langium/lsp: Added 'getReferenceCandidates(...)' hook to 'DefaultCompletionProvider' (#1385)

* allows to add completion-specific scope computation refinements
* removed the 'filterCrossReference' hook method, as its functionality is covered by the new hook

Author: sailingKieler

packages/langium/src/lsp/completion/completion-provider.ts

@@ -24,7 +24,7 @@ import * as ast from '../../languages/generated/ast.js';
 import { assignMandatoryProperties, getContainerOfType } from '../../utils/ast-utils.js';
 import { findDeclarationNodeAtOffset, findLeafNodeBeforeOffset } from '../../utils/cst-utils.js';
 import { getEntryRule, getExplicitRuleType } from '../../utils/grammar-utils.js';
-import { stream } from '../../utils/stream.js';
+import { stream, type Stream } from '../../utils/stream.js';
 import { findFirstFeatures, findNextFeatures } from './follow-element-computation.js';
 
 export type CompletionAcceptor = (context: CompletionContext, value: CompletionValueItem) => void
@@ -428,18 +428,28 @@ export class DefaultCompletionProvider implements CompletionProvider {
                 property: assignment.feature
             };
             try {
-                const scope = this.scopeProvider.getScope(refInfo);
-                scope.getAllElements().forEach(e => {
-                    if (this.filterCrossReference(context, e)) {
-                        acceptor(context, this.createReferenceCompletionItem(e));
-                    }
-                });
+                this.getReferenceCandidates(refInfo, context).forEach(
+                    c => acceptor(context, this.createReferenceCompletionItem(c))
+                );
             } catch (err) {
                 console.error(err);
             }
         }
     }
 
+    /**
+     * Override this method to change how the stream of candidates is determined for a reference.
+     * This way completion-specific modifications and refinements can be added to the proposals computation
+     *  beyond the rules being implemented in the scope provider, e.g. filtering.
+     *
+     * @param refInfo Information about the reference for which the candidates are requested.
+     * @param _context Information about the completion request including document, cursor position, token under cursor, etc.
+     * @returns A stream of all elements being valid for the given reference.
+     */
+    protected getReferenceCandidates(refInfo: ReferenceInfo, _context: CompletionContext): Stream<AstNodeDescription> {
+        return this.scopeProvider.getScope(refInfo).getAllElements();
+    }
+
     /**
      * Override this method to change how reference completion items are created.
      * To change the `kind` of a completion item, override the `NodeKindProvider` service instead.
@@ -456,10 +466,6 @@ export class DefaultCompletionProvider implements CompletionProvider {
         };
     }
 
-    protected filterCrossReference(_context: CompletionContext, _nodeDescription: AstNodeDescription): boolean {
-        return true;
-    }
-
     protected completionForKeyword(context: CompletionContext, keyword: ast.Keyword, acceptor: CompletionAcceptor): MaybePromise<void> {
         if (!this.filterKeyword(context, keyword)) {
             return;