[tool] Refactor WebTemplate to be immutable (#168201)

This flows better. No uncertainty about calling the substitutions
function more than once, etc.

packages/flutter_tools/lib/src/build_system/targets/web.dart

@@ -571,7 +571,7 @@ _flutter.buildConfig = ${jsonEncode(buildConfig)};
     // in question.
     final String? serviceWorkerVersion =
         includeServiceWorkerSettings ? Random().nextInt(1 << 32).toString() : null;
-    bootstrapTemplate.applySubstitutions(
+    final String bootstrapContent = bootstrapTemplate.withSubstitutions(
       baseHref: '',
       serviceWorkerVersion: serviceWorkerVersion,
       flutterJsFile: flutterJsFile,
@@ -581,7 +581,7 @@ _flutter.buildConfig = ${jsonEncode(buildConfig)};
     final File outputFlutterBootstrapJs = fileSystem.file(
       fileSystem.path.join(environment.outputDir.path, 'flutter_bootstrap.js'),
     );
-    await outputFlutterBootstrapJs.writeAsString(bootstrapTemplate.content);
+    await outputFlutterBootstrapJs.writeAsString(bootstrapContent);
 
     await for (final FileSystemEntity file in webResources.list(recursive: true)) {
       if (file is File && file.basename == 'index.html') {
@@ -592,18 +592,18 @@ _flutter.buildConfig = ${jsonEncode(buildConfig)};
           _emitWebTemplateWarning(environment, relativePath, warning);
         }
 
-        indexHtmlTemplate.applySubstitutions(
+        final String indexHtmlContent = indexHtmlTemplate.withSubstitutions(
           baseHref: environment.defines[kBaseHref] ?? '/',
           serviceWorkerVersion: serviceWorkerVersion,
           flutterJsFile: flutterJsFile,
           buildConfig: buildConfig,
-          flutterBootstrapJs: bootstrapTemplate.content,
+          flutterBootstrapJs: bootstrapContent,
         );
         final File outputIndexHtml = fileSystem.file(
           fileSystem.path.join(environment.outputDir.path, relativePath),
         );
         await outputIndexHtml.create(recursive: true);
-        await outputIndexHtml.writeAsString(indexHtmlTemplate.content);
+        await outputIndexHtml.writeAsString(indexHtmlContent);
       }
     }
   }

packages/flutter_tools/lib/src/isolated/devfs_web.dart

@@ -132,7 +132,7 @@ class WebAssetServer implements AssetReader {
     this._canaryFeatures, {
     required this.webRenderer,
     required this.useLocalCanvasKit,
-  }) : basePath = _getWebTemplate('index.html', _kDefaultIndex).getBaseHref() {
+  }) : basePath = WebTemplate.baseHref(_htmlTemplate('index.html', _kDefaultIndex)) {
     // TODO(srujzs): Remove this assertion when the library bundle format is
     // supported without canary mode.
     if (_ddcModuleSystem) {
@@ -671,13 +671,12 @@ _flutter.buildConfig = ${jsonEncode(buildConfig)};
       'flutter_bootstrap.js',
       generateDefaultFlutterBootstrapScript(includeServiceWorkerSettings: false),
     );
-    bootstrapTemplate.applySubstitutions(
+    return bootstrapTemplate.withSubstitutions(
       baseHref: '/',
       serviceWorkerVersion: null,
       buildConfig: _buildConfigString,
       flutterJsFile: _flutterJsFile,
     );
-    return bootstrapTemplate.content;
   }
 
   shelf.Response _serveFlutterBootstrapJs() {
@@ -689,16 +688,15 @@ _flutter.buildConfig = ${jsonEncode(buildConfig)};
 
