Add flutter widget-preview {start, clean} commands (#159510)

This is the initial tooling work for Flutter Widget Previews, adding two
commands: `flutter widget-preview start` and `flutter widget-preview
clean`.

The `start` command currently only checks to see if
`.dart_tool/widget_preview_scaffold/` exists and creates a new Flutter
project using the widget_preview_scaffold template if one isn't found.
The widget_preview_scaffold template currently only contains some
placeholder files and will be updated to include additional code
required by the scaffold.

The `clean` command simply deletes `.dart_tool/widget_preview_scaffold/`
if it's found.

This change also includes some refactoring of the `create` command in
order to share some project creation logic without requiring `flutter
widget-preview start` to spawn a new process simply to run `flutter
create -t widget_preview .dart_tool/widget_preview_scaffold`.

Related issue: flutter/flutter#115704

---------

Co-authored-by: Andrew Kolos <andrewrkolos@gmail.com>

Author: bkonyi

packages/flutter_tools/lib/executable.dart

@@ -43,6 +43,7 @@ import 'src/commands/symbolize.dart';
 import 'src/commands/test.dart';
 import 'src/commands/update_packages.dart';
 import 'src/commands/upgrade.dart';
+import 'src/commands/widget_preview.dart';
 import 'src/devtools_launcher.dart';
 import 'src/features.dart';
 import 'src/globals.dart' as globals;
@@ -250,6 +251,7 @@ List<FlutterCommand> generateCommands({
     verbose: verbose,
     nativeAssetsBuilder: globals.nativeAssetsBuilder,
   ),
+  WidgetPreviewCommand(),
   UpgradeCommand(verboseHelp: verboseHelp),
   SymbolizeCommand(
     stdio: globals.stdio,

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

@@ -488,7 +488,7 @@ class Environment {
   /// The path to the package configuration file to use for compilation.
   ///
   /// This is used by package:package_config to locate the actual package_config.json
-  /// file. If not provided, defaults to `.dart_tool/package_config.json`.
+  /// file. If not provided in tests, defaults to `.dart_tool/package_config.json`.
   final String packageConfigPath;
 
   /// The `BUILD_DIR` environment variable.

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

@@ -35,10 +35,73 @@ const String kPlatformHelp =
   'When adding platforms to a plugin project, the pubspec.yaml will be updated with the requested platform. '
   'Adding desktop platforms requires the corresponding desktop config setting to be enabled.';
 
-class CreateCommand extends CreateBase {
-  CreateCommand({
-    super.verboseHelp = false,
-  }) {
+class CreateCommand extends FlutterCommand with CreateBase {
+  CreateCommand({bool verboseHelp = false}) {
+    addPubOptions();
+    argParser.addFlag(
+      'with-driver-test',
+      help: '(deprecated) Historically, this added a flutter_driver dependency and generated a '
+            'sample "flutter drive" test. Now it does nothing. Consider using the '
+            '"integration_test" package: https://pub.dev/packages/integration_test',
+      hide: !verboseHelp,
+    );
+    argParser.addFlag(
+      'overwrite',
+      help: 'When performing operations, overwrite existing files.',
+    );
+    argParser.addOption(
+      'description',
+      defaultsTo: 'A new Flutter project.',
+      help:
+          'The description to use for your new Flutter project. This string ends up in the pubspec.yaml file.',
+    );
+    argParser.addOption(
+      'org',
+      defaultsTo: 'com.example',
+      help:
+          'The organization responsible for your new Flutter project, in reverse domain name notation. '
+          'This string is used in Java package names and as prefix in the iOS bundle identifier.',
+    );
+    argParser.addOption(
+      'project-name',
+      help:
+          'The project name for this new Flutter project. This must be a valid dart package name.',
+    );
+    argParser.addOption(
+      'ios-language',
+      abbr: 'i',
+      defaultsTo: 'swift',
+      allowed: <String>['objc', 'swift'],
+      help: '(deprecated) The language to use for iOS-specific code, either Swift (recommended) or Objective-C (legacy).',
+      hide: !verboseHelp,
+    );
+    argParser.addOption(
+      'android-language',
+      abbr: 'a',
+      defaultsTo: 'kotlin',
+      allowed: <String>['java', 'kotlin'],
+      help: 'The language to use for Android-specific code, either Kotlin (recommended) or Java (legacy).',
+    );
+    argParser.addFlag(
+      'skip-name-checks',
+      help:
+          'Allow the creation of applications and plugins with invalid names. '
+          'This is only intended to enable testing of the tool itself.',
+      hide: !verboseHelp,
+    );
+    argParser.addFlag(
+      'implementation-tests',
+      help:
+          'Include implementation tests that verify the template functions correctly. '
+          'This is only intended to enable testing of the tool itself.',
+      hide: !verboseHelp,
+    );
+    argParser.addOption(
+      'initial-create-revision',
+      help: 'The Flutter SDK git commit hash to store in .migrate_config. This parameter is used by the tool '
+            'internally and should generally not be used manually.',
+      hide: !verboseHelp,
+    );
     addPlatformsOptions(customHelp: kPlatformHelp);
     argParser.addOption(
       'template',
@@ -437,12 +500,12 @@ class CreateCommand extends CreateBase {
         pubContext = PubContext.createPackage;
     }
 
