Allow material button to grow as wide as its constraints allow (#19416)

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

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:math' as math;
+
 import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
@@ -166,7 +168,7 @@ class _RawMaterialButtonState extends State<RawMaterialButton> {
       ? (_highlight ? widget.highlightElevation : widget.elevation)
       : widget.disabledElevation;
 
-    Widget result = new ConstrainedBox(
+    final Widget result = new ConstrainedBox(
       constraints: widget.constraints,
       child: new Material(
         elevation: elevation,
@@ -194,29 +196,24 @@ class _RawMaterialButtonState extends State<RawMaterialButton> {
         ),
       ),
     );
-    BoxConstraints constraints;
+    Size minSize;
     switch (widget.materialTapTargetSize) {
       case MaterialTapTargetSize.padded:
-        constraints = const BoxConstraints(minWidth: 48.0, minHeight: 48.0);
+        minSize = const Size(48.0, 48.0);
         break;
       case MaterialTapTargetSize.shrinkWrap:
-        constraints = const BoxConstraints();
+        minSize = Size.zero;
         break;
     }
-    result = new _ButtonRedirectingHitDetectionWidget(
-      constraints: constraints,
-      child: new Center(
-        child: result,
-        widthFactor: 1.0,
-        heightFactor: 1.0,
-      ),
-    );
 
     return new Semantics(
       container: true,
       button: true,
       enabled: widget.enabled,
-      child: result,
+      child: new _InputPadding(
+        minSize: minSize,
+        child: result,
+      ),
     );
   }
 }
@@ -446,40 +443,88 @@ class MaterialButton extends StatelessWidget {
   }
 }
 
-/// Redirects the position passed to [RenderBox.hitTest] to the center of the
-/// widget if the child hit test would have failed otherwise.
-///
-/// The primary purpose of this widget is to allow padding around [Material] widgets
-/// to trigger the child ink feature without increasing the size of the material.
-class _ButtonRedirectingHitDetectionWidget extends SingleChildRenderObjectWidget {
-  const _ButtonRedirectingHitDetectionWidget({
+/// A widget to pad the area around a [MaterialButton]'s inner [Material].
+/// 
+/// Redirect taps that occur in the padded area around the child to the center 
+/// of the child. This increases the size of the button and the button's 
+/// "tap target", but not its material or its ink splashes.
+class _InputPadding extends SingleChildRenderObjectWidget {
+  const _InputPadding({
     Key key,
     Widget child,
-    this.constraints
+    this.minSize,
   }) : super(key: key, child: child);
 
-  final BoxConstraints constraints;
+  final Size minSize;
 
   @override
   RenderObject createRenderObject(BuildContext context) {
-    return new _RenderButtonRedirectingHitDetection(constraints);
+    return new _RenderInputPadding(minSize);
   }
 
   @override
-  void updateRenderObject(BuildContext context, covariant _RenderButtonRedirectingHitDetection renderObject) {
-    renderObject.additionalConstraints = constraints;
+  void updateRenderObject(BuildContext context, covariant _RenderInputPadding renderObject) {
+    renderObject.minSize = minSize;
   }
 }
 
