feat(language-service): add definitions for templateUrl (#32238)

Adds support for `getDefinitionAt` when called on a templateUrl
property assignment.

The currrent architecture for getting definitions is designed to be
called on templates, so we have to introduce a new
`getTsDefinitionAndBoundSpan` method to get Angular-specific definitions
in TypeScript files and pass a `readTemplate` closure that will read the
contents of a template using `TypeScriptServiceHost#getTemplates`. We
can probably go in and make this nicer in a future PR, though I'm not
sure what the best architecture should be yet.

Part of angular/vscode-ng-language-service#111

PR Close #32238

packages/language-service/src/definitions.ts

@@ -6,10 +6,13 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import * as path from 'path';
 import * as ts from 'typescript'; // used as value and is provided at runtime
 import {AstResult} from './common';
 import {locateSymbol} from './locate_symbol';
-import {Span} from './types';
+import {getPropertyAssignmentFromValue, isClassDecoratorProperty} from './template';
+import {Span, TemplateSource} from './types';
+import {findTightestNode} from './utils';
 
 /**
  * Convert Angular Span to TypeScript TextSpan. Angular Span has 'start' and
@@ -59,3 +62,67 @@ export function getDefinitionAndBoundSpan(
       definitions, textSpan,
   };
 }
+
+/**
+ * Gets an Angular-specific definition in a TypeScript source file.
+ */
+export function getTsDefinitionAndBoundSpan(
+    sf: ts.SourceFile, position: number,
+    tsLsHost: Readonly<ts.LanguageServiceHost>): ts.DefinitionInfoAndBoundSpan|undefined {
+  const node = findTightestNode(sf, position);
+  if (!node) return;
+  switch (node.kind) {
+    case ts.SyntaxKind.StringLiteral:
+    case ts.SyntaxKind.NoSubstitutionTemplateLiteral:
+      // Attempt to extract definition of a URL in a property assignment.
+      return getUrlFromProperty(node as ts.StringLiteralLike, tsLsHost);
+    default:
+      return undefined;
+  }
+}
+
+/**
+ * Attempts to get the definition of a file whose URL is specified in a property assignment in a
+ * directive decorator.
+ * Currently applies to `templateUrl` properties.
+ */
+function getUrlFromProperty(
+    urlNode: ts.StringLiteralLike,
+    tsLsHost: Readonly<ts.LanguageServiceHost>): ts.DefinitionInfoAndBoundSpan|undefined {
+  const asgn = getPropertyAssignmentFromValue(urlNode);
+  if (!asgn) return;
+  // If the URL is not a property of a class decorator, don't generate definitions for it.
+  if (!isClassDecoratorProperty(asgn)) return;
+
+  const sf = urlNode.getSourceFile();
+  switch (asgn.name.getText()) {
+    case 'templateUrl':
+      // Extract definition of the template file specified by this `templateUrl` property.
+      const url = path.join(path.dirname(sf.fileName), urlNode.text);
+
+      // If the file does not exist, bail. It is possible that the TypeScript language service host
+      // does not have a `fileExists` method, in which case optimistically assume the file exists.
+      if (tsLsHost.fileExists && !tsLsHost.fileExists(url)) return;
+
+      const templateDefinitions: ts.DefinitionInfo[] = [{
+        kind: ts.ScriptElementKind.externalModuleName,
+        name: url,
+        containerKind: ts.ScriptElementKind.unknown,
+        containerName: '',
+        // Reading the template is expensive, so don't provide a preview.
+        textSpan: {start: 0, length: 0},
+        fileName: url,
+      }];
+
+      return {
+        definitions: templateDefinitions,
+        textSpan: {
+          // Exclude opening and closing quotes in the url span.
+          start: urlNode.getStart() + 1,
+          length: urlNode.getWidth() - 2,
+        },
+      };
+    default:
+      return undefined;
+  }
+}