Make DPI_CHANGED event handling async

With this commit, all the widgets scale themselves asynchronously
independent of the order saving the wait time over their children to
scale.

Author: amartya4256

bundles/org.eclipse.swt/Eclipse SWT Tests/win32/org/eclipse/swt/internal/DPITestUtil.java

@@ -13,6 +13,9 @@
  *******************************************************************************/
 package org.eclipse.swt.internal;
 
+import java.time.*;
+import java.util.concurrent.atomic.*;
+
 import org.eclipse.swt.*;
 import org.eclipse.swt.widgets.*;
 
@@ -21,14 +24,34 @@ public final class DPITestUtil {
 	private DPITestUtil() {
 	}
 
+	private static final int TIMEOUT_MILLIS = 10000;
+
 	public static void changeDPIZoom (Shell shell, int nativeZoom) {
 		DPIUtil.setDeviceZoom(nativeZoom);
 		Event event = new Event();
 		event.type = SWT.ZoomChanged;
 		event.widget = shell;
 		event.detail = nativeZoom;
 		event.doit = true;
+		AtomicInteger scalingCounter = new AtomicInteger(0);
+		event.data = scalingCounter;
 		shell.notifyListeners(SWT.ZoomChanged, event);
+		waitForDPIChange(shell, TIMEOUT_MILLIS, scalingCounter);
+	}
+
+	public static void waitForPassCondition(Shell shell, int timeout, AtomicInteger scalingCounter) {
+		final Instant timeOut = Instant.now().plusMillis(timeout);
+		final Display display = shell == null ? Display.getDefault() : shell.getDisplay();
+
+		while (Instant.now().isBefore(timeOut) && scalingCounter.get() != 0) {
+			if (!display.isDisposed()) {
+				display.readAndDispatch();
+			}
+		}
+	}
+
+	public static void waitForDPIChange(Shell shell, int timeout, AtomicInteger scalingCounter) {
+		waitForPassCondition(shell, timeout, scalingCounter);
 	}
 
 }

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Composite.java

@@ -1977,7 +1977,7 @@ public String toString() {
 void handleDPIChange(Event event, float scalingFactor) {
 	super.handleDPIChange(event, scalingFactor);
 	for (Control child : getChildren()) {
-		child.notifyListeners(SWT.ZoomChanged, event);
+		child.sendZoomChangedEvent(event, getShell());
 	}
 }
 }

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/widgets/Control.java

@@ -16,6 +16,7 @@
 
 
 import java.util.*;
+import java.util.concurrent.atomic.*;
 import java.util.stream.*;
 
 import org.eclipse.swt.*;
@@ -78,6 +79,9 @@ public abstract class Control extends Widget implements Drawable {
 	Font font;
 	int drawCount, foreground, background, backgroundAlpha = 255;
 
+	/** Cache for currently processed DPI change event to be able to cancel it if a new one is triggered */
+	private Event currentDpiChangeEvent;
+
 /**
  * Prevents uninitialized instances from being created outside the package.
  */
@@ -4757,7 +4761,7 @@ public boolean setParent (Composite parent) {
 	if (parent.nativeZoom != nativeZoom) {
 		int newZoom = parent.nativeZoom;
 		Event zoomChangedEvent = createZoomChangedEvent(newZoom);
-		notifyListeners(SWT.ZoomChanged, zoomChangedEvent);
+		sendZoomChangedEvent(zoomChangedEvent, getShell());
 	}
 	int flags = OS.SWP_NOSIZE | OS.SWP_NOMOVE | OS.SWP_NOACTIVATE;
 	OS.SetWindowPos (topHandle, OS.HWND_BOTTOM, 0, 0, 0, 0, flags);
@@ -4950,11 +4954,15 @@ LRESULT WM_DESTROY (long wParam, long lParam) {
 	return null;
 }
 
-void handleMonitorSpecificDpiChange(int newNativeZoom, Rectangle newBoundsInPixels) {
+private void handleMonitorSpecificDpiChange(int newNativeZoom, Rectangle newBoundsInPixels) {
 	DPIUtil.setDeviceZoom (newNativeZoom);
 	Event zoomChangedEvent = createZoomChangedEvent(newNativeZoom);
-	notifyListeners(SWT.ZoomChanged, zoomChangedEvent);
+	if (currentDpiChangeEvent != null) {
+		currentDpiChangeEvent.doit = false;
+	}
+	currentDpiChangeEvent = zoomChangedEvent;
 	this.setBoundsInPixels(newBoundsInPixels.x, newBoundsInPixels.y, newBoundsInPixels.width, newBoundsInPixels.height);
+	sendZoomChangedEvent(zoomChangedEvent, getShell());
 }
 
 private Event createZoomChangedEvent(int zoom) {
@@ -4963,6 +4971,7 @@ private Event createZoomChangedEvent(int zoom) {
 	event.widget = this;
 	event.detail = zoom;
 	event.doit = true;
+	event.data = new AtomicInteger(0);
 	return event;
 }
 
@@ -5876,6 +5885,27 @@ LRESULT wmScrollChild (long wParam, long lParam) {
 	return null;
 }
 
+void sendZoomChangedEvent(Event event, Shell shell) {
+	AtomicInteger handleDPIChangedScheduledTasksCount = (AtomicInteger) event.data;
+	handleDPIChangedScheduledTasksCount.incrementAndGet();
+	getDisplay().asyncExec(() -> {
+		try {
+			if (!this.isDisposed() && event.doit) {
+				notifyListeners(SWT.ZoomChanged, event);
+			}
+		} finally {
+			if (shell.isDisposed()) {
+				return;
+			}
+			if (handleDPIChangedScheduledTasksCount.decrementAndGet() <= 0) {
+				currentDpiChangeEvent = null;
+				if (event.doit) {
+					shell.WM_SIZE(0, 0);
+				}
+			}
+		}
+	});
+}
 
 @Override
 void handleDPIChange(Event event, float scalingFactor) {