   shelf.Response _serveIndexHtml() {
     final WebTemplate indexHtml = _getWebTemplate('index.html', _kDefaultIndex);
-    indexHtml.applySubstitutions(
-      // Currently, we don't support --base-href for the "run" command.
-      baseHref: '/',
-      serviceWorkerVersion: null,
-      buildConfig: _buildConfigString,
-      flutterJsFile: _flutterJsFile,
-      flutterBootstrapJs: _flutterBootstrapJsContent,
-    );
     return shelf.Response.ok(
-      indexHtml.content,
+      indexHtml.withSubstitutions(
+        // Currently, we don't support --base-href for the "run" command.
+        baseHref: '/',
+        serviceWorkerVersion: null,
+        buildConfig: _buildConfigString,
+        flutterJsFile: _flutterJsFile,
+        flutterBootstrapJs: _flutterBootstrapJsContent,
+      ),
       headers: <String, String>{HttpHeaders.contentTypeHeader: 'text/html'},
     );
   }
@@ -1375,7 +1373,11 @@ String? _stripBasePath(String path, String basePath) {
 }
 
 WebTemplate _getWebTemplate(String filename, String fallbackContent) {
-  final File template = globals.fs.currentDirectory.childDirectory('web').childFile(filename);
+  final String htmlContent = _htmlTemplate(filename, fallbackContent);
-  final String htmlContent = template.existsSync() ? template.readAsStringSync() : fallbackContent;
   return WebTemplate(htmlContent);
 }
+
+String _htmlTemplate(String filename, String fallbackContent) {
+  final File template = globals.fs.currentDirectory.childDirectory('web').childFile(filename);
+  return template.existsSync() ? template.readAsStringSync() : fallbackContent;
+}

packages/flutter_tools/lib/src/web_template.dart

@@ -4,6 +4,7 @@
 
 import 'package:html/dom.dart';
 import 'package:html/parser.dart';
+import 'package:meta/meta.dart';
 
 import 'base/common.dart';
 import 'base/file_system.dart';
@@ -29,16 +30,12 @@ class WebTemplateWarning {
 /// }
 /// ```
 class WebTemplate {
-  WebTemplate(this._content);
+  const WebTemplate(this._content);
 
-  String get content => _content;
+  final String _content;
-  String _content;
 
-  Document _getDocument() => parse(_content);
+  static String baseHref(String html) {
-
+    final Element? baseElement = parse(html).querySelector('base');
-  /// Parses the base href from the index.html file.
-  String getBaseHref() {
-    final Element? baseElement = _getDocument().querySelector('base');
     final String? baseHref =
         baseElement?.attributes == null ? null : baseElement!.attributes['href'];
 
@@ -94,20 +91,23 @@ class WebTemplate {
     return WebTemplateWarning(warningText, lineCount + 1);
   }
 
-  /// Applies substitutions to the content of the index.html file.
+  /// Applies substitutions to the content of the index.html file and returns the result.
-  void applySubstitutions({
+  @useResult
+  String withSubstitutions({
     required String baseHref,
     required String? serviceWorkerVersion,
     required File flutterJsFile,
     String? buildConfig,
     String? flutterBootstrapJs,
   }) {
-    if (_content.contains(kBaseHrefPlaceholder)) {
+    String newContent = _content;
-      _content = _content.replaceAll(kBaseHrefPlaceholder, baseHref);
+
+    if (newContent.contains(kBaseHrefPlaceholder)) {
+      newContent = newContent.replaceAll(kBaseHrefPlaceholder, baseHref);
     }
 
     if (serviceWorkerVersion != null) {
-      _content = _content
+      newContent = newContent
           .replaceFirst(
             // Support older `var` syntax as well as new `const` syntax
             RegExp('(const|var) serviceWorkerVersion = null'),
@@ -120,21 +120,22 @@ class WebTemplate {
             "navigator.serviceWorker.register('flutter_service_worker.js?v=$serviceWorkerVersion')",
           );
     }
-    _content = _content.replaceAll(
+    newContent = newContent.replaceAll(
       '{{flutter_service_worker_version}}',
       serviceWorkerVersion != null ? '"$serviceWorkerVersion"' : 'null',
     );
     if (buildConfig != null) {
-      _content = _content.replaceAll('{{flutter_build_config}}', buildConfig);
+      newContent = newContent.replaceAll('{{flutter_build_config}}', buildConfig);
     }
 
