Fix null pointer when drawable of a GC is null

[arunjose696]

The drawable on a GC can be null this would result in a null pointer error in GC#calculateZoomForImage as we check if the drawable is not autoScalable.

Expand for Code to trigger nullpointer

package org.eclipse.swt.snippets;

import org.eclipse.swt.*;
import org.eclipse.swt.custom.*;
import org.eclipse.swt.graphics.*;
import org.eclipse.swt.layout.*;
import org.eclipse.swt.widgets.*;

public class Snippet386 {

    public static void main(String[] args) {
        Display display = new Display();

        Image[] images = new Image[] {
            new Image(display, createImageDataProvider())
        };

        String[] descriptions = new String[] {

            "ImageDataProvider with fixed font size for a given zoom",

        };

        Slice[] slices = {
            new Slice("Full", 0.0, 0.0, 1.0, 1.0)
        };

        createShellWithImages(display, images, descriptions, slices, "Snippet 386 - Flipped Layout");
    }



    private static ImageDataProvider createImageDataProvider() {
        return new ImageDataProvider() {
            @Override
            public ImageData getImageData(int zoomLevel) {
                int scaleFactor = zoomLevel / 100;
                return createScaledTextImageData(100 * scaleFactor, 100 * scaleFactor);
            }
        };
    }



    private static ImageData createScaledTextImageData(int width, int height) {
        Display display = Display.getDefault();
        String text = "abcd";

        int fontSize = Math.max(1, height / 100);
        Font font = new Font(display, "Arial", fontSize, SWT.NORMAL);

        Image tmp = new Image(display, 1, 1);
        GC measureGC = new GC(tmp);
        measureGC.setFont(font);
        Point textExtent = measureGC.textExtent(text);
        measureGC.dispose();
        tmp.dispose();

        double scale = Math.min((double) width / textExtent.x, (double) height / textExtent.y);
        font.dispose();
        font = new Font(display, "Arial", Math.max(1, (int) (fontSize * scale)), SWT.NORMAL);

        Image image = new Image(display, width, height);
        GC gc = new GC(image);
        gc.setFont(font);
        gc.setForeground(display.getSystemColor(SWT.COLOR_BLACK));
        gc.setBackground(display.getSystemColor(SWT.COLOR_WHITE));
        gc.fillRectangle(image.getBounds());

        gc.setLineWidth(Math.max(1, width / 20));
        gc.drawLine(0, 0, width / 2, height);

        Point newTextExtent = gc.textExtent(text);
        gc.drawText(text, (width - newTextExtent.x) / 2, (height - newTextExtent.y) / 2, true);

        gc.dispose();
        ImageData data = image.getImageData();

        image.dispose();
        font.dispose();

        return data;
    }

    static class Slice {
        String name;
        double xFrac, yFrac, wFrac, hFrac;
        Slice(String name, double x, double y, double w, double h) {
            this.name = name;
            this.xFrac = x;
            this.yFrac = y;
            this.wFrac = w;
            this.hFrac = h;
        }
    }

    private static void createShellWithImages(Display display, Image[] images, String[] descriptions, Slice[] slices, String title) {
        Shell shell = new Shell(display, SWT.SHELL_TRIM | SWT.MAX | SWT.RESIZE);
        shell.setText(title);
        shell.setLayout(new FillLayout());

        ScrolledComposite scrolledComposite = new ScrolledComposite(shell, SWT.V_SCROLL | SWT.H_SCROLL | SWT.BORDER);
        Canvas canvas = new Canvas(scrolledComposite, SWT.DOUBLE_BUFFERED);
        scrolledComposite.setContent(canvas);
        scrolledComposite.setExpandHorizontal(true);
        scrolledComposite.setExpandVertical(true);

        int boxW = 500, boxH = 500, gap = 20;
        int titleHeight = 20, descHeight = 40, sliceHeaderHeight = 30;

        int rows = images.length;
        int cols = slices.length;

        int canvasWidth = (boxW + gap) * cols + gap;
        int canvasHeight = (boxH + titleHeight + gap) * rows + descHeight + sliceHeaderHeight;
        canvas.setSize(canvasWidth, canvasHeight);
        scrolledComposite.setMinSize(canvasWidth, canvasHeight);

        canvas.addListener(SWT.Paint, e -> {
            // Column headers
            for (int col = 0; col < cols; col++) {
                int x = col * (boxW + gap) + gap;
                Font font = new Font(display, "Arial", 18, SWT.NORMAL);
                e.gc.setFont(font);
                e.gc.drawText(slices[col].name, x, 0, true);
                font.dispose();

            }

            // Rows
            for (int row = 0; row < rows; row++) {
                Image image = images[row];
                Rectangle rect = image.getBounds();
                int y = row * (boxH + titleHeight + gap) + descHeight + sliceHeaderHeight;

                Font font = new Font(display, "Arial", 18, SWT.NORMAL);
                e.gc.setFont(font);
                e.gc.drawText(descriptions[row], 0, y - 10, true);
                font.dispose();

                for (int col = 0; col < cols; col++) {
                    Slice s = slices[col];
                    int x = col * (boxW + gap) + gap;

                    int srcX = (int) (rect.width * s.xFrac);
                    int srcY = (int) (rect.height * s.yFrac);
                    int srcW = (int) (rect.width * s.wFrac);
                    int srcH = (int) (rect.height * s.hFrac);

                    int boxTop = y + titleHeight;
                    e.gc.drawRectangle(x, boxTop, boxW, boxH);


                    e.gc.drawImage(image, srcX, srcY, srcW, srcH, x, boxTop, boxW, boxH);

                }
            }
        });

        shell.setMaximized(true);
        shell.open();

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

        for (Image img : images) img.dispose();
        display.dispose();
    }
}

Issue was caused by #2507

[github-actions]

Test Results

  118 files  + 7    118 suites  +7   11m 32s ⏱️ -21s
4 577 tests +55  4 554 ✅ +48  17 💤 +3  6 ❌ +5 
  298 runs  +56    288 ✅ +47   4 💤 +3  6 ❌ +6 

For more details on these failures, see this check.

Results for commit c5bfc29. ± Comparison against base commit 00106b4.

This pull request removes 1 and adds 56 tests. Note that renamed tests count towards both.

org.eclipse.swt.tests.junit.Test_org_eclipse_swt_dnd_Clipboard ‑ Unknown test

AllWin32Tests ImageWin32Tests ‑ testDisposeDrawnImageBeforeRequestingTargetForOtherZoom
AllWin32Tests ImageWin32Tests ‑ testDrawImageAtDifferentZooms(boolean)[1] true
AllWin32Tests ImageWin32Tests ‑ testDrawImageAtDifferentZooms(boolean)[2] false
AllWin32Tests ImageWin32Tests ‑ testImageDataForDifferentFractionalZoomsShouldBeDifferent
AllWin32Tests ImageWin32Tests ‑ testImageShouldHaveDimesionAsPerZoomLevel
AllWin32Tests ImageWin32Tests ‑ testRetrieveImageDataAtDifferentZooms(boolean)[1] true
AllWin32Tests ImageWin32Tests ‑ testRetrieveImageDataAtDifferentZooms(boolean)[2] false
AllWin32Tests ImageWin32Tests ‑ test_getImageData_fromCopiedImage
AllWin32Tests ImageWin32Tests ‑ test_getImageData_fromImageForImageDataFromImage
AllWin32Tests TestTreeColumn ‑ test_ColumnOrder
…

[akoch-yatta]

Change is very simple and makes sense. Test failures are unrelated