[Docs] Create 'center' snippets template (#40367)

* Create 'center' template

dev/snippets/config/templates/README.md

@@ -97,6 +97,15 @@ follows:
   `stateful_widget_material`, except that it wraps the stateful widget with a
   `Scaffold`.
 
+- [`stateful_widget_scaffold_center`](stateful_widget_scaffold_center.tmpl) : Similar to
+  `stateful_widget_scaffold`, except that it wraps the stateful widget with a
+  `Scaffold` _and_ a `Center`.
+
 - [`stateless_widget_scaffold`](stateless_widget_scaffold.tmpl) : Similar to
   `stateless_widget_material`, except that it wraps the stateless widget with a
   `Scaffold`.
+
+- [`stateless_widget_scaffold_center`](stateless_widget_scaffold_center.tmpl) : Similar to
+  `stateless_widget_scaffold`, except that it wraps the stateless widget with a
+  `Scaffold` _and_ a `Center`.
+

dev/snippets/config/templates/stateful_widget_scaffold_center.tmpl

+// Flutter code sample for {{id}}
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This Widget is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: _title,
+      home: Scaffold(
+        appBar: AppBar(title: const Text(_title)),
+        body: Center(
+          child: MyStatefulWidget(),
+        ),
+      ),
+    );
+  }
+}
+
+{{code-preamble}}
+
+class MyStatefulWidget extends StatefulWidget {
+  MyStatefulWidget({Key key}) : super(key: key);
+
+  @override
+  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
+}
+
+class _MyStatefulWidgetState extends State<MyStatefulWidget> {
+  {{code}}
+}

dev/snippets/config/templates/stateless_widget_scaffold_center.tmpl

+// Flutter code sample for {{id}}
+
+{{description}}
+
+import 'package:flutter/material.dart';
+
+{{code-imports}}
+
+void main() => runApp(new MyApp());
+
+/// This Widget is the main application widget.
+class MyApp extends StatelessWidget {
+  static const String _title = 'Flutter Code Sample';
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: _title,
+      home: Scaffold(
+        appBar: AppBar(title: const Text(_title)),
+        body: Center(
+          child: MyStatelessWidget(),
+        )
+      ),
+    );
+  }
+}
+
+
+{{code-preamble}}
+
+/// This is the stateless widget that the main application instantiates.
+class MyStatelessWidget extends StatelessWidget {
+  MyStatelessWidget({Key key}) : super(key: key);
+
+  @override
+  {{code}}
+}

packages/flutter/lib/src/material/checkbox_list_tile.dart

@@ -37,7 +37,7 @@ import 'theme_data.dart';
 /// To show the [CheckboxListTile] as disabled, pass null as the [onChanged]
 /// callback.
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 ///
 /// ![CheckboxListTile sample](https://flutter.github.io/assets-for-api-docs/assets/material/checkbox_list_tile.png)
 ///
@@ -53,15 +53,13 @@ import 'theme_data.dart';
 /// ```dart
 /// @override
 /// Widget build(BuildContext context) {
-///   return Center(
-///     child: CheckboxListTile(
+///   return CheckboxListTile(
 ///     title: const Text('Animate Slowly'),
 ///     value: timeDilation != 1.0,
 ///     onChanged: (bool value) {
 ///       setState(() { timeDilation = value ? 10.0 : 1.0; });
 ///     },
 ///     secondary: const Icon(Icons.hourglass_empty),
-///     ),
 ///   );
 /// }
 /// ```
@@ -84,7 +82,7 @@ import 'theme_data.dart';
 /// into one. Therefore, it may be necessary to create a custom radio tile
 /// widget to accommodate similar use cases.
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 ///
 /// ![Checkbox list tile semantics sample](https://flutter.github.io/assets-for-api-docs/assets/material/checkbox_list_tile_semantics.png)
 ///
@@ -147,9 +145,7 @@ import 'theme_data.dart';
 ///
 /// @override
 /// Widget build(BuildContext context) {
-///   return Scaffold(
-///     body: Center(
-///       child: LinkedLabelCheckbox(
+///   return LinkedLabelCheckbox(
 ///     label: 'Linked, tappable label text',
 ///     padding: const EdgeInsets.symmetric(horizontal: 20.0),
 ///     value: _isSelected,
@@ -158,8 +154,6 @@ import 'theme_data.dart';
 ///         _isSelected = newValue;
 ///       });
 ///     },
-///       ),
-///     ),
 ///   );
 /// }
 /// ```
@@ -172,7 +166,7 @@ import 'theme_data.dart';
 /// combining [Checkbox] with other widgets, such as [Text], [Padding] and
 /// [InkWell].
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 ///
 /// ![Custom checkbox list tile sample](https://flutter.github.io/assets-for-api-docs/assets/material/checkbox_list_tile_custom.png)
 ///
@@ -222,9 +216,7 @@ import 'theme_data.dart';
 ///
 /// @override
 /// Widget build(BuildContext context) {
-///   return Scaffold(
-///     body: Center(
-///       child: LabeledCheckbox(
+///   return LabeledCheckbox(
 ///     label: 'This is the label text',
 ///     padding: const EdgeInsets.symmetric(horizontal: 20.0),
 ///     value: _isSelected,
@@ -233,8 +225,6 @@ import 'theme_data.dart';
 ///         _isSelected = newValue;
 ///       });
 ///     },
-///       ),
-///     ),
 ///   );
 /// }
 /// ```

