Callback for dynamic drawing with a GC on images

bundles/org.eclipse.swt/Eclipse SWT Custom Widgets/common/org/eclipse/swt/custom/CTabFolder.java

@@ -17,6 +17,7 @@
 import org.eclipse.swt.accessibility.*;
 import org.eclipse.swt.events.*;
 import org.eclipse.swt.graphics.*;
+import org.eclipse.swt.internal.*;
 import org.eclipse.swt.widgets.*;
 
 /**
@@ -717,26 +718,25 @@ public Rectangle computeTrim (int x, int y, int width, int height) {
 	}
 	return trim;
 }
+
 Image createButtonImage(Display display, int button) {
-	return new Image(display, (ImageDataProvider) zoom -> {
-		GC tempGC = new GC (CTabFolder.this);
-		Point size = renderer.computeSize(button, SWT.NONE, tempGC, SWT.DEFAULT, SWT.DEFAULT);
-		tempGC.dispose();
-
-		Rectangle trim = renderer.computeTrim(button, SWT.NONE, 0, 0, 0, 0);
-		Image image = new Image (display, size.x - trim.width, size.y - trim.height);
-		GC gc = new GC (image);
-		Color transColor = renderer.parent.getBackground();
-		gc.setBackground(transColor);
-		gc.fillRectangle(image.getBounds());
-		renderer.draw(button, SWT.NONE, new Rectangle(trim.x, trim.y, size.x, size.y), gc);
-		gc.dispose ();
-
-		final ImageData imageData = image.getImageData (zoom);
-		imageData.transparentPixel = imageData.palette.getPixel(transColor.getRGB());
-		image.dispose();
-		return imageData;
-	});
+	final GC tempGC = new GC (CTabFolder.this);
+	final Point size = renderer.computeSize(button, SWT.NONE, tempGC, SWT.DEFAULT, SWT.DEFAULT);
+	tempGC.dispose();
+
+	final Rectangle trim = renderer.computeTrim(button, SWT.NONE, 0, 0, 0, 0);
+	final Point imageSize = new Point(size.x - trim.width, size.y - trim.height);
+	Color transColor = renderer.parent.getBackground();
+	final ImageGcDrawer imageGcDrawer = new TransparencyColorImageGcDrawer(transColor) {
+		@Override
+		public void drawOn(GC gc, int imageWidth, int imageHeight) {
+			Rectangle imageBounds = new Rectangle(0, 0, imageWidth, imageHeight);
+			gc.setBackground(transColor);
+			gc.fillRectangle(imageBounds);
+			renderer.draw(button, SWT.NONE, imageBounds, gc);
+		}
+	};
+	return new Image(display, imageGcDrawer, imageSize.x, imageSize.y);
 }
 
 private void notifyItemCountChange() {

bundles/org.eclipse.swt/Eclipse SWT/cocoa/org/eclipse/swt/graphics/Image.java

@@ -15,6 +15,7 @@
 
 
 import java.io.*;
+import java.util.*;
 
 import org.eclipse.swt.*;
 import org.eclipse.swt.internal.*;
@@ -135,6 +136,11 @@ public final class Image extends Resource implements Drawable {
 	 */
 	private ImageDataProvider imageDataProvider;
 
+	/**
+	 * ImageGcDrawer to provide a callback to draw on a GC for various zoom levels
+	 */
+	private ImageGcDrawer imageGcDrawer;
+
 	/**
 	 * Style flag used to differentiate normal, gray-scale and disabled images based
 	 * on image data providers. Without this, a normal and a disabled image of the
@@ -384,8 +390,9 @@ public Image(Device device, Image srcImage, int flag) {
 
 		imageFileNameProvider = srcImage.imageFileNameProvider;
 		imageDataProvider = srcImage.imageDataProvider;
+		imageGcDrawer = srcImage.imageGcDrawer;
 		this.styleFlag = srcImage.styleFlag | flag;
-		if (imageFileNameProvider != null || imageDataProvider != null) {
+		if (imageFileNameProvider != null || imageDataProvider != null ||srcImage.imageGcDrawer != null) {
 			/* If source image has 200% representation then create the 200% representation for the new image & apply flag */
 			NSBitmapImageRep rep200 = srcImage.getRepresentation (200);
 			if (rep200 != null) createRepFromSourceAndApplyFlag(rep200, srcWidth * 2, srcHeight * 2, flag);
@@ -843,6 +850,54 @@ public Image(Device device, ImageDataProvider imageDataProvider) {
 	}
 }
 
