[GTK] Stop decoding the image file on every draw at zoom != 100

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/graphics/GC.java

@@ -913,9 +913,14 @@ void drawImage(Image srcImage, int srcX, int srcY, int srcWidth, int srcHeight,
 	/* Refresh Image as per zoom level, if required. */
 	srcImage.refreshImageForZoom ();
 
-	ImageData srcImageData = srcImage.getImageData();
-	int imgWidth = srcImageData.width;
-	int imgHeight = srcImageData.height;
+	int imgWidth = srcImage.width;
+	int imgHeight = srcImage.height;
+	if (imgWidth == -1 || imgHeight == -1) {
+		/* Images wrapped around a native handle carry no dimensions, see Image.gtk_new. */
+		ImageData srcImageData = srcImage.getImageData();
+		imgWidth = srcImageData.width;
+		imgHeight = srcImageData.height;
+	}
 	if (srcWidth == 0 && srcHeight == 0) {
 		srcWidth = imgWidth;
 		srcHeight = imgHeight;

tests/org.eclipse.swt.tests/JUnit Tests/org/eclipse/swt/tests/junit/Test_org_eclipse_swt_graphics_GC.java

@@ -30,6 +30,9 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.lang.ref.WeakReference;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Arrays;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 
@@ -59,6 +62,7 @@
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.ValueSource;
 
@@ -72,6 +76,9 @@ public class Test_org_eclipse_swt_graphics_GC {
 
 private static final int IMAGE_SIZE = 200;
 
+@TempDir
+static Path tempFolder;
+
 @BeforeEach
 public void setUp() {
 	display = Display.getDefault();
@@ -1208,6 +1215,60 @@ RGB getRealRGB(Color color) {
 	return palette.getRGB(pixel);
 }
 
+/**
+ * Drawing reads the image dimensions from the image itself. Obtaining them through
+ * ImageData used to decode the file again on every draw at a device zoom other than 100.
+ */
+@Test
+public void test_drawImage_doesNotReReadImageFileAtNonDefaultZoom() throws IOException {
+	Path file = tempFolder.resolve("volatile-collapseall.png");
+	Files.copy(SwtTestUtil.getPath("collapseall.png", tempFolder), file);
+	int previousDeviceZoom = DPIUtil.getDeviceZoom();
+	Image fileImage = null;
+	try {
+		DPIUtil.setDeviceZoom(200);
+		gc.dispose();
+		gc = new GC(image);
+		fileImage = new Image(display, file.toString());
+		ImageData beforeDelete = drawToFreshTarget(fileImage);
+		assertFalse(Arrays.equals(blankTargetData(), beforeDelete.data), "the reference draw produced no pixels");
+
+		Files.delete(file);
+
+		ImageData afterDelete = drawToFreshTarget(fileImage);
+		ImageDataTestHelper.assertImageDataEqual(beforeDelete, afterDelete, beforeDelete);
+		gc.drawImage(fileImage, 0, 0);
+	} finally {
+		if (fileImage != null) {
+			fileImage.dispose();
+		}
+		DPIUtil.setDeviceZoom(previousDeviceZoom);
+		Files.deleteIfExists(file);
+	}
+}
+
+private byte[] blankTargetData() {
+	Image target = new Image(display, IMAGE_SIZE, IMAGE_SIZE);
+	try {
+		return target.getImageData().data;
+	} finally {
+		target.dispose();
+	}
+}
+
+private ImageData drawToFreshTarget(Image source) {
+	Rectangle bounds = source.getBounds();
+	Image target = new Image(display, IMAGE_SIZE, IMAGE_SIZE);
+	GC targetGc = new GC(target);
+	try {
+		targetGc.drawImage(source, 0, 0, bounds.width, bounds.height, 0, 0, bounds.width * 2, bounds.height * 2);
+		return target.getImageData();
+	} finally {
+		targetGc.dispose();
+		target.dispose();
+	}
+}
+
 private void executeWithNonDefaultDeviceZoom(Runnable executable) {
 	int previousDeviceZoom = DPIUtil.getDeviceZoom();
 	DPIUtil.setDeviceZoom(200);