packages/flutter/lib/src/material/chip.dart

@@ -175,7 +175,7 @@ abstract class DeletableChipAttributes {
   /// that the user tapped the delete button. In order to delete the chip, you
   /// have to do something similar to the following sample:
   ///
-  /// {@tool snippet --template=stateful_widget_scaffold}
+  /// {@tool snippet --template=stateful_widget_scaffold_center}
   ///
   /// This sample shows how to use [onDeleted] to remove an entry when the
   /// delete button is tapped.
@@ -231,7 +231,7 @@ abstract class DeletableChipAttributes {
   /// ```dart
   /// @override
   /// Widget build(BuildContext context) {
-  ///   return Center(child: CastList());
+  ///   return CastList();
   /// }
   /// ```
   /// {@end-tool}

packages/flutter/lib/src/material/dropdown.dart

@@ -532,7 +532,7 @@ class DropdownButtonHideUnderline extends InheritedWidget {
 /// dropdown's value. It should also call [State.setState] to rebuild the
 /// dropdown with the new value.
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 ///
 /// This sample shows a `DropdownButton` with a customized icon, text style,
 /// and underline and whose value is one of "One", "Two", "Free", or "Four".
@@ -544,9 +544,7 @@ class DropdownButtonHideUnderline extends InheritedWidget {
 ///
 /// @override
 /// Widget build(BuildContext context) {
-///   return Scaffold(
-///     body: Center(
-///       child: DropdownButton<String>(
+///   return DropdownButton<String>(
 ///     value: dropdownValue,
 ///     icon: Icon(Icons.arrow_downward),
 ///     iconSize: 24,
@@ -571,8 +569,6 @@ class DropdownButtonHideUnderline extends InheritedWidget {
 ///         );
 ///       })
 ///       .toList(),
-///       ),
-///     ),
 ///   );
 /// }
 /// ```

packages/flutter/lib/src/material/icon_button.dart

@@ -38,7 +38,7 @@ const double _kMinButtonSize = kMinInteractiveDimension;
 /// requirements in the Material Design specification. The [alignment] controls
 /// how the icon itself is positioned within the hit region.
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 ///
 /// This sample shows an `IconButton` that uses the Material icon "volume_up" to
 /// increase the volume.
@@ -49,9 +49,7 @@ const double _kMinButtonSize = kMinInteractiveDimension;
 ///
 /// ```dart
 /// Widget build(BuildContext context) {
-///   return Scaffold(
-///     body: Center(
-///       child: Column(
+///   return Column(
 ///     mainAxisSize: MainAxisSize.min,
 ///     children: <Widget>[
 ///       IconButton(
@@ -65,8 +63,6 @@ const double _kMinButtonSize = kMinInteractiveDimension;
 ///       ),
 ///       Text('Volume : $_volume')
 ///     ],
-///       ),
-///     ),
 ///   );
 /// }
 /// ```

