Support setting the elevation of disabled floating action buttons (#24728)

Previously, a disabled floating action button always had zero
elevation, which looks dumb.

This also fixes the issue whereby highlightElevation was not honoured
on floating action buttons.

This also fixes an issue I found during testing whereby setState was
being called during build when onHighlightChanged fired due to
onPressed becoming null while a gesture is ongoing (which triggers an
onTapCancel synchronously during build).

bin/internal/goldens.version

-034b2a540bc46375cf0c175a0fd512dcd46971e0
+7e3f945a906f9f1ffdcb1edba870f4533d2c2b86

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

@@ -68,6 +68,10 @@ class RawMaterialButton extends StatefulWidget {
 
   /// Called by the underlying [InkWell] widget's [InkWell.onHighlightChanged]
   /// callback.
+  ///
+  /// If [onPressed] changes from null to non-null while a gesture is ongoing,
+  /// this can fire during the build phase (in which case calling
+  /// [State.setState] is not allowed).
   final ValueChanged<bool> onHighlightChanged;
 
   /// Defines the default text style, with [Material.textStyle], for the
@@ -110,6 +114,8 @@ class RawMaterialButton extends StatefulWidget {
   ///
   /// Defaults to 0.0. The value is always non-negative.
   ///
+  /// See also:
+  ///
   ///  * [elevation], the default elevation.
   ///  * [highlightElevation], the elevation when the button is pressed.
   final double disabledElevation;
@@ -161,12 +167,24 @@ class RawMaterialButton extends StatefulWidget {
 class _RawMaterialButtonState extends State<RawMaterialButton> {
   bool _highlight = false;
   void _handleHighlightChanged(bool value) {
+    if (_highlight != value) {
       setState(() {
         _highlight = value;
         if (widget.onHighlightChanged != null)
           widget.onHighlightChanged(value);
       });
     }
+  }
+
+  @override
+  void didUpdateWidget(RawMaterialButton oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    if (_highlight && !widget.enabled) {
+      _highlight = false;
+      if (widget.onHighlightChanged != null)
+        widget.onHighlightChanged(false);
+    }
+  }
 
   @override
   Widget build(BuildContext context) {

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

@@ -444,7 +444,7 @@ class ButtonThemeData extends Diagnosticable {
   }
 
   /// The [button]'s background color when [MaterialButton.onPressed] is null
-  /// (when MaterialButton.enabled is false).
+  /// (when [MaterialButton.enabled] is false).
   ///
   /// Returns the button's [MaterialButton.disabledColor] if it is non-null.
   ///

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

@@ -118,7 +118,6 @@ class FlatButton extends MaterialButton {
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
     final ButtonThemeData buttonTheme = ButtonTheme.of(context);
-
     return RawMaterialButton(
       onPressed: onPressed,
       onHighlightChanged: onHighlightChanged,

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

@@ -43,10 +43,15 @@ class _DefaultHeroTag {
 ///
 /// Use at most a single floating action button per screen. Floating action
 /// buttons should be used for positive actions such as "create", "share", or
-/// "navigate".
+/// "navigate". (If more than one floating action button is used within a
+/// [Route], then make sure that each button has a unique [heroTag], otherwise
+/// an exception will be thrown.)
 ///
 /// If the [onPressed] callback is null, then the button will be disabled and
-/// will not react to touch.
+/// will not react to touch. It is highly discouraged to disable a floating
+/// action button as there is no indication to the user that the button is
+/// disabled. Consider changing the [backgroundColor] if disabling the floating
+/// action button.
 ///
 /// See also:
 ///
@@ -54,12 +59,13 @@ class _DefaultHeroTag {
 ///  * [RaisedButton], another kind of button that appears to float above the
 ///    content.
 ///  * <https://material.io/design/components/buttons-floating-action-button.html>
-class FloatingActionButton extends StatefulWidget {
+class FloatingActionButton extends StatelessWidget {
   /// Creates a circular floating action button.
   ///
   /// The [elevation], [highlightElevation], [mini], [shape], and [clipBehavior]
-  /// arguments must not be null. Additionally, [elevation] and
-  /// [highlightElevation] must be non-negative.
+  /// arguments must not be null. Additionally, [elevation],
+  /// [highlightElevation], and [disabledElevation] (if specified) must be
+  /// non-negative.
   const FloatingActionButton({
     Key key,
     this.child,
@@ -69,6 +75,7 @@ class FloatingActionButton extends StatefulWidget {
     this.heroTag = const _DefaultHeroTag(),
     this.elevation = 6.0,
     this.highlightElevation = 12.0,
+    double disabledElevation,
     @required this.onPressed,
     this.mini = false,
     this.shape = const CircleBorder(),
@@ -77,18 +84,21 @@ class FloatingActionButton extends StatefulWidget {
     this.isExtended = false,
   }) :  assert(elevation != null && elevation >= 0.0),
         assert(highlightElevation != null && highlightElevation >= 0.0),
+        assert(disabledElevation == null || disabledElevation >= 0.0),
         assert(mini != null),
         assert(shape != null),
         assert(isExtended != null),
         _sizeConstraints = mini ? _kMiniSizeConstraints : _kSizeConstraints,
+        disabledElevation = disabledElevation ?? elevation,
         super(key: key);
 