-    if (_content.contains('{{flutter_js}}')) {
+    if (newContent.contains('{{flutter_js}}')) {
-      _content = _content.replaceAll('{{flutter_js}}', flutterJsFile.readAsStringSync());
+      newContent = newContent.replaceAll('{{flutter_js}}', flutterJsFile.readAsStringSync());
     }
 
     if (flutterBootstrapJs != null) {
-      _content = _content.replaceAll('{{flutter_bootstrap_js}}', flutterBootstrapJs);
+      newContent = newContent.replaceAll('{{flutter_bootstrap_js}}', flutterBootstrapJs);
     }
+    return newContent;
   }
 }
 

packages/flutter_tools/test/general.shard/web_template_test.dart

@@ -221,93 +221,96 @@ void main() {
   flutterJs.writeAsStringSync('(flutter.js content)');
 
   test('can parse baseHref', () {
-    expect(WebTemplate('<base href="/foo/111/">').getBaseHref(), 'foo/111');
+    expect(WebTemplate.baseHref('<base href="/foo/111/">'), 'foo/111');
-    expect(WebTemplate(htmlSample1).getBaseHref(), 'foo/222');
+    expect(WebTemplate.baseHref(htmlSample1), 'foo/222');
-    expect(WebTemplate(htmlSample2).getBaseHref(), ''); // Placeholder base href.
+    expect(WebTemplate.baseHref(htmlSample2), ''); // Placeholder base href.
   });
 
   test('handles missing baseHref', () {
-    expect(WebTemplate('').getBaseHref(), '');
+    expect(WebTemplate.baseHref(''), '');
-    expect(WebTemplate('<base>').getBaseHref(), '');
+    expect(WebTemplate.baseHref('<base>'), '');
-    expect(WebTemplate(htmlSample3).getBaseHref(), '');
+    expect(WebTemplate.baseHref(htmlSample3), '');
   });
 
   test('throws on invalid baseHref', () {
-    expect(() => WebTemplate('<base href>').getBaseHref(), throwsToolExit());
+    expect(() => WebTemplate.baseHref('<base href>'), throwsToolExit());
-    expect(() => WebTemplate('<base href="">').getBaseHref(), throwsToolExit());
+    expect(() => WebTemplate.baseHref('<base href="">'), throwsToolExit());
-    expect(() => WebTemplate('<base href="foo/111">').getBaseHref(), throwsToolExit());
+    expect(() => WebTemplate.baseHref('<base href="foo/111">'), throwsToolExit());
-    expect(() => WebTemplate('<base href="foo/111/">').getBaseHref(), throwsToolExit());
+    expect(() => WebTemplate.baseHref('<base href="foo/111/">'), throwsToolExit());
-    expect(() => WebTemplate('<base href="/foo/111">').getBaseHref(), throwsToolExit());
+    expect(() => WebTemplate.baseHref('<base href="/foo/111">'), throwsToolExit());
   });
 
   test('applies substitutions', () {
-    final WebTemplate indexHtml = WebTemplate(htmlSample2);
+    const WebTemplate indexHtml = WebTemplate(htmlSample2);
-    indexHtml.applySubstitutions(
+
-      baseHref: '/foo/333/',
-      serviceWorkerVersion: 'v123xyz',
-      flutterJsFile: flutterJs,
-    );
     expect(
-      indexHtml.content,
+      indexHtml.withSubstitutions(
+        baseHref: '/foo/333/',
+        serviceWorkerVersion: 'v123xyz',
+        flutterJsFile: flutterJs,
+      ),
       htmlSample2Replaced(baseHref: '/foo/333/', serviceWorkerVersion: 'v123xyz'),
     );
   });
 
   test('applies substitutions with legacy var version syntax', () {
-    final WebTemplate indexHtml = WebTemplate(htmlSampleLegacyVar);
+    const WebTemplate indexHtml = WebTemplate(htmlSampleLegacyVar);
-    indexHtml.applySubstitutions(
-      baseHref: '/foo/333/',
-      serviceWorkerVersion: 'v123xyz',
-      flutterJsFile: flutterJs,
-    );
     expect(
-      indexHtml.content,
+      indexHtml.withSubstitutions(
+        baseHref: '/foo/333/',
+        serviceWorkerVersion: 'v123xyz',
+        flutterJsFile: flutterJs,
+      ),
       htmlSample2Replaced(baseHref: '/foo/333/', serviceWorkerVersion: 'v123xyz'),
     );
   });
 