packages/flutter/lib/src/material/ink_well.dart

@@ -791,7 +791,7 @@ class _InkResponseState<T extends InkResponse> extends State<T> with AutomaticKe
 ///
 /// An example of this situation is as follows:
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 ///
 /// Tap the container to cause it to grow. Then, tap it again and hold before
 /// the widget reaches its maximum size to observe the clipped ink splash.
@@ -800,8 +800,7 @@ class _InkResponseState<T extends InkResponse> extends State<T> with AutomaticKe
 /// double sideLength = 50;
 ///
 /// Widget build(BuildContext context) {
-///   return Center(
-///     child: AnimatedContainer(
+///   return AnimatedContainer(
 ///     height: sideLength,
 ///     width: sideLength,
 ///     duration: Duration(seconds: 2),
@@ -816,7 +815,6 @@ class _InkResponseState<T extends InkResponse> extends State<T> with AutomaticKe
 ///         },
 ///       ),
 ///     ),
-///     ),
 ///   );
 /// }
 /// ```

packages/flutter/lib/src/material/radio.dart

@@ -27,7 +27,7 @@ const double _kInnerRadius = 4.5;
 /// will respond to [onChanged] by calling [State.setState] to update the
 /// radio button's [groupValue].
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 ///
 /// Here is an example of Radio widgets wrapped in ListTiles, which is similar
 /// to what you could get with the RadioListTile widget.
@@ -52,8 +52,7 @@ const double _kInnerRadius = 4.5;
 /// SingingCharacter _character = SingingCharacter.lafayette;
 ///
 /// Widget build(BuildContext context) {
-///   return Center(
-///     child: Column(
+///   return Column(
 ///     children: <Widget>[
 ///       ListTile(
 ///         title: const Text('Lafayette'),
@@ -76,7 +75,6 @@ const double _kInnerRadius = 4.5;
 ///         ),
 ///       ),
 ///     ],
-///     ),
 ///   );
 /// }
 /// ```

packages/flutter/lib/src/material/switch_list_tile.dart

@@ -46,7 +46,7 @@ enum _SwitchListTileType { material, adaptive }
 /// To show the [SwitchListTile] as disabled, pass null as the [onChanged]
 /// callback.
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 ///
 /// ![SwitchListTile sample](https://flutter.github.io/assets-for-api-docs/assets/material/switch_list_tile.png)
 ///
@@ -58,13 +58,11 @@ enum _SwitchListTileType { material, adaptive }
 ///
 /// @override
 /// Widget build(BuildContext context) {
-///   return Center(
-///     child: SwitchListTile(
+///   return SwitchListTile(
 ///     title: const Text('Lights'),
 ///     value: _lights,
 ///     onChanged: (bool value) { setState(() { _lights = value; }); },
 ///     secondary: const Icon(Icons.lightbulb_outline),
-///     ),
 ///   );
 /// }
 /// ```
@@ -87,7 +85,7 @@ enum _SwitchListTileType { material, adaptive }
 /// into one. Therefore, it may be necessary to create a custom radio tile
 /// widget to accommodate similar use cases.
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 ///
 /// ![Switch list tile semantics sample](https://flutter.github.io/assets-for-api-docs/assets/material/switch_list_tile_semantics.png)
 ///
@@ -150,9 +148,7 @@ enum _SwitchListTileType { material, adaptive }
 ///
 /// @override
 /// Widget build(BuildContext context) {
-///   return Scaffold(
-///     body: Center(
-///       child: LinkedLabelSwitch(
+///   return LinkedLabelSwitch(
 ///     label: 'Linked, tappable label text',
 ///     padding: const EdgeInsets.symmetric(horizontal: 20.0),
 ///     value: _isSelected,
@@ -161,8 +157,6 @@ enum _SwitchListTileType { material, adaptive }
 ///         _isSelected = newValue;
 ///       });
 ///     },
-///       ),
-///     ),
 ///   );
 /// }
 /// ```
@@ -175,7 +169,7 @@ enum _SwitchListTileType { material, adaptive }
 /// combining [Switch] with other widgets, such as [Text], [Padding] and
 /// [InkWell].
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 ///
 /// ![Custom switch list tile sample](https://flutter.github.io/assets-for-api-docs/assets/material/switch_list_tile_custom.png)
 ///
@@ -227,9 +221,7 @@ enum _SwitchListTileType { material, adaptive }
 ///
 /// @override
 /// Widget build(BuildContext context) {
-///   return Scaffold(
-///     body: Center(
-///       child: LabeledSwitch(
+///   return LabeledSwitch(
 ///     label: 'This is the label text',
 ///     padding: const EdgeInsets.symmetric(horizontal: 20.0),
 ///     value: _isSelected,
@@ -238,8 +230,6 @@ enum _SwitchListTileType { material, adaptive }
 ///         _isSelected = newValue;
 ///       });
 ///     },
-///       ),
-///     ),
 ///   );
 /// }
 /// ```

