Add outer gesture detector to increase mini-fab size to 48x48 (#17921)

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

@@ -38,6 +38,7 @@ class RawMaterialButton extends StatefulWidget {
     this.elevation: 2.0,
     this.highlightElevation: 8.0,
     this.disabledElevation: 0.0,
+    this.outerPadding,
     this.padding: EdgeInsets.zero,
     this.constraints: const BoxConstraints(minWidth: 88.0, minHeight: 36.0),
     this.shape: const RoundedRectangleBorder(),
@@ -57,6 +58,10 @@ class RawMaterialButton extends StatefulWidget {
   /// If this is set to null, the button will be disabled, see [enabled].
   final VoidCallback onPressed;
 
+  /// Padding to increase the size of the gesture detector which doesn't
+  /// increase the visible material of the button.
+  final EdgeInsets outerPadding;
+
   /// Called by the underlying [InkWell] widget's [InkWell.onHighlightChanged]
   /// callback.
   final ValueChanged<bool> onHighlightChanged;
@@ -153,11 +158,7 @@ class _RawMaterialButtonState extends State<RawMaterialButton> {
       ? (_highlight ? widget.highlightElevation : widget.elevation)
       : widget.disabledElevation;
 
-    return new Semantics(
-      container: true,
-      button: true,
-      enabled: widget.enabled,
-      child: new ConstrainedBox(
+    Widget result = new ConstrainedBox(
       constraints: widget.constraints,
       child: new Material(
         elevation: elevation,
@@ -184,9 +185,27 @@ class _RawMaterialButtonState extends State<RawMaterialButton> {
           ),
         ),
       ),
+    );
+
+    if (widget.outerPadding != null) {
+      result = new GestureDetector(
+        behavior: HitTestBehavior.translucent,
+        excludeFromSemantics: true,
+        onTap: widget.onPressed,
+        child: new Padding(
+          padding: widget.outerPadding,
+          child: result
         ),
       );
     }
+
+    return new Semantics(
+      container: true,
+      button: true,
+      enabled: widget.enabled,
+      child: result,
+    );
+  }
 }
 
 /// A utility class for building Material buttons that depend on the

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

@@ -187,7 +187,8 @@ class FloatingActionButton extends StatefulWidget {
   ///
   /// By default, floating action buttons are non-mini and have a height and
   /// width of 56.0 logical pixels. Mini floating action buttons have a height
-  /// and width of 40.0 logical pixels.
+  /// and width of 40.0 logical pixels with a layout width and height of 48.0
+  /// logical pixels.
   final bool mini;
 
   /// The margin to keep around the floating action button when creating a
@@ -267,6 +268,7 @@ class _FloatingActionButtonState extends State<FloatingActionButton> {
       onHighlightChanged: _handleHighlightChanged,
       elevation: _highlight ? widget.highlightElevation : widget.elevation,
       constraints: widget._sizeConstraints,
+      outerPadding: widget.mini ? const EdgeInsets.all(4.0) : null,
       fillColor: widget.backgroundColor ?? theme.accentColor,
       textStyle: theme.accentTextTheme.button.copyWith(
         color: foregroundColor,

packages/flutter/test/material/raw_material_button_test.dart

+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../widgets/semantics_tester.dart';
+
+void main() {
+  testWidgets('outerPadding expands hit test area', (WidgetTester tester) async {
+    int pressed = 0;
+
+    await tester.pumpWidget(new RawMaterialButton(
+      onPressed: () {
+        pressed++;
+      },
+      constraints: new BoxConstraints.tight(const Size(10.0, 10.0)),
+      outerPadding: const EdgeInsets.all(50.0),
+      child: const Text('+', textDirection: TextDirection.ltr),
+    ));
+
+    await tester.tapAt(const Offset(100.0, 100.0));
+
+    expect(pressed, 1);
+  });
+
+  testWidgets('outerPadding expands semantics area', (WidgetTester tester) async {
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    await tester.pumpWidget(
+      new Center(
+        child: new RawMaterialButton(
+          onPressed: () {},
+          constraints: new BoxConstraints.tight(const Size(10.0, 10.0)),
+          outerPadding: const EdgeInsets.all(50.0),
+          child: const Text('+', textDirection: TextDirection.ltr),
+        ),
+      ),
+    );
+
+    expect(semantics, hasSemantics(
+      new TestSemantics.root(
+        children: <TestSemantics>[
+        new TestSemantics(
+          id: 1,
+          flags: <SemanticsFlag>[
+            SemanticsFlag.isButton,
+            SemanticsFlag.hasEnabledState,
+            SemanticsFlag.isEnabled,
+          ],
+          actions: <SemanticsAction>[
+            SemanticsAction.tap,
+          ],
+          label: '+',
+          textDirection: TextDirection.ltr,
+          rect: Rect.fromLTRB(0.0, 0.0, 110.0, 110.0),
+          children: <TestSemantics>[],
+        ),
+      ]
+    ), ignoreTransform: true));
+
+    semantics.dispose();
+  });
+}
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