+/**
+ * The provided ImageGcDrawer will be called on demand whenever a new variant of the
+ * Image for an additional zoom is required. Depending on the OS specific implementation
+ * these calls will be done during the instantiation or later when a new variant is
+ * requested.
+ *
+ * @param device the device on which to create the image
+ * @param imageGcDrawer the ImageGcDrawer object to be called when a new image variant
+ * for another zoom is required.
+ * @param width the width of the new image in points
+ * @param height the height of the new image in points
+ *
+ * @exception IllegalArgumentException <ul>
+ *    <li>ERROR_NULL_ARGUMENT - if device is null and there is no current device</li>
+ *    <li>ERROR_NULL_ARGUMENT - if the ImageGcDrawer is null</li>
+ * </ul>
+ * @since 3.129
+ */
+public Image(Device device, ImageGcDrawer imageGcDrawer, int width, int height) {
+	super(device);
+	if (imageGcDrawer == null) SWT.error(SWT.ERROR_NULL_ARGUMENT);
+	this.imageGcDrawer = imageGcDrawer;
+	ImageData data = drawWithImageGcDrawer(imageGcDrawer, width, height, 100);
+	if (data == null) SWT.error(SWT.ERROR_INVALID_ARGUMENT);
+	NSAutoreleasePool pool = null;
+	if (!NSThread.isMainThread()) pool = (NSAutoreleasePool) new NSAutoreleasePool().alloc().init();
+	try {
+		init (data);
+		init ();		
+	} finally {
+		if (pool != null) pool.release();
+	}
+}
+
+private ImageData drawWithImageGcDrawer(ImageGcDrawer imageGcDrawer, int width, int height, int zoom) {
+	Image image = new Image(device, width, height);
+	GC gc = new GC(image);
+	try {
+		imageGcDrawer.drawOn(gc, width, height);
+		ImageData imageData = image.getImageData(zoom);
+		imageGcDrawer.postProcess(imageData);
+		return imageData;
+	} finally {
+		gc.dispose();
+		image.dispose();
+	}
+}
+
 private AlphaInfo _getAlphaInfoAtCurrentZoom (NSBitmapImageRep rep) {
 	int deviceZoom = DPIUtil.getDeviceZoom();
 	if (deviceZoom != 100 && (imageFileNameProvider != null || imageDataProvider != null)) {
@@ -1121,6 +1176,9 @@ public boolean equals (Object object) {
 		return styleFlag == image.styleFlag && imageDataProvider.equals (image.imageDataProvider);
 	} else if (imageFileNameProvider != null && image.imageFileNameProvider != null) {
 		return styleFlag == image.styleFlag && imageFileNameProvider.equals (image.imageFileNameProvider);
+	} else if (imageGcDrawer != null && image.imageGcDrawer != null) {
+		return styleFlag == image.styleFlag && imageGcDrawer.equals(image.imageGcDrawer) && width == image.width
+				&& height == image.height;
 	} else {
 		return handle == image.handle;
 	}
@@ -1357,6 +1415,8 @@ public int hashCode () {
 		return imageDataProvider.hashCode();
 	} else if (imageFileNameProvider != null) {
 		return imageFileNameProvider.hashCode();
+	} else if (imageGcDrawer != null) {
+		return Objects.hash(imageGcDrawer, height, width);
 	} else {
 		return handle != null ? (int)handle.id : 0;
 	}

bundles/org.eclipse.swt/Eclipse SWT/common/org/eclipse/swt/graphics/ImageGcDrawer.java