packages/flutter/lib/src/widgets/basic.dart

@@ -5496,7 +5496,7 @@ class WidgetToRenderBoxAdapter extends LeafRenderObjectWidget {
 /// If it has a child, this widget defers to the child for sizing behavior. If
 /// it does not have a child, it grows to fit the parent instead.
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 /// This example makes a [Container] react to being touched, showing a count of
 /// the number of pointer downs and ups.
 ///
@@ -5531,8 +5531,7 @@ class WidgetToRenderBoxAdapter extends LeafRenderObjectWidget {
 ///
 /// @override
 /// Widget build(BuildContext context) {
-///   return Center(
-///     child: ConstrainedBox(
+///   return ConstrainedBox(
 ///     constraints: new BoxConstraints.tight(Size(300.0, 200.0)),
 ///     child: Listener(
 ///       onPointerDown: _incrementDown,
@@ -5555,7 +5554,6 @@ class WidgetToRenderBoxAdapter extends LeafRenderObjectWidget {
 ///         ),
 ///       ),
 ///     ),
-///     ),
 ///   );
 /// }
 /// ```
@@ -5745,7 +5743,7 @@ class _PointerListener extends SingleChildRenderObjectWidget {
 /// If it has a child, this widget defers to the child for sizing behavior. If
 /// it does not have a child, it grows to fit the parent instead.
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 /// This example makes a [Container] react to being entered by a mouse
 /// pointer, showing a count of the number of entries and exits.
 ///
@@ -5778,8 +5776,7 @@ class _PointerListener extends SingleChildRenderObjectWidget {
 ///
 /// @override
 /// Widget build(BuildContext context) {
-///   return Center(
-///     child: ConstrainedBox(
+///   return ConstrainedBox(
 ///     constraints: new BoxConstraints.tight(Size(300.0, 200.0)),
 ///     child: MouseRegion(
 ///       onEnter: _incrementEnter,
@@ -5802,7 +5799,6 @@ class _PointerListener extends SingleChildRenderObjectWidget {
 ///         ),
 ///       ),
 ///     ),
-///     ),
 ///   );
 /// }
 /// ```

packages/flutter/lib/src/widgets/tween_animation_builder.dart

@@ -59,7 +59,7 @@ import 'value_listenable_builder.dart';
 ///
 /// ## Example Code
 ///
-/// {@tool snippet --template=stateful_widget_scaffold}
+/// {@tool snippet --template=stateful_widget_scaffold_center}
 /// This example shows an [IconButton] that "zooms" in when the widget first
 /// builds (its size smoothly increases from 0 to 24) and whenever the button
 /// is pressed, it smoothly changes its size to the new target value of either
@@ -70,8 +70,7 @@ import 'value_listenable_builder.dart';
 ///
 /// @override
 /// Widget build(BuildContext context) {
-///   return Center(
-///     child: TweenAnimationBuilder(
+///   return TweenAnimationBuilder(
 ///     tween: Tween<double>(begin: 0, end: targetValue),
 ///     duration: Duration(seconds: 1),
 ///     builder: (BuildContext context, double size, Widget child) {
@@ -87,7 +86,6 @@ import 'value_listenable_builder.dart';
 ///       );
 ///     },
 ///     child: Icon(Icons.aspect_ratio),
-///     ),
 ///   );
 /// }
 /// ```