[ Widget Previews ] Add widget_preview_scaffold.shard to test the widget_preview_scaffold template contents (flutter#166358)

Adds a new `widget_preview_scaffold.shard` directory which contains a
hydrated `widget_preview_scaffold` template. This will allow for us to
write widget tests against the widgets defined in the templates.

This PR doesn't add any widget tests and is only adding the ability to
run these tests in follow up changes.

Fixes flutter#166416

Author: bkonyi

.ci.yaml

@@ -1589,6 +1589,29 @@ targets:
       - engine/**
       - DEPS
 
+  - name: Linux tool_tests_widget_preview_scaffold
+    recipe: flutter/flutter_drone
+    timeout: 60
+    bringup: true
+    properties:
+      add_recipes_cq: "true"
+      dependencies: >-
+        [
+          {"dependency": "android_sdk", "version": "version:35v1"},
+          {"dependency": "open_jdk", "version": "version:21"}
+        ]
+      shard: tool_tests
+      subshard: widget_preview_scaffold
+      tags: >
+        ["framework", "hostonly", "shard", "linux"]
+    runIf:
+      - dev/**
+      - packages/flutter_tools/**
+      - bin/**
+      - .ci.yaml
+      - engine/**
+      - DEPS
+
   - name: Linux_android_emu android_engine_vulkan_tests
     recipe: flutter/flutter_drone
     timeout: 60

dev/bots/test.dart

@@ -236,10 +236,17 @@ Future<void> _runIntegrationToolTests() async {
   );
 }
 
+Future<void> _runWidgetPreviewScaffoldToolTests() async {
+  await runFlutterTest(
+    path.join(_toolsPath, 'test', 'widget_preview_scaffold.shard', 'widget_preview_scaffold'),
+  );
+}
+
 Future<void> _runToolTests() async {
   await selectSubshard(<String, ShardRunner>{
     'general': _runGeneralToolTests,
     'commands': _runCommandsToolTests,
+    'widget_preview_scaffold': _runWidgetPreviewScaffoldToolTests,
   });
 }
 

packages/flutter_tools/lib/src/commands/widget_preview.dart

@@ -137,13 +137,20 @@ final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with C
         kHeadlessWeb,
         help: 'Launches Chrome in headless mode for testing.',
         hide: !verboseHelp,
+      )
+      ..addOption(
+        kWidgetPreviewScaffoldOutputDir,
+        help:
+            'Generated the widget preview environment scaffolding at a given location '
+            'for testing purposes.',
       );
   }
 
   static const String kWidgetPreviewScaffoldName = 'widget_preview_scaffold';
   static const String kLaunchPreviewer = 'launch-previewer';
   static const String kUseFlutterDesktop = 'desktop';
   static const String kHeadlessWeb = 'headless-web';
+  static const String kWidgetPreviewScaffoldOutputDir = 'scaffold-output-dir';
 
   @override
   Future<Set<DevelopmentArtifact>> get requiredArtifacts async => const <DevelopmentArtifact>{
@@ -189,30 +196,36 @@ final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with C
   );
 
   late final PreviewCodeGenerator _previewCodeGenerator;
-  late final PreviewManifest _previewManifest;
+  late final PreviewManifest _previewManifest = PreviewManifest(
+    logger: logger,
+    rootProject: rootProject,
+    fs: fs,
+    cache: cache,
+  );
 
   /// The currently running instance of the widget preview scaffold.
   AppInstance? _widgetPreviewApp;
 
   @override
   Future<FlutterCommandResult> runCommand() async {
-    final Directory widgetPreviewScaffold = rootProject.widgetPreviewScaffold;
-    _previewManifest = PreviewManifest(
-      logger: logger,
-      rootProject: rootProject,
-      fs: fs,
-      cache: cache,
-    );
+    final String? customPreviewScaffoldOutput = stringArg(kWidgetPreviewScaffoldOutputDir);
+    final Directory widgetPreviewScaffold =
+        customPreviewScaffoldOutput != null
+            ? fs.directory(customPreviewScaffoldOutput)
+            : rootProject.widgetPreviewScaffold;
 
