Perform element changes always in a non UI thread

Currently PackageExplorerContentProvider receives events for changed
elements from different threads and already has handling to perform only
certain actions in the UI thread. But it does not really account for the
case when the event itself is triggered from the UI thread in which case
lengthy operation can take place in the UI.

This now checks if the change is using the UI and otherwise schedules a
job to perform the required actions in the background.

org.eclipse.jdt.ui.tests/ui/org/eclipse/jdt/ui/tests/packageview/ContentProviderTests3.java

@@ -21,6 +21,8 @@
 import static org.junit.jupiter.api.Assertions.fail;
 
 import java.io.File;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
@@ -29,7 +31,10 @@
 import org.eclipse.jdt.testplugin.JavaProjectHelper;
 import org.eclipse.jdt.testplugin.JavaTestPlugin;
 
+import org.eclipse.core.runtime.NullProgressMonitor;
+import org.eclipse.core.runtime.OperationCanceledException;
 import org.eclipse.core.runtime.Path;
+import org.eclipse.core.runtime.jobs.Job;
 
 import org.eclipse.core.resources.IFile;
 import org.eclipse.core.resources.IFolder;
@@ -54,6 +59,7 @@
 import org.eclipse.jdt.core.IPackageFragment;
 import org.eclipse.jdt.core.IPackageFragmentRoot;
 
+import org.eclipse.jdt.internal.ui.packageview.PackageExplorerContentProvider;
 import org.eclipse.jdt.internal.ui.util.CoreUtility;
 
 
@@ -212,9 +218,25 @@ public void testDeleteBottomLevelFragmentFolding() throws Exception {
 	protected void sendEvent(IJavaElementDelta delta) {
 		IElementChangedListener listener= (IElementChangedListener) fProvider;
 		listener.elementChanged(new ElementChangedEvent(delta, ElementChangedEvent.POST_CHANGE));
-
-		//force events from dispaly
-		while(fMyPart.getTreeViewer().getControl().getDisplay().readAndDispatch()) {
+		CountDownLatch latch = new CountDownLatch(1);
+		new Thread(new Runnable() {
+
+			@Override
+			public void run() {
+				try {
+					Job.getJobManager().join(PackageExplorerContentProvider.class, new NullProgressMonitor());
+				} catch (OperationCanceledException | InterruptedException e) {
+				}
+				latch.countDown();
+			}
+		}).start();
+		//force events from display
+		try {
+			while(!latch.await(10, TimeUnit.MILLISECONDS)) {
+				while(fMyPart.getTreeViewer().getControl().getDisplay().readAndDispatch()) {
+				}
+			}
+		} catch (InterruptedException e) {
 		}
 	}
 

org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/packageview/PackageExplorerContentProvider.java

@@ -18,6 +18,8 @@
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.List;
+import java.util.Queue;
+import java.util.concurrent.ConcurrentLinkedQueue;
 
 import org.eclipse.swt.widgets.Control;
 import org.eclipse.swt.widgets.Display;
@@ -28,6 +30,7 @@
 import org.eclipse.core.runtime.IProgressMonitor;
 import org.eclipse.core.runtime.IStatus;
 import org.eclipse.core.runtime.Status;
+import org.eclipse.core.runtime.jobs.Job;
 
 import org.eclipse.core.resources.IFolder;
 import org.eclipse.core.resources.IProject;
@@ -93,6 +96,8 @@ public class PackageExplorerContentProvider extends StandardJavaElementContentPr
 
 	private UIJob fUpdateJob;
 
+	private final DeltaJob fdeltaJob;
+
 	/**
 	 * We use a cache to know whether a package has a single child for the hierarchical representation.
 	 * This avoids looping over all packages for each call to
@@ -107,6 +112,7 @@ public class PackageExplorerContentProvider extends StandardJavaElementContentPr
 	 */
 	public PackageExplorerContentProvider(boolean provideMembers) {
 		super(provideMembers);
+		fdeltaJob= new DeltaJob();
 		fShowLibrariesNode= false;
 		fIsFlatLayout= false;
 		fFoldPackages= arePackagesFoldedInHierarchicalLayout();
@@ -127,6 +133,12 @@ protected Object getViewerInput() {
 
 	@Override
 	public void elementChanged(final ElementChangedEvent event) {
+		IJavaElementDelta delta= event.getDelta();
+		fdeltaJob.queue.add(delta);
+		fdeltaJob.schedule();
+	}
+
+	protected void processDelta(IJavaElementDelta delta) {
 		final ArrayList<Runnable> runnables= new ArrayList<>();
 		try {
 			clearPackageCache();
@@ -136,7 +148,7 @@ public void elementChanged(final ElementChangedEvent event) {
 			if (inputDeleted(runnables))
 				return;
 
-			processDelta(event.getDelta(), runnables);
+			processDelta(delta, runnables);
 		} catch (JavaModelException e) {
 			JavaPlugin.log(e);
 		} finally {
@@ -1006,4 +1018,29 @@ public void propertyChange(PropertyChangeEvent event) {
 			}
 		}
 	}
+
+	private final class DeltaJob extends Job {
+		private Queue<IJavaElementDelta> queue= new ConcurrentLinkedQueue<>();
+
+		DeltaJob() {
+			super(PackagesMessages.PackageExplorerContentProvider_update_job_description);
+			setSystem(true);
+		}
+
+		@Override
+		public boolean belongsTo(Object family) {
+			return family == PackageExplorerContentProvider.class;
+		}
+
+		@Override
+		protected IStatus run(IProgressMonitor monitor) {
+
+			IJavaElementDelta delta;
+			while ((delta= queue.poll()) != null) {
+				processDelta(delta);
+			}
+			return Status.OK_STATUS;
+		}
+
+	}
 }