@@ -0,0 +1,48 @@
+/*******************************************************************************
+ * Copyright (c) 2025 Yatta and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *     Yatta - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.swt.graphics;
+
+/**
+ * Interface to provide a callback mechanism to draw on different GC instances
+ * depending on the zoom the image will be used for. A common use case is when
+ * the application is moved from a low DPI monitor to a high DPI monitor. This
+ * provides API which will be called by SWT during the image rendering.
+ *
+ * This interface needs to be implemented by client code to provide logic that
+ * draws on the empty GC on demand.
+ *
+ * @since 3.129
+ */
+public interface ImageGcDrawer {
+
+	/**
+	 * Draws an image on a GC for a requested zoom level.
+	 *
+	 * @param gc          The GC will draw on the underlying Image and is configured
+	 *                    for the targeted zoom
+	 * @param imageWidth  The width of the image in points to draw on
+	 * @param imageHeight The height of the image in points to draw on
+	 */
+	void drawOn(GC gc, int imageWidth, int imageHeight);
+
+	/**
+	 * Executes post processing on ImageData. This method will always be called
+	 * after <code>drawOn</code> and contain the resulting ImageData.
+	 *
+	 * @param imageData The resulting ImageData after <code>drawOn</code> was called
+	 */
+	default void postProcess(ImageData imageData) {
+	}
+
+}

bundles/org.eclipse.swt/Eclipse SWT/common/org/eclipse/swt/internal/TransparencyColorImageGcDrawer.java

@@ -0,0 +1,31 @@
+/*******************************************************************************
+ * Copyright (c) 2025 Yatta and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *     Yatta - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.swt.internal;
+
+import org.eclipse.swt.graphics.*;
+
+public abstract class TransparencyColorImageGcDrawer implements ImageGcDrawer {
+
+	private final Color transparencyColor;
+
+	public TransparencyColorImageGcDrawer(Color transparencyColor) {
+		this.transparencyColor = transparencyColor;
+	}
+
+	@Override
+	public void postProcess(ImageData imageData) {
+		imageData.transparentPixel = imageData.palette.getPixel(transparencyColor.getRGB());
+	}
+
+}

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

@@ -15,6 +15,7 @@
 
 
 import java.io.*;
+import java.util.*;
 
 import org.eclipse.swt.*;
 import org.eclipse.swt.internal.*;
@@ -151,6 +152,11 @@ public final class Image extends Resource implements Drawable {
 	 */
 	private ImageDataProvider imageDataProvider;
 
+	/**
+	 * ImageGcDrawer to provide a callback to draw on a GC for various zoom levels
+	 */
+	private ImageGcDrawer imageGcDrawer;
+
 	/**
 	 * Style flag used to differentiate normal, gray-scale and disabled images based
 	 * on image data providers. Without this, a normal and a disabled image of the
@@ -263,6 +269,7 @@ public Image(Device device, Image srcImage, int flag) {
 	this.type = srcImage.type;
 	this.imageDataProvider = srcImage.imageDataProvider;
 	this.imageFileNameProvider = srcImage.imageFileNameProvider;
+	this.imageGcDrawer = srcImage.imageGcDrawer;
 	this.styleFlag = srcImage.styleFlag | flag;
 	this.currentDeviceZoom = srcImage.currentDeviceZoom;
 
@@ -661,6 +668,36 @@ public Image(Device device, ImageDataProvider imageDataProvider) {
 	init ();
 }
 