-class _RenderButtonRedirectingHitDetection extends RenderConstrainedBox {
-  _RenderButtonRedirectingHitDetection (BoxConstraints additionalConstraints) : super(additionalConstraints: additionalConstraints);
+class _RenderInputPadding extends RenderShiftedBox {
+  _RenderInputPadding(this._minSize, [RenderBox child]) : super(child) ;
+
+  Size get minSize => _minSize;
+  Size _minSize;
+  set minSize(Size value) {
+    if (_minSize == value)
+      return;
+    _minSize = value;
+    markNeedsLayout();
+  }
+
+  @override
+  double computeMinIntrinsicWidth(double height) {
+    if (child != null)
+      return math.max(child.computeMinIntrinsicWidth(height), minSize.width);
+    return 0.0;
+  }
+
+  @override
+  double computeMinIntrinsicHeight(double width) {
+    if (child != null)
+      return math.max(child.computeMinIntrinsicHeight(width), minSize.height);
+    return 0.0;
+  }
+
+  @override
+  double computeMaxIntrinsicWidth(double height) {
+    if (child != null)
+      return math.max(child.computeMaxIntrinsicWidth(height), minSize.width);
+    return 0.0;
+  }
+
+  @override
+  double computeMaxIntrinsicHeight(double width) {
+    if (child != null)
+      return math.max(child.computeMaxIntrinsicHeight(width), minSize.height);
+    return 0.0;
+  }
+
+  @override
+  void performLayout() {
+    if (child != null) {
+      child.layout(constraints, parentUsesSize: true);
+      final double height = math.max(child.size.width, minSize.width);
+      final double width = math.max(child.size.height, minSize.height);
+      size = constraints.constrain(new Size(height, width));
+      final BoxParentData childParentData = child.parentData;
+      childParentData.offset = Alignment.center.alongOffset(size - child.size);
+    } else {
+      size = Size.zero;
+    }
+  }
 
   @override
   bool hitTest(HitTestResult result, {Offset position}) {
-    if (!size.contains(position))
-      return false;
-    if (child.hitTest(result, position: position))
-      return true;
-    return child.hitTest(result, position: size.center(Offset.zero));
+    return super.hitTest(result, position: position) || 
+      child.hitTest(result, position: child.size.center(Offset.zero));
   }
 }

packages/flutter/test/material/raw_material_button_test.dart

@@ -66,14 +66,16 @@ void main() {
     const Color fillColor = const Color(0xFFEF5350);
 
     await tester.pumpWidget(
-      new RawMaterialButton(
-        materialTapTargetSize: MaterialTapTargetSize.padded,
-        onPressed: () {},
-        fillColor: fillColor,
-        highlightColor: highlightColor,
-        splashColor: splashColor,
-        child: const SizedBox(),
-      )
+      new Center(
+        child: new RawMaterialButton(
+          materialTapTargetSize: MaterialTapTargetSize.padded,
+          onPressed: () {},
+          fillColor: fillColor,
+          highlightColor: highlightColor,
+          splashColor: splashColor,
+          child: const SizedBox(),
+        ),
+      ),
     );
 
     final Offset center = tester.getCenter(find.byType(InkWell));
@@ -93,14 +95,16 @@ void main() {
     const Color fillColor = const Color(0xFFEF5350);
 
     await tester.pumpWidget(
-      new RawMaterialButton(
-        materialTapTargetSize: MaterialTapTargetSize.padded,
-        onPressed: () {},
-        fillColor: fillColor,
-        highlightColor: highlightColor,
-        splashColor: splashColor,
-        child: const SizedBox(),
-      )
+      new Center(
+        child: new RawMaterialButton(
+          materialTapTargetSize: MaterialTapTargetSize.padded,
+          onPressed: () {},
+          fillColor: fillColor,
+          highlightColor: highlightColor,
+          splashColor: splashColor,
+          child: const SizedBox(),
+        ),
+      ),
     );
 
     final Offset top = tester.getRect(find.byType(InkWell)).topCenter;
@@ -117,6 +121,7 @@ void main() {
     await tester.pumpWidget(
       new MaterialApp(
         home: new Column(
+          crossAxisAlignment: CrossAxisAlignment.center,
           children: <Widget>[
             new RawMaterialButton(
             materialTapTargetSize: MaterialTapTargetSize.padded,
@@ -165,4 +170,24 @@ void main() {
     );
     expect(find.byKey(key).hitTestable(), findsOneWidget);
   });
-}
\ No newline at end of file
+  
+  testWidgets('RawMaterialButton can be expanded by parent constraints', (WidgetTester tester) async {
+    const Key key = const Key('test');
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Column(
+          crossAxisAlignment: CrossAxisAlignment.stretch,
+          children: <Widget>[
+            new RawMaterialButton(
+              key: key,
+              onPressed: () {},
+              child: const SizedBox(),
+            )
+          ],
+        ),
+      ),
+    );
+
+    expect(tester.getSize(find.byKey(key)), const Size(800.0, 48.0));
+  });
+}