[Sticky Scrolling] Throttle sticky scrolling calculation

The calculation of the sticky-scrolling lines is now throttled, executing only every 100 ms, to prevent potential performance bottlenecks.

Follow up to #1894

Author: Christopher-Hermann

bundles/org.eclipse.ui.editors/src/org/eclipse/ui/internal/texteditor/stickyscroll/StickyScrollingHandler.java

@@ -19,13 +19,15 @@
 import static org.eclipse.ui.texteditor.AbstractDecoratedTextEditorPreferenceConstants.EDITOR_STICKY_SCROLLING_MAXIMUM_COUNT;
 import static org.eclipse.ui.texteditor.AbstractDecoratedTextEditorPreferenceConstants.EDITOR_TAB_WIDTH;
 
+import java.time.Duration;
 import java.util.List;
 
 import org.eclipse.swt.graphics.Color;
 
 import org.eclipse.jface.preference.IPreferenceStore;
 import org.eclipse.jface.preference.PreferenceConverter;
 import org.eclipse.jface.util.IPropertyChangeListener;
+import org.eclipse.jface.util.Throttler;
 
 import org.eclipse.jface.text.IViewportListener;
 import org.eclipse.jface.text.source.ISourceViewer;
@@ -37,6 +39,8 @@
  */
 public class StickyScrollingHandler implements IViewportListener {
 
+	private final static int THROTTLER_DELAY= 100;
+
 	private ISourceViewer sourceViewer;
 
 	private StickyScrollingControl stickyScrollingControl;
@@ -49,6 +53,10 @@ public class StickyScrollingHandler implements IViewportListener {
 
 	private StickyLinesProvider stickyLinesProvider;
 
+	private Throttler throttler;
+
+	private int verticalOffset;
+
 	/**
 	 * Creates a StickyScrollingHandlerIndentation that will be linked to the given source viewer.
 	 * The sticky scrolling will be computed by the default {@link StickyLinesProvider}.
@@ -73,6 +81,7 @@ public StickyScrollingHandler(ISourceViewer sourceViewer, IVerticalRuler vertica
 			StickyLinesProvider stickyLinesProvider) {
 		this.sourceViewer= sourceViewer;
 
+		throttler= new Throttler(sourceViewer.getTextWidget().getDisplay(), Duration.ofMillis(THROTTLER_DELAY), this::calculateAndShowStickyLines);
 		this.stickyLinesProvider= stickyLinesProvider;
 
 		StickyScrollingControlSettings settings= loadAndListenForProperties(preferenceStore);
@@ -117,7 +126,15 @@ private StickyScrollingControlSettings loadSettings(IPreferenceStore store) {
 	}
 
 	@Override
-	public void viewportChanged(int verticalOffset) {
+	public void viewportChanged(int newVerticalOffset) {
+		if (this.verticalOffset == newVerticalOffset) {
+			return;
+		}
+		verticalOffset= newVerticalOffset;
+		throttler.throttledExec();
+	}
+
+	private void calculateAndShowStickyLines() {
 		List<StickyLine> stickyLines= stickyLinesProvider.get(verticalOffset, sourceViewer);
 		stickyScrollingControl.setStickyLines(stickyLines);
 	}
@@ -130,6 +147,7 @@ public void uninstall() {
 		this.sourceViewer.removeViewportListener(this);
 		preferenceStore.removePropertyChangeListener(propertyChangeListener);
 		preferenceStore= null;
+		throttler= null;
 
 		stickyScrollingControl.dispose();
 	}

tests/org.eclipse.ui.editors.tests/src/org/eclipse/ui/internal/texteditor/stickyscroll/StickyScrollingHandlerTest.java

@@ -21,6 +21,8 @@
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertTrue;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
 import java.util.List;
@@ -121,6 +123,32 @@ public void testPreferencesUpdated() {
 		assertEquals(expStickyLineText, stickyLineText.getText());
 	}
 
+	@Test
+	public void testThrottledExecution() throws InterruptedException {
+		when(linesProvider.get(100, sourceViewer)).thenReturn(List.of(new StickyLine("line 10", 9)));
+
+		stickyScrollingHandler.viewportChanged(100);
+		stickyScrollingHandler.viewportChanged(200);
+		stickyScrollingHandler.viewportChanged(300);
+		stickyScrollingHandler.viewportChanged(400);
+
+		waitInUi(200);
+
+		// Call to lines provider should be throttled
+		verify(linesProvider, times(1)).get(100, sourceViewer);
+		verify(linesProvider, times(0)).get(200, sourceViewer);
+		verify(linesProvider, times(0)).get(300, sourceViewer);
+		verify(linesProvider, times(1)).get(400, sourceViewer);
+	}
+
+	private void waitInUi(int ms) throws InterruptedException {
+		while (shell.getDisplay().readAndDispatch()) {
+		}
+		Thread.sleep(ms);
+		while (shell.getDisplay().readAndDispatch()) {
+		}
+	}
+
 	private IPreferenceStore createPreferenceStore() {
 		store = new PreferenceStore();
 		store.setValue(EDITOR_TAB_WIDTH, 4);