     // Check to see if a preview scaffold has already been generated. If not,
     // generate one.
-    final bool generateScaffoldProject = _previewManifest.shouldGenerateProject();
+    final bool generateScaffoldProject =
+        customPreviewScaffoldOutput != null || _previewManifest.shouldGenerateProject();
     // TODO(bkonyi): can this be moved?
     widgetPreviewScaffold.createSync();
 
     if (generateScaffoldProject) {
       // WARNING: this log message is used by test/integration.shard/widget_preview_test.dart
-      logger.printStatus('Creating widget preview scaffolding at: ${widgetPreviewScaffold.path}');
+      logger.printStatus(
+        'Creating widget preview scaffolding at: ${widgetPreviewScaffold.absolute.path}',
+      );
       await generateApp(
         <String>['app', kWidgetPreviewScaffoldName],
         widgetPreviewScaffold,
@@ -230,6 +243,9 @@ final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with C
         overwrite: true,
         generateMetadata: false,
       );
+      if (customPreviewScaffoldOutput != null) {
+        return FlutterCommandResult.success();
+      }
       _previewManifest.generate();
 
       // WARNING: this access of widgetPreviewScaffoldProject needs to happen

packages/flutter_tools/templates/widget_preview_scaffold/README.md.tmpl

@@ -1,4 +1,4 @@
-# {{titleCaseProjectName}}
+# Widget Preview Scaffold
 
 This project is generated by `flutter widget-preview` and is used to host Widgets
 to be previewed in the widget previewer.

packages/flutter_tools/templates/widget_preview_scaffold/lib/src/generated_preview.dart.tmpl

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter/widgets.dart';
 import 'widget_preview.dart';
 
 List<WidgetPreview> previews() => <WidgetPreview>[];

packages/flutter_tools/templates/widget_preview_scaffold/lib/src/widget_preview.dart.tmpl

@@ -50,4 +50,4 @@ class WidgetPreview {
   /// If not provided, the default text scaling factor provided by [MediaQuery]
   /// will be used.
   final double? textScaleFactor;
-}
+}

packages/flutter_tools/templates/widget_preview_scaffold/pubspec.yaml.tmpl

@@ -1,4 +1,4 @@
-name: {{projectName}}
+name: widget_preview_scaffold
 description: Scaffolding for Flutter Widget Previews
 publish_to: 'none'
 version: 0.0.1
@@ -11,6 +11,9 @@ dependencies:
     sdk: flutter
   flutter_test:
     sdk: flutter
+  # These will be replaced with proper constraints after the template is hydrated.
+  flutter_lints: any
+  stack_trace: any
 
 flutter:
   uses-material-design: true

packages/flutter_tools/test/widget_preview_scaffold.shard/.gitignore

@@ -0,0 +1,2 @@
+# The generated web assets aren't needed for widget_preview_scaffold widget tests
+/widget_preview_scaffold/web/

packages/flutter_tools/test/widget_preview_scaffold.shard/README.md

@@ -0,0 +1,7 @@
+# Widget Preview Scaffold Testing
+
+This directory contains a hydrated instance of the `widget_preview_scaffold` project template that's used for generating the environment used by `flutter widget-preview start` to host Widget Previews, as well as utilities to detect if the hydrated instance is outdated when compared to the template files.
+
+# Updating the Hydrated Template
+
+If any of the `widget_preview_scaffold` template files are updated, `widget_preview_scaffold/test/template_change_detection_smoke_test.dart` will fail to indicate that the hydrated scaffold needs to be regenerated. To do this, run `dart test/widget_preview_scaffold.shard/update_widget_preview_scaffold.dart` to regenerate the project.

packages/flutter_tools/test/widget_preview_scaffold.shard/update_widget_preview_scaffold.dart

@@ -0,0 +1,36 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:io';
+
+import 'package:path/path.dart' as path; // flutter_ignore: package_path_import
+
+import 'widget_preview_scaffold/test/widget_preview_scaffold_test_utils.dart';
+
+/// Regenerates the widget_preview_scaffold if needed.
+void main() {
+  if (WidgetPreviewScaffoldTestUtils.checkForTemplateUpdates(
+    widgetPreviewScaffoldProject: Directory(
+      Platform.script.resolve('widget_preview_scaffold/').path,
+    ),
+    widgetPreviewScaffoldTemplateDir: Directory(
+      Platform.script.resolve(path.join('..', '..', 'templates', 'widget_preview_scaffold')).path,
+    ),
+  )) {
+    stdout.writeln('Changes detected in the widget_preview_scaffold project templates.');
+    stdout.writeln('Regenerating...');
+    final List<String> args = <String>[
+      'widget-preview',
+      'start',
+      '--scaffold-output-dir=${Platform.script.resolve('widget_preview_scaffold').path}',
+    ];
+    stdout.writeln('Executing: flutter ${args.join(' ')}');
+    final ProcessResult result = Process.runSync('flutter', args);
+    stdout.writeln(result.stdout);
+    stderr.writeln(result.stderr);
+    stdout.writeln('Regenerated widget_preview_scaffold.');
+  } else {
+    stdout.writeln('No changes detected in the widget_preview_scaffold project templates.');
+  }
+}