[ Widget Preview ] Update generated scaffold project to include early preview rendering (flutter#162847)

With this change, `flutter widget-preview start` will launch a working
widget preview environment that can render previews from a target
project.

Also fixes an issue where `--offline` wasn't being respected by some pub
operations.

Author: bkonyi

packages/flutter_tools/lib/src/base/signals.dart

@@ -124,7 +124,7 @@ class LocalSignals implements Signals {
 
     // If _handlersList[signal] is empty, then lookup the cached stream
     // controller and unsubscribe from the stream.
-    if (_handlersList.isEmpty) {
+    if (_handlersList[signal]!.isEmpty) {
       await _streamSubscriptions[signal]?.cancel();
     }
     return true;

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

@@ -491,9 +491,26 @@ final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with C
     await pub.interactively(
       <String>[
         pubAdd,
+        if (offline) '--offline',
         '--directory',
         widgetPreviewScaffoldProject.directory.path,
-        '${rootProject.manifest.appName}:{"path":${rootProject.directory.path}}',
+        // Ensure the path using POSIX separators, otherwise the "path_not_posix" check will fail.
+        '${rootProject.manifest.appName}:{"path":${rootProject.directory.path.replaceAll(r"\", "/")}}',
+      ],
+      context: PubContext.pubAdd,
+      command: pubAdd,
+      touchesPackageConfig: true,
+    );
+
+    // Adds a dependency on flutter_lints, which is referenced by the
+    // analysis_options.yaml generated by the 'app' template.
+    await pub.interactively(
+      <String>[
+        pubAdd,
+        if (offline) '--offline',
+        '--directory',
+        widgetPreviewScaffoldProject.directory.path,
+        'flutter_lints',
       ],
       context: PubContext.pubAdd,
       command: pubAdd,

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

@@ -19,7 +19,7 @@ class PreviewCodeGenerator {
   /// project.
   final FlutterProject widgetPreviewScaffoldProject;
 
-  static const String generatedPreviewFilePath = 'lib/generated_preview.dart';
+  static const String generatedPreviewFilePath = 'lib/src/generated_preview.dart';
 
   /// Generates code used by the widget preview scaffold based on the preview instances listed in
   /// [previews].
@@ -43,10 +43,7 @@ class PreviewCodeGenerator {
   void populatePreviewsInGeneratedPreviewScaffold(PreviewMapping previews) {
     final Library lib = Library(
       (LibraryBuilder b) => b.body.addAll(<Spec>[
-        Directive.import(
-          // TODO(bkonyi): update with actual location in the framework
-          'package:widget_preview/widget_preview.dart',
-        ),
+        Directive.import('package:flutter/widgets.dart'),
         Method(
           (MethodBuilder b) =>
               b

packages/flutter_tools/templates/template_manifest.json

@@ -353,6 +353,10 @@
     "templates/plugin_swift_package_manager/macos.tmpl/projectName.tmpl/Sources/projectName.tmpl/PrivacyInfo.xcprivacy",
 
     "templates/widget_preview_scaffold/lib/main.dart.tmpl",
+    "templates/widget_preview_scaffold/lib/src/widget_preview_rendering.dart.tmpl",
+    "templates/widget_preview_scaffold/lib/src/controls.dart.tmpl",
+    "templates/widget_preview_scaffold/lib/src/generated_preview.dart.tmpl",
+    "templates/widget_preview_scaffold/lib/src/utils.dart.tmpl",
     "templates/widget_preview_scaffold/pubspec.yaml.tmpl",
     "templates/widget_preview_scaffold/README.md.tmpl",
 

packages/flutter_tools/templates/widget_preview_scaffold/lib/main.dart.tmpl

@@ -2,25 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// TODO(bkonyi): Implement.
+import 'src/widget_preview_rendering.dart';
+import 'src/generated_preview.dart';
 
-import 'package:flutter/material.dart';
-
-void main() {
-  runApp(const MainApp());
-}
-
-class MainApp extends StatelessWidget {
-  const MainApp({super.key});
-
-  @override
-  Widget build(BuildContext context) {
-    return const MaterialApp(
-      home: Scaffold(
-        body: Center(
-          child: Text('Hello World!'),
-        ),
-      ),
-    );
-  }
+Future<void> main() async {
+  await mainImpl(previewsProvider: previews);
 }

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

@@ -0,0 +1,102 @@
+// 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 'package:flutter/material.dart';
+
+class _WidgetPreviewIconButton extends StatelessWidget {
+  const _WidgetPreviewIconButton({
+    required this.tooltip,
+    required this.onPressed,
+    required this.icon,
+  });
+
+  final String tooltip;
+  final void Function()? onPressed;
+  final IconData icon;
+
+  @override
+  Widget build(BuildContext context) {
+    return Tooltip(
+      message: tooltip,
+      child: Ink(
+        decoration: ShapeDecoration(
+          shape: const CircleBorder(),
+          color: onPressed != null ? Colors.lightBlue : Colors.grey,
+        ),
+        child: IconButton(
+          onPressed: onPressed,
+          icon: Icon(
+            color: Colors.white,
+            icon,
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+/// Provides controls to change the zoom level of a [WidgetPreview].
+class ZoomControls extends StatelessWidget {
+  /// Provides controls to change the zoom level of a [WidgetPreview].
+  const ZoomControls({
+    super.key,
+    required TransformationController transformationController,
+  }) : _transformationController = transformationController;
+
+  final TransformationController _transformationController;
+
+  @override
+  Widget build(BuildContext context) {
+    return Row(
+      mainAxisSize: MainAxisSize.min,
+      children: <Widget>[
+        _WidgetPreviewIconButton(
+          tooltip: 'Zoom in',
+          onPressed: _zoomIn,
+          icon: Icons.zoom_in,
+        ),
+        const SizedBox(
+          width: 10,
+        ),
+        _WidgetPreviewIconButton(
+          tooltip: 'Zoom out',
+          onPressed: _zoomOut,
+          icon: Icons.zoom_out,
+        ),
+        const SizedBox(
+          width: 10,
+        ),
+        _WidgetPreviewIconButton(
+          tooltip: 'Reset zoom',
+          onPressed: _reset,
+          icon: Icons.refresh,
+        ),
+      ],
+    );
+  }
+
+  void _zoomIn() {
+    _transformationController.value = Matrix4.copy(
+      _transformationController.value,
+    ).scaled(1.1);
+  }
+
+  void _zoomOut() {
+    final Matrix4 updated = Matrix4.copy(
+      _transformationController.value,
+    ).scaled(0.9);
+
+    // Don't allow for zooming out past the original size of the widget.
+    // Assumes scaling is evenly applied to the entire matrix.
+    if (updated.entry(0, 0) < 1.0) {
+      updated.setIdentity();
+    }
+
+    _transformationController.value = updated;
+  }
+
+  void _reset() {
+    _transformationController.value = Matrix4.identity();
+  }
+}

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

@@ -0,0 +1,7 @@
+// 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 'package:flutter/widgets.dart';
+
+List<WidgetPreview> previews() => <WidgetPreview>[];

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

@@ -0,0 +1,18 @@
+// 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 'package:flutter/material.dart';
+
+/// A basic vertical spacer.
+class VerticalSpacer extends StatelessWidget {
+  /// Creates a basic vertical spacer.
+  const VerticalSpacer({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const SizedBox(
+      height: 10,
+    );
+  }
+}