-  /// Creates a wider [StadiumBorder] shaped floating action button with both
+  /// Creates a wider [StadiumBorder]-shaped floating action button with both
   /// an [icon] and a [label].
   ///
-  /// The [label], [icon], [elevation], [highlightElevation], [clipBehavior]
-  /// and [shape] arguments must not be null. Additionally, [elevation] and
-  //  [highlightElevation] must be non-negative.
+  /// The [label], [icon], [elevation], [highlightElevation], [clipBehavior] and
+  /// [shape] arguments must not be null. Additionally, [elevation]
+  /// [highlightElevation], and [disabledElevation] (if specified) must be
+  /// non-negative.
   FloatingActionButton.extended({
     Key key,
     this.tooltip,
@@ -97,6 +107,7 @@ class FloatingActionButton extends StatefulWidget {
     this.heroTag = const _DefaultHeroTag(),
     this.elevation = 6.0,
     this.highlightElevation = 12.0,
+    double disabledElevation,
     @required this.onPressed,
     this.shape = const StadiumBorder(),
     this.isExtended = true,
@@ -106,10 +117,12 @@ class FloatingActionButton extends StatefulWidget {
     @required Widget label,
   }) :  assert(elevation != null && elevation >= 0.0),
         assert(highlightElevation != null && highlightElevation >= 0.0),
+        assert(disabledElevation == null || disabledElevation >= 0.0),
         assert(shape != null),
         assert(isExtended != null),
         assert(clipBehavior != null),
         _sizeConstraints = _kExtendedSizeConstraints,
+        disabledElevation = disabledElevation ?? elevation,
         mini = false,
         child = _ChildOverflowBox(
           child: Row(
@@ -167,11 +180,15 @@ class FloatingActionButton extends StatefulWidget {
 
   /// The z-coordinate at which to place this button releative to its parent.
   ///
-  ///
   /// This controls the size of the shadow below the floating action button.
   ///
   /// Defaults to 6, the appropriate elevation for floating action buttons. The
   /// value is always non-negative.
+  ///
+  /// See also:
+  ///
+  ///  * [highlightElevation], the elevation when the button is pressed.
+  ///  * [disabledElevation], the elevation when the button is disabled.
   final double elevation;
 
   /// The z-coordinate at which to place this button relative to its parent when
@@ -187,6 +204,21 @@ class FloatingActionButton extends StatefulWidget {
   ///  * [elevation], the default elevation.
   final double highlightElevation;
 
+  /// The z-coordinate at which to place this button when the button is disabled
+  /// ([onPressed] is null).
+  ///
+  /// This controls the size of the shadow below the floating action button.
+  ///
+  /// Defaults to the same value as [elevation]. Setting this to zero makes the
+  /// floating action button work similar to a [RaisedButton] but the titular
+  /// "floating" effect is lost. The value is always non-negative.
+  ///
+  /// See also:
+  ///
+  ///  * [elevation], the default elevation.
+  ///  * [highlightElevation], the elevation when the button is pressed.
+  final double disabledElevation;
+
   /// Controls the size of this button.
   ///
   /// By default, floating action buttons are non-mini and have a height and
@@ -229,62 +261,50 @@ class FloatingActionButton extends StatefulWidget {
 
   final BoxConstraints _sizeConstraints;
 
-  @override
-  _FloatingActionButtonState createState() => _FloatingActionButtonState();
-}
-
-class _FloatingActionButtonState extends State<FloatingActionButton> {
-  bool _highlight = false;
-
-  void _handleHighlightChanged(bool value) {
-    setState(() {
-      _highlight = value;
-    });
-  }
-
   @override
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
-    final Color foregroundColor = widget.foregroundColor ?? theme.accentIconTheme.color;
+    final Color foregroundColor = this.foregroundColor ?? theme.accentIconTheme.color;
     Widget result;
 
