feat: support dynamic registration for textDocument/completion

org.eclipse.lsp4e.test/src/org/eclipse/lsp4e/test/completion/DynamicCompletionRegistrationTest.java

@@ -0,0 +1,104 @@
+/*******************************************************************************
+ * Copyright (c) 2025 Vegard IT GmbH and others.
+ * This program and the accompanying materials are made
+ * available under the terms of the Eclipse Public License 2.0
+ * which is available at https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ * Contributors:
+ *  Sebastian Thomschke (Vegard IT GmbH) - initial implementation
+ *******************************************************************************/
+package org.eclipse.lsp4e.test.completion;
+
+import static org.eclipse.lsp4e.test.utils.TestUtils.waitForAndAssertCondition;
+import static org.junit.Assert.*;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.TimeUnit;
+
+import org.eclipse.core.resources.IFile;
+import org.eclipse.jface.text.ITextViewer;
+import org.eclipse.jface.text.contentassist.ICompletionProposal;
+import org.eclipse.lsp4e.operations.completion.LSContentAssistProcessor;
+import org.eclipse.lsp4e.test.utils.AbstractTestWithProject;
+import org.eclipse.lsp4e.test.utils.TestUtils;
+import org.eclipse.lsp4e.tests.mock.MockLanguageServer;
+import org.eclipse.lsp4j.CompletionItem;
+import org.eclipse.lsp4j.CompletionItemKind;
+import org.eclipse.lsp4j.CompletionList;
+import org.eclipse.lsp4j.CompletionOptions;
+import org.eclipse.lsp4j.Position;
+import org.eclipse.lsp4j.Range;
+import org.eclipse.lsp4j.Registration;
+import org.eclipse.lsp4j.RegistrationParams;
+import org.eclipse.lsp4j.TextEdit;
+import org.eclipse.lsp4j.jsonrpc.messages.Either;
+import org.eclipse.lsp4j.services.LanguageClient;
+import org.junit.Before;
+import org.junit.Test;
+
+import com.google.gson.Gson;
+
+/**
+ * Verifies that dynamic registration of completion updates LSP4E server
+ * capabilities and enables content assist proposals.
+ */
+public class DynamicCompletionRegistrationTest extends AbstractTestWithProject {
+
+	private LSContentAssistProcessor contentAssistProcessor;
+
+	@Before
+	public void setup() {
+		contentAssistProcessor = new LSContentAssistProcessor(true, false);
+	}
+
+    @Test
+    public void testDynamicCompletionRegistrationProvidesProposalsAndTriggers() throws Exception {
+        // Prepare a file and open a viewer
+        IFile file = TestUtils.createUniqueTestFile(project, "");
+        ITextViewer viewer = TestUtils.openTextViewer(file);
+
+        // Ensure the mock LS is up
+        waitForAndAssertCondition(5_000, () -> !MockLanguageServer.INSTANCE.getRemoteProxies().isEmpty());
+        LanguageClient client = getMockClient();
+        assertNotNull(client);
+
+        // Provide a simple completion item in the mock LS
+        var items = new ArrayList<CompletionItem>();
+        var item = new CompletionItem();
+        item.setLabel("Alpha");
+        item.setKind(CompletionItemKind.Text);
+        item.setTextEdit(Either.forLeft(new TextEdit(new Range(new Position(0, 0), new Position(0, 0)), "Alpha")));
+        items.add(item);
+        MockLanguageServer.INSTANCE.setCompletionList(new CompletionList(false, items));
+
+        // Dynamically register completion with trigger characters (server-like behavior)
+        var registration = new Registration();
+        registration.setId("test-completion-reg");
+        registration.setMethod("textDocument/completion");
+        var opts = new CompletionOptions();
+        opts.setTriggerCharacters(List.of(".", "/", "#"));
+        registration.setRegisterOptions(new Gson().toJsonTree(opts));
+        client.registerCapability(new RegistrationParams(List.of(registration))).get(2, TimeUnit.SECONDS);
+
+        // Compute proposals (manual invocation). Should return the mock item.
+        ICompletionProposal[] proposals = contentAssistProcessor.computeCompletionProposals(viewer, 0);
+        assertEquals(1, proposals.length);
+        assertEquals("Alpha", proposals[0].getDisplayString());
+
+        // Ask processor for auto-activation triggers and assert they include the registered ones
+        char[] triggers = contentAssistProcessor.getCompletionProposalAutoActivationCharacters();
+        String trig = new String(triggers != null ? triggers : new char[0]);
+        assertTrue(trig.indexOf('.') >= 0);
+        assertTrue(trig.indexOf('/') >= 0);
+        assertTrue(trig.indexOf('#') >= 0);
+    }
+
+	private LanguageClient getMockClient() {
+		var proxies = MockLanguageServer.INSTANCE.getRemoteProxies();
+		assertEquals(1, proxies.size());
+		return proxies.get(0);
+	}
+}

org.eclipse.lsp4e/src/org/eclipse/lsp4e/LanguageServerWrapper.java

@@ -83,6 +83,7 @@
 import org.eclipse.lsp4j.ClientCapabilities;
 import org.eclipse.lsp4j.ClientInfo;
 import org.eclipse.lsp4j.CodeActionOptions;
+import org.eclipse.lsp4j.CompletionOptions;
 import org.eclipse.lsp4j.DidChangeWorkspaceFoldersParams;
 import org.eclipse.lsp4j.DocumentFormattingOptions;
 import org.eclipse.lsp4j.DocumentOnTypeFormattingOptions;
@@ -1107,6 +1108,18 @@ public void registerCapability(RegistrationParams params) {
 				serverCapabilities.setCodeActionProvider(Boolean.TRUE);
 				addRegistration(reg, () -> serverCapabilities.setCodeActionProvider(beforeRegistration));
 				break;
+			case "textDocument/completion": { //$NON-NLS-1$
+				CompletionOptions previous = serverCapabilities.getCompletionProvider();
+				try {
+					final var completionOpts = new Gson().fromJson((JsonObject) reg.getRegisterOptions(),
+							CompletionOptions.class);
+					serverCapabilities.setCompletionProvider(completionOpts);
+					addRegistration(reg, () -> serverCapabilities.setCompletionProvider(previous));
+				} catch (final Exception ex) {
+					LanguageServerPlugin.logError(ex);
+				}
+				break;
+			}
 			case "workspace/symbol": //$NON-NLS-1$
 				final Either<Boolean, WorkspaceSymbolOptions> workspaceSymbolBeforeRegistration = serverCapabilities.getWorkspaceSymbolProvider();
 				serverCapabilities.setWorkspaceSymbolProvider(Boolean.TRUE);

org.eclipse.lsp4e/src/org/eclipse/lsp4e/internal/SupportedFeatures.java

@@ -96,6 +96,7 @@ public static TextDocumentClientCapabilities getTextDocumentClientCapabilities()
 				"detail", //$NON-NLS-1$
 				"additionalTextEdits"))); //$NON-NLS-1$
 		final var completionCapabilities = new CompletionCapabilities(completionItemCapabilities);
+		completionCapabilities.setDynamicRegistration(Boolean.TRUE);
 		completionCapabilities.setContextSupport(true);
 		completionCapabilities.setCompletionList(new CompletionListCapabilities(List.of( //
 				"commitCharacters", //$NON-NLS-1$