-    if (boolArg('pub')) {
+    if (shouldCallPubGet) {
       final FlutterProject project = FlutterProject.fromDirectory(relativeDir);
       await pub.get(
         context: pubContext,
         project: project,
-        offline: boolArg('offline'),
+        offline: offline,
         outputMode: PubOutputMode.summaryOnly,
       );
       // Setting `includeIos` etc to false as with FlutterProjectType.package

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

@@ -48,96 +48,15 @@ const String _kDefaultPlatformArgumentHelp =
     'Platform folders (e.g. android/) will be generated in the target project. '
     'Adding desktop platforms requires the corresponding desktop config setting to be enabled.';
 
-/// Common behavior for `flutter create` commands.
-abstract class CreateBase extends FlutterCommand {
-  CreateBase({
-    required bool verboseHelp,
-  }) {
-    argParser.addFlag(
-      'pub',
-      defaultsTo: true,
-      help:
-          'Whether to run "flutter pub get" after the project has been created.',
-    );
-    argParser.addFlag(
-      'offline',
-      help:
-          'When "flutter pub get" is run by the create command, this indicates '
-          'whether to run it in offline mode or not. In offline mode, it will need to '
-          'have all dependencies already available in the pub cache to succeed.',
-    );
-    argParser.addFlag(
-      'with-driver-test',
-      help: '(deprecated) Historically, this added a flutter_driver dependency and generated a '
-            'sample "flutter drive" test. Now it does nothing. Consider using the '
-            '"integration_test" package: https://pub.dev/packages/integration_test',
-      hide: !verboseHelp,
-    );
-    argParser.addFlag(
-      'overwrite',
-      help: 'When performing operations, overwrite existing files.',
-    );
-    argParser.addOption(
-      'description',
-      defaultsTo: 'A new Flutter project.',
-      help:
-          'The description to use for your new Flutter project. This string ends up in the pubspec.yaml file.',
-    );
-    argParser.addOption(
-      'org',
-      defaultsTo: 'com.example',
-      help:
-          'The organization responsible for your new Flutter project, in reverse domain name notation. '
-          'This string is used in Java package names and as prefix in the iOS bundle identifier.',
-    );
-    argParser.addOption(
-      'project-name',
-      help:
-          'The project name for this new Flutter project. This must be a valid dart package name.',
-    );
-    argParser.addOption(
-      'ios-language',
-      abbr: 'i',
-      defaultsTo: 'swift',
-      allowed: <String>['objc', 'swift'],
-      help: '(deprecated) The language to use for iOS-specific code, either Swift (recommended) or Objective-C (legacy).',
-      hide: !verboseHelp,
-    );
-    argParser.addOption(
-      'android-language',
-      abbr: 'a',
-      defaultsTo: 'kotlin',
-      allowed: <String>['java', 'kotlin'],
-      help: 'The language to use for Android-specific code, either Kotlin (recommended) or Java (legacy).',
-    );
-    argParser.addFlag(
-      'skip-name-checks',
-      help:
-          'Allow the creation of applications and plugins with invalid names. '
-          'This is only intended to enable testing of the tool itself.',
-      hide: !verboseHelp,
-    );
-    argParser.addFlag(
-      'implementation-tests',
-      help:
-          'Include implementation tests that verify the template functions correctly. '
-          'This is only intended to enable testing of the tool itself.',
-      hide: !verboseHelp,
-    );
-    argParser.addOption(
-      'initial-create-revision',
-      help: 'The Flutter SDK git commit hash to store in .migrate_config. This parameter is used by the tool '
-            'internally and should generally not be used manually.',
-      hide: !verboseHelp,
-    );
-  }
-
+/// Common behavior for `flutter create` and `flutter widget-preview start` commands.
+mixin CreateBase on FlutterCommand {
   /// Pattern for a Windows file system drive (e.g. "D:").
   ///
   /// `dart:io` does not recognize strings matching this pattern as absolute
   /// paths, as they have no top level back-slash; however, users often specify
   /// this
   @visibleForTesting
+  @protected
   static final RegExp kWindowsDrivePattern = RegExp(r'^[a-zA-Z]:$');
 
   /// The output directory of the command.
@@ -162,6 +81,35 @@ abstract class CreateBase extends FlutterCommand {
     return globals.fs.path.normalize(projectDir.absolute.path);
   }
 
+  @protected
+  bool get shouldCallPubGet {
+    return boolArg('pub');
+  }
+
+  @protected
+  bool get offline {
+    return boolArg('offline');
+  }
+
+  /// Adds `--pub` and `--offline` options.
+  @protected
+  void addPubOptions() {
+    argParser
+      ..addFlag(
+        'pub',
+        defaultsTo: true,
+        help:
+            'Whether to run "flutter pub get" after the project has been created.',
+      )
+      ..addFlag(
+        'offline',
+        help:
+            'When "flutter pub get" is run by the create command, this indicates '
+            'whether to run it in offline mode or not. In offline mode, it will need to '
+            'have all dependencies already available in the pub cache to succeed.',
+      );
+  }
+
   /// Adds a `--platforms` argument.
   ///
   /// The help message of the argument is replaced with `customHelp` if `customHelp` is not null.
@@ -558,7 +506,7 @@ abstract class CreateBase extends FlutterCommand {
     final bool windowsPlatform = templateContext['windows'] as bool? ?? false;
     final bool webPlatform = templateContext['web'] as bool? ?? false;
 
-    if (boolArg('pub')) {
+    if (shouldCallPubGet) {
       final Environment environment = Environment(
         artifacts: globals.artifacts!,
         logger: globals.logger,