-    if (widget.child != null) {
+    if (child != null) {
       result = IconTheme.merge(
         data: IconThemeData(
           color: foregroundColor,
         ),
-        child: widget.child,
+        child: child,
       );
     }
 
     result = RawMaterialButton(
-      onPressed: widget.onPressed,
-      onHighlightChanged: _handleHighlightChanged,
-      elevation: _highlight ? widget.highlightElevation : widget.elevation,
-      constraints: widget._sizeConstraints,
-      materialTapTargetSize: widget.materialTapTargetSize ?? theme.materialTapTargetSize,
-      fillColor: widget.backgroundColor ?? theme.accentColor,
+      onPressed: onPressed,
+      elevation: elevation,
+      highlightElevation: highlightElevation,
+      disabledElevation: disabledElevation,
+      constraints: _sizeConstraints,
+      materialTapTargetSize: materialTapTargetSize ?? theme.materialTapTargetSize,
+      fillColor: backgroundColor ?? theme.accentColor,
       textStyle: theme.accentTextTheme.button.copyWith(
         color: foregroundColor,
         letterSpacing: 1.2,
       ),
-      shape: widget.shape,
-      clipBehavior: widget.clipBehavior,
+      shape: shape,
+      clipBehavior: clipBehavior,
       child: result,
     );
 
-    if (widget.tooltip != null) {
+    if (tooltip != null) {
       result = MergeSemantics(
         child: Tooltip(
-          message: widget.tooltip,
+          message: tooltip,
           child: result,
         ),
       );
     }
 
-    if (widget.heroTag != null) {
+    if (heroTag != null) {
       result = Hero(
-        tag: widget.heroTag,
+        tag: heroTag,
         child: result,
       );
     }

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

@@ -242,6 +242,12 @@ class InkResponse extends StatefulWidget {
   /// The value passed to the callback is true if this part of the material has
   /// become highlighted and false if this part of the material has stopped
   /// being highlighted.
+  ///
+  /// If all of [onTap], [onDoubleTap], and [onLongPress] become null while a
+  /// gesture is ongoing, then [onTapCancel] will be fired and
+  /// [onHighlightChanged] will be fired with the value false _during the
+  /// build_. This means, for instance, that in that scenario [State.setState]
+  /// cannot be called.
   final ValueChanged<bool> onHighlightChanged;
 
   /// Whether this ink response should be clipped its bounds.

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

@@ -73,6 +73,10 @@ class MaterialButton extends StatelessWidget {
 
   /// Called by the underlying [InkWell] widget's [InkWell.onHighlightChanged]
   /// callback.
+  ///
+  /// If [onPressed] changes from null to non-null while a gesture is ongoing,
+  /// this can fire during the build phase (in which case calling
+  /// [State.setState] is not allowed).
   final ValueChanged<bool> onHighlightChanged;
 
   /// Defines the button's base colors, and the defaults for the button's minimum
@@ -255,6 +259,7 @@ class MaterialButton extends StatelessWidget {
 
     return RawMaterialButton(
       onPressed: onPressed,
+      onHighlightChanged: onHighlightChanged,
       fillColor: color,
       textStyle: theme.textTheme.button.copyWith(color: buttonTheme.getTextColor(this)),
       highlightColor: highlightColor ?? theme.highlightColor,

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

@@ -315,6 +315,27 @@ class _OutlineButtonState extends State<_OutlineButton> with SingleTickerProvide
     );
   }
 
+  @override
+  void didUpdateWidget(_OutlineButton oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    if (_pressed && !widget.enabled) {
+      _pressed = false;
+      _controller.reverse();
+    }
+  }
+
+  void _handleHighlightChanged(bool value) {
+    if (_pressed == value)
+      return;
+    setState(() {
+      _pressed = value;
+      if (value)
+        _controller.forward();
+      else
+        _controller.reverse();
+    });
+  }
+
   @override
   void dispose() {
     _controller.dispose();
@@ -375,15 +396,7 @@ class _OutlineButtonState extends State<_OutlineButton> with SingleTickerProvide
           elevation: 0.0,
           disabledElevation: 0.0,
           highlightElevation: _getHighlightElevation(),
-          onHighlightChanged: (bool value) {
-            setState(() {
-              _pressed = value;
-              if (value)
-                _controller.forward();
-              else
-                _controller.reverse();
-            });
-          },
+          onHighlightChanged: _handleHighlightChanged,
           padding: widget.padding,
           shape: _OutlineBorder(
             shape: widget.shape,

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

@@ -128,7 +128,6 @@ class RaisedButton extends MaterialButton {
   Widget build(BuildContext context) {
     final ThemeData theme = Theme.of(context);
     final ButtonThemeData buttonTheme = ButtonTheme.of(context);
-
     return RawMaterialButton(
       onPressed: onPressed,
       onHighlightChanged: onHighlightChanged,

packages/flutter/test/material/floating_action_button_test.dart

@@ -110,6 +110,137 @@ void main() {
     expect(find.text('Add'), findsOneWidget);
   });
 
