[ExpansionPanelList] adds elevation property (#62840)

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

@@ -12,6 +12,7 @@ import 'expand_icon.dart';
 import 'ink_well.dart';
 import 'material_localizations.dart';
 import 'mergeable_material.dart';
+import 'shadows.dart';
 import 'theme.dart';
 
 const double _kPanelHeaderCollapsedHeight = kMinInteractiveDimension;
@@ -231,6 +232,7 @@ class ExpansionPanelList extends StatefulWidget {
     this.animationDuration = kThemeAnimationDuration,
     this.expandedHeaderPadding = _kPanelHeaderExpandedDefaultPadding,
     this.dividerColor,
+    this.elevation = 2,
   }) : assert(children != null),
        assert(animationDuration != null),
        _allowOnlyOnePanelOpen = false,
@@ -321,6 +323,7 @@ class ExpansionPanelList extends StatefulWidget {
     this.initialOpenPanelValue,
     this.expandedHeaderPadding = _kPanelHeaderExpandedDefaultPadding,
     this.dividerColor,
+    this.elevation = 2,
   }) : assert(children != null),
        assert(animationDuration != null),
        _allowOnlyOnePanelOpen = true,
@@ -371,6 +374,17 @@ class ExpansionPanelList extends StatefulWidget {
   /// is null, then [ThemeData.dividerColor] