Polish SnackBar padding (#6071)

Previously, we were getting double padding: both inside and outside the button.
This patch moves all the padding inside the button so that the whole region is
hittable and the text is positioned correctly.

Fixes #6059

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

@@ -14,7 +14,7 @@ import 'theme.dart';
 import 'typography.dart';
 
 // https://www.google.com/design/spec/components/snackbars-toasts.html#snackbars-toasts-specs
-const double _kSideMargins = 24.0;
+const double _kSnackBarPadding = 24.0;
 const double _kSingleLineVerticalPadding = 14.0;
 const double _kMultiLineVerticalTopPadding = 24.0;
 const double _kMultiLineVerticalSpaceBetweenTextAndButtons = 10.0;
@@ -83,13 +83,9 @@ class _SnackBarActionState extends State<SnackBarAction> {
 
   @override
   Widget build(BuildContext context) {
-    return new Container(
-      margin: const EdgeInsets.only(left: _kSideMargins),
-      child: new FlatButton(
-        onPressed: _haveTriggeredAction ? null : _handlePressed,
-        textTheme: ButtonTextTheme.accent,
-        child: new Text(config.label)
-      )
+    return new FlatButton(
+      onPressed: _haveTriggeredAction ? null : _handlePressed,
+      child: new Text(config.label)
     );
   }
 }
@@ -139,9 +135,10 @@ class SnackBar extends StatelessWidget {
   Widget build(BuildContext context) {
     assert(animation != null);
     List<Widget> children = <Widget>[
+      const SizedBox(width: _kSnackBarPadding),
       new Flexible(
         child: new Container(
-          margin: const EdgeInsets.symmetric(vertical: _kSingleLineVerticalPadding),
+          padding: const EdgeInsets.symmetric(vertical: _kSingleLineVerticalPadding),
           child: new DefaultTextStyle(
             style: Typography.white.subhead,
             child: content
@@ -149,8 +146,15 @@ class SnackBar extends StatelessWidget {
         )
       )
     ];
-    if (action != null)
-      children.add(action);
+    if (action != null) {
+      children.add(new ButtonTheme.bar(
+        padding: const EdgeInsets.symmetric(horizontal: _kSnackBarPadding),
+        textTheme: ButtonTextTheme.accent,
+        child: action
+      ));
+    } else {
+      children.add(const SizedBox(width: _kSnackBarPadding));
+    }
     CurvedAnimation heightAnimation = new CurvedAnimation(parent: animation, curve: _snackBarHeightCurve);
     CurvedAnimation fadeAnimation = new CurvedAnimation(parent: animation, curve: _snackBarFadeCurve, reverseCurve: const Threshold(0.0));
     ThemeData theme = Theme.of(context);
@@ -176,21 +180,18 @@ class SnackBar extends StatelessWidget {
             child: new Material(
               elevation: 6,
               color: _kSnackBackground,
-              child: new Container(
-                margin: const EdgeInsets.symmetric(horizontal: _kSideMargins),
-                child: new Theme(
-                  data: new ThemeData(
-                    brightness: Brightness.dark,
-                    accentColor: theme.accentColor,
-                    accentColorBrightness: theme.accentColorBrightness,
-                    textTheme: Typography.white
-                  ),
-                  child: new FadeTransition(
-                    opacity: fadeAnimation,
-                    child: new Row(
-                      children: children,
-                      crossAxisAlignment: CrossAxisAlignment.center
-                    )
+              child: new Theme(
+                data: new ThemeData(
+                  brightness: Brightness.dark,
+                  accentColor: theme.accentColor,
+                  accentColorBrightness: theme.accentColorBrightness,
+                  textTheme: Typography.white
+                ),
+                child: new FadeTransition(
+                  opacity: fadeAnimation,
+                  child: new Row(
+                    children: children,
+                    crossAxisAlignment: CrossAxisAlignment.center
                   )
                 )
               )

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

@@ -782,7 +782,7 @@ class CustomMultiChildLayout extends MultiChildRenderObjectWidget {
 /// height, treating nulls as zero.
 class SizedBox extends SingleChildRenderObjectWidget {
   /// Creates a box of a specific size.
-  SizedBox({ Key key, this.width, this.height, Widget child })
+  const SizedBox({ Key key, this.width, this.height, Widget child })
     : super(key: key, child: child);
 
   /// If non-null, requires the child to have exactly this width.

packages/flutter/test/material/snack_bar_test.dart

@@ -295,4 +295,43 @@ void main() {
     await tester.tap(find.text('ACTION'));
     expect(tapCount, equals(1));
   });
+
+  testWidgets('SnackBar button text alignment', (WidgetTester tester) async {
+    await tester.pumpWidget(new MaterialApp(
+      home: new Scaffold(
+        body: new Builder(
+          builder: (BuildContext context) {
+            return new GestureDetector(
+              onTap: () {
+                Scaffold.of(context).showSnackBar(new SnackBar(
+                  content: new Text('I am a snack bar.'),
+                  duration: new Duration(seconds: 2),
+                  action: new SnackBarAction(label: 'ACTION', onPressed: () {})
+                ));
+              },
+              child: new Text('X')
+            );
+          }
+        )
+      )
+    ));
+    await tester.tap(find.text('X'));
+    await tester.pump(); // start animation
+    await tester.pump(const Duration(milliseconds: 750));
+
+    RenderBox textBox = tester.firstRenderObject(find.text('I am a snack bar.'));
+    RenderBox actionTextBox = tester.firstRenderObject(find.text('ACTION'));
+    RenderBox snackBarBox = tester.firstRenderObject(find.byType(SnackBar));
+
+    Point textBottomLeft = textBox.localToGlobal(textBox.size.bottomLeft(Point.origin));
+    Point textBottomRight = textBox.localToGlobal(textBox.size.bottomRight(Point.origin));
+    Point actionTextBottomLeft = actionTextBox.localToGlobal(actionTextBox.size.bottomLeft(Point.origin));
+    Point actionTextBottomRight = actionTextBox.localToGlobal(actionTextBox.size.bottomRight(Point.origin));
+    Point snackBarBottomLeft = snackBarBox.localToGlobal(snackBarBox.size.bottomLeft(Point.origin));
+    Point snackBarBottomRight = snackBarBox.localToGlobal(snackBarBox.size.bottomRight(Point.origin));
+
+    expect(textBottomLeft.x - snackBarBottomLeft.x, 24.0);
+    expect(actionTextBottomLeft.x - textBottomRight.x, 24.0);
+    expect(snackBarBottomRight.x - actionTextBottomRight.x, 24.0);
+  });
 }