing the image to both a canvas and the composite its self. Same outcome.
        Canvas canvas = new Canvas(this, SWT.NONE);
        canvas.addPaintListener(new PaintListener() {
            public void paintControl(PaintEvent e) {
                e.gc.drawImage(img, 0, 0); // If I comment this out, it runs fine.
            }
        });

        img.dispose();
    }

    @Override
    protected void checkSubclass() {}
}

Any help would be appreciated.

Answer 1

You get the error because your image is disposed when paintControl tries to draw it. You dispose it yourself at the end of the ImageTest constructor before the paint listener gets the chance to even be called.

You can avoid that by making img a member variable of your class and then override the dispose methos to do the clean up:

@Override
public void dispose() {
    this.img.dispose();
    super.dispose();
}

Don't forget to remove the line

img.dispose();

from your constructor.

Answer 2

GC.drawImage API document says that in the following cases IllegalArgumentException is being thrown. The possibility is that image object is null.

IllegalArgumentException -
ERROR_NULL_ARGUMENT - if the image is null
ERROR_INVALID_ARGUMENT - if the image has been disposed
ERROR_INVALID_ARGUMENT - if the given coordinates are outside the bounds of the image