Enable extended FAB hero transition resizing (#19534)

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

@@ -2,7 +2,10 @@
 // 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/painting.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 
 import 'button.dart';
@@ -103,7 +106,8 @@ class FloatingActionButton extends StatefulWidget {
         assert(isExtended != null),
         _sizeConstraints = _kExtendedSizeConstraints,
         mini = false,
-        child = new Row(
+        child = new _ChildOverflowBox(
+          child: new Row(
             mainAxisSize: MainAxisSize.min,
             children: <Widget>[
               const SizedBox(width: 16.0),
@@ -113,6 +117,7 @@ class FloatingActionButton extends StatefulWidget {
               const SizedBox(width: 20.0),
             ],
           ),
+        ),
         super(key: key);
 
   /// The widget below this widget in the tree.
@@ -273,3 +278,55 @@ class _FloatingActionButtonState extends State<FloatingActionButton> {
     return result;
   }
 }
+
+// This widget's size matches its child's size unless its constraints
+// force it to be larger or smaller. The child is centered.
+//
+// Used to encapsulate extended FABs whose size is fixed, using Row
+// and MainAxisSize.min, to be as wide as their label and icon.
+class _ChildOverflowBox extends SingleChildRenderObjectWidget {
+  const _ChildOverflowBox({
+    Key key,
+    Widget child,
+  }) : super(key: key, child: child);
+
+  @override
+  _RenderChildOverflowBox createRenderObject(BuildContext context) {
+    return new _RenderChildOverflowBox(
+      textDirection: Directionality.of(context),
+    );
+  }
+
+  @override
+  void updateRenderObject(BuildContext context, _RenderChildOverflowBox renderObject) {
+    renderObject
+      ..textDirection = Directionality.of(context);
+  }
+}
+
+class _RenderChildOverflowBox extends RenderAligningShiftedBox {
+  _RenderChildOverflowBox({
+    RenderBox child,
+    TextDirection textDirection,
+  }) : super(child: child, alignment: Alignment.center, textDirection: textDirection);
+
+  @override
+  double computeMinIntrinsicWidth(double height) => 0.0;
+
+  @override
+  double computeMinIntrinsicHeight(double width) => 0.0;
+
+  @override
+  void performLayout() {
+    if (child != null) {
+      child.layout(const BoxConstraints(), parentUsesSize: true);
+      size = new Size(
+        math.max(constraints.minWidth, math.min(constraints.maxWidth, child.size.width)),
+        math.max(constraints.minHeight, math.min(constraints.maxHeight, child.size.height)),
+      );
+      alignChild();
+    } else {
+      size = constraints.biggest;
+    }
+  }
+}

packages/flutter/test/material/floating_action_button_test.dart

@@ -344,5 +344,68 @@ void main() {
     semantics.dispose();
   });
 
+  testWidgets('extended FAB hero transitions succeed', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/18782
 
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Scaffold(
+          floatingActionButton: new Builder(
+            builder: (BuildContext context) { // define context of Navigator.push()
+              return new FloatingActionButton.extended(
+                icon: const Icon(Icons.add),
+                label: const Text('A long FAB label'),
+                onPressed: () {
+                  Navigator.push(context, new MaterialPageRoute<void>(
+                    builder: (BuildContext context) {
+                      return new Scaffold(
+                        floatingActionButton: new FloatingActionButton.extended(
+                          icon: const Icon(Icons.add),
+                          label: const Text('X'),
+                          onPressed: () { },
+                        ),
+                        body: new Center(
+                          child: new RaisedButton(
+                            child: const Text('POP'),
+                            onPressed: () {
+                              Navigator.pop(context);
+                            },
+                          ),
+                        ),
+                      );
+                    },
+                  ));
+                },
+              );
+            },
+          ),
+          body: const Center(
+            child: const Text('Hello World'),
+          ),
+        ),
+      ),
+    );
+
+    final Finder longFAB = find.text('A long FAB label');
+    final Finder shortFAB = find.text('X');
+    final Finder helloWorld = find.text('Hello World');
+
+    expect(longFAB, findsOneWidget);
+    expect(shortFAB, findsNothing);
+    expect(helloWorld, findsOneWidget);
+
+    await tester.tap(longFAB);
+    await tester.pumpAndSettle();
+
+    expect(shortFAB, findsOneWidget);
+    expect(longFAB, findsNothing);
+
+    // Trigger a hero transition from shortFAB to longFAB.
+    await tester.tap(find.text('POP'));
+    await tester.pumpAndSettle();
+
+    expect(longFAB, findsOneWidget);
+    expect(shortFAB, findsNothing);
+    expect(helloWorld, findsOneWidget);
+  });
 }