+  testWidgets('Floating Action Button elevation when highlighted - defaults', (WidgetTester tester) async {
+    expect(const FloatingActionButton(onPressed: null).highlightElevation, 12.0);
+    expect(const FloatingActionButton(onPressed: null, highlightElevation: 0.0).highlightElevation, 0.0);
+  });
+
+  testWidgets('Floating Action Button elevation when highlighted - effect', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          floatingActionButton: FloatingActionButton(
+            onPressed: () { },
+          ),
+        ),
+      ),
+    );
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 6.0);
+    final TestGesture gesture = await tester.press(find.byType(PhysicalShape));
+    await tester.pump();
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 6.0);
+    await tester.pump(const Duration(seconds: 1));
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 12.0);
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          floatingActionButton: FloatingActionButton(
+            onPressed: () { },
+            highlightElevation: 20.0
+          ),
+        ),
+      ),
+    );
+    await tester.pump();
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 12.0);
+    await tester.pump(const Duration(seconds: 1));
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 20.0);
+    await gesture.up();
+    await tester.pump();
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 20.0);
+    await tester.pump(const Duration(seconds: 1));
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 6.0);
+  });
+
+  testWidgets('Floating Action Button elevation when disabled - defaults', (WidgetTester tester) async {
+    expect(FloatingActionButton(onPressed: () { }).disabledElevation, 6.0);
+    expect(const FloatingActionButton(onPressed: null).disabledElevation, 6.0);
+    expect(FloatingActionButton(onPressed: () { }, disabledElevation: 0.0).disabledElevation, 0.0);
+  });
+
+  testWidgets('Floating Action Button elevation when disabled - effect', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Scaffold(
+          floatingActionButton: FloatingActionButton(
+            onPressed: null,
+          ),
+        ),
+      ),
+    );
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 6.0);
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Scaffold(
+          floatingActionButton: FloatingActionButton(
+            onPressed: null,
+            disabledElevation: 3.0,
+          ),
+        ),
+      ),
+    );
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 6.0);
+    await tester.pump(const Duration(seconds: 1));
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 3.0);
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          floatingActionButton: FloatingActionButton(
+            onPressed: () { },
+            disabledElevation: 3.0,
+          ),
+        ),
+      ),
+    );
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 3.0);
+    await tester.pump(const Duration(seconds: 1));
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 6.0);
+  });
+
+  testWidgets('Floating Action Button elevation when disabled while highlighted - effect', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          floatingActionButton: FloatingActionButton(
+            onPressed: () { },
+          ),
+        ),
+      ),
+    );
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 6.0);
+    await tester.press(find.byType(PhysicalShape));
+    await tester.pump();
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 6.0);
+    await tester.pump(const Duration(seconds: 1));
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 12.0);
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Scaffold(
+          floatingActionButton: FloatingActionButton(
+            onPressed: null,
+          ),
+        ),
+      ),
+    );
+    await tester.pump();
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 12.0);
+    await tester.pump(const Duration(seconds: 1));
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 6.0);
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          floatingActionButton: FloatingActionButton(
+            onPressed: () { },
+          ),
+        ),
+      ),
+    );
+    await tester.pump();
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 6.0);
+    await tester.pump(const Duration(seconds: 1));
+    expect(tester.widget<PhysicalShape>(find.byType(PhysicalShape)).elevation, 6.0);
+  });
+
   testWidgets('FlatActionButton mini size is configurable by ThemeData.materialTapTargetSize', (WidgetTester tester) async {
     final Key key1 = UniqueKey();
     await tester.pumpWidget(
@@ -476,7 +607,7 @@ void main() {
     await tester.pump(const Duration(milliseconds: 1000));
     await expectLater(
       find.byKey(key),
-      matchesGoldenFile('floating_action_button_test.clip.1.png'),
+      matchesGoldenFile('floating_action_button_test.clip.2.png'), // .clip.1.png is obsolete and can be removed
       skip: !Platform.isLinux,
     );
   });

packages/flutter/test/material/outline_button_test.dart

@@ -43,6 +43,25 @@ void main() {
     expect(pressedCount, 1);
   });
 
+  testWidgets('Outline button doesn\'t crash if disabled during a gesture', (WidgetTester tester) async {
+    Widget buildFrame(VoidCallback onPressed) {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: Theme(
+          data: ThemeData(),
+          child: Center(
+            child: OutlineButton(onPressed: onPressed),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildFrame(() { }));
+    await tester.press(find.byType(OutlineButton));
+    await tester.pumpAndSettle();
+    await tester.pumpWidget(buildFrame(null));
+    await tester.pumpAndSettle();
+  });
 
   testWidgets('OutlineButton shape and border component overrides', (WidgetTester tester) async {
     const Color fillColor = Color(0xFF00FF00);