   test('applies substitutions to inline flutter.js bootstrap script', () {
-    final WebTemplate indexHtml = WebTemplate(htmlSampleInlineFlutterJsBootstrap);
+    const WebTemplate indexHtml = WebTemplate(htmlSampleInlineFlutterJsBootstrap);
     expect(indexHtml.getWarnings(), isEmpty);
 
-    indexHtml.applySubstitutions(
+    expect(
-      baseHref: '/',
+      indexHtml.withSubstitutions(
-      serviceWorkerVersion: '(service worker version)',
+        baseHref: '/',
-      flutterJsFile: flutterJs,
+        serviceWorkerVersion: '(service worker version)',
-      buildConfig: '(build config)',
+        flutterJsFile: flutterJs,
+        buildConfig: '(build config)',
+      ),
+      htmlSampleInlineFlutterJsBootstrapOutput,
     );
-    expect(indexHtml.content, htmlSampleInlineFlutterJsBootstrapOutput);
   });
 
   test('applies substitutions to full flutter_bootstrap.js replacement', () {
-    final WebTemplate indexHtml = WebTemplate(htmlSampleFullFlutterBootstrapReplacement);
+    const WebTemplate indexHtml = WebTemplate(htmlSampleFullFlutterBootstrapReplacement);
     expect(indexHtml.getWarnings(), isEmpty);
 
-    indexHtml.applySubstitutions(
+    expect(
-      baseHref: '/',
+      indexHtml.withSubstitutions(
-      serviceWorkerVersion: '(service worker version)',
+        baseHref: '/',
-      flutterJsFile: flutterJs,
+        serviceWorkerVersion: '(service worker version)',
-      buildConfig: '(build config)',
+        flutterJsFile: flutterJs,
-      flutterBootstrapJs: '(flutter bootstrap script)',
+        buildConfig: '(build config)',
+        flutterBootstrapJs: '(flutter bootstrap script)',
+      ),
+      htmlSampleFullFlutterBootstrapReplacementOutput,
     );
-    expect(indexHtml.content, htmlSampleFullFlutterBootstrapReplacementOutput);
   });
 
   test('re-parses after substitutions', () {
-    final WebTemplate indexHtml = WebTemplate(htmlSample2);
+    const WebTemplate indexHtml = WebTemplate(htmlSample2);
-    expect(indexHtml.getBaseHref(), ''); // Placeholder base href.
+    expect(WebTemplate.baseHref(htmlSample2), ''); // Placeholder base href.
 
-    indexHtml.applySubstitutions(
+    final String substituted = indexHtml.withSubstitutions(
       baseHref: '/foo/333/',
       serviceWorkerVersion: 'v123xyz',
       flutterJsFile: flutterJs,
     );
     // The parsed base href should be updated after substitutions.
-    expect(indexHtml.getBaseHref(), 'foo/333');
+    expect(WebTemplate.baseHref(substituted), 'foo/333');
   });
 
   test('warns on legacy service worker patterns', () {
-    final WebTemplate indexHtml = WebTemplate(htmlSampleLegacyVar);
+    const WebTemplate indexHtml = WebTemplate(htmlSampleLegacyVar);
     final List<WebTemplateWarning> warnings = indexHtml.getWarnings();
     expect(warnings.length, 2);
 
@@ -316,7 +319,7 @@ void main() {
   });
 
   test('warns on legacy FlutterLoader.loadEntrypoint', () {
-    final WebTemplate indexHtml = WebTemplate(htmlSampleLegacyLoadEntrypoint);
+    const WebTemplate indexHtml = WebTemplate(htmlSampleLegacyLoadEntrypoint);
     final List<WebTemplateWarning> warnings = indexHtml.getWarnings();
 
     expect(warnings.length, 1);