+/**
+ * The provided ImageGcDrawer will be called on demand whenever a new variant of the
+ * Image for an additional zoom is required. Depending on the OS specific implementation
+ * these calls will be done during the instantiation or later when a new variant is
+ * requested.
+ *
+ * @param device the device on which to create the image
+ * @param imageGcDrawer the ImageGcDrawer object to be called when a new image variant
+ * for another zoom is required.
+ * @param width the width of the new image in points
+ * @param height the height of the new image in points
+ *
+ * @exception IllegalArgumentException <ul>
+ *    <li>ERROR_NULL_ARGUMENT - if device is null and there is no current device</li>
+ *    <li>ERROR_NULL_ARGUMENT - if the ImageGcDrawer is null</li>
+ * </ul>
+ * @since 3.129
+ */
+public Image(Device device, ImageGcDrawer imageGcDrawer, int width, int height) {
+	super(device);
+	if (imageGcDrawer == null) {
+		SWT.error(SWT.ERROR_NULL_ARGUMENT);
+	}
+	this.imageGcDrawer = imageGcDrawer;
+	currentDeviceZoom = DPIUtil.getDeviceZoom();
+	ImageData imageData = drawWithImageGcDrawer(width, height, currentDeviceZoom);
+	init (imageData);
+	init ();
+}
+
 /**
  * Refreshes the image for the current device scale factor.
  * <p>
@@ -722,6 +759,17 @@ boolean refreshImageForZoom () {
 			refreshed = true;
 			currentDeviceZoom = deviceZoomLevel;
 		}
+	} else if (imageGcDrawer != null) {
+		int deviceZoomLevel = deviceZoom;
+		if (deviceZoomLevel != currentDeviceZoom) {
+			ImageData data = drawWithImageGcDrawer(width, height, deviceZoomLevel);
+			/* Release current native resources */
+			destroy ();
+			init(data);
+			init();
+			refreshed = true;
+			currentDeviceZoom = deviceZoomLevel;
+		}
 	} else {
 		if (!DPIUtil.useCairoAutoScale()) {
 			int deviceZoomLevel = deviceZoom;
@@ -904,6 +952,9 @@ public boolean equals (Object object) {
 		return (styleFlag == image.styleFlag) && imageDataProvider.equals (image.imageDataProvider);
 	} else if (imageFileNameProvider != null && image.imageFileNameProvider != null) {
 		return (styleFlag == image.styleFlag) && imageFileNameProvider.equals (image.imageFileNameProvider);
+	} else if (imageGcDrawer != null && image.imageGcDrawer != null) {
+		return styleFlag == image.styleFlag && imageGcDrawer.equals(image.imageGcDrawer) && width == image.width
+				&& height == image.height;
 	} else {
 		return surface == image.surface;
 	}
@@ -1110,11 +1161,27 @@ public ImageData getImageData (int zoom) {
 	} else if (imageFileNameProvider != null) {
 		ElementAtZoom<String> fileName = DPIUtil.validateAndGetImagePathAtZoom (imageFileNameProvider, zoom);
 		return DPIUtil.scaleImageData (device, new ImageData (fileName.element()), zoom, fileName.zoom());
+	} else if (imageGcDrawer != null) {
+		return drawWithImageGcDrawer(width, height, zoom);
 	} else {
 		return DPIUtil.scaleImageData (device, getImageDataAtCurrentZoom (), zoom, currentDeviceZoom);
 	}
 }
 
+private ImageData drawWithImageGcDrawer(int width, int height, int zoom) {
+	Image image = new Image(device, width, height);
+	GC gc = new GC(image);
+	try {
+		imageGcDrawer.drawOn(gc, width, height);
+		ImageData imageData = image.getImageData(zoom);
+		imageGcDrawer.postProcess(imageData);
+		return imageData;
+	} finally {
+		gc.dispose();
+		image.dispose();
+	}
+}
+
 /**
  * Invokes platform specific functionality to allocate a new image.
  * <p>
@@ -1179,6 +1246,8 @@ public int hashCode () {
 		return imageDataProvider.hashCode();
 	} else if (imageFileNameProvider != null) {
 		return imageFileNameProvider.hashCode();
+	} else if (imageGcDrawer != null) {
+		return Objects.hash(imageGcDrawer, width, height);
 	} else {
 		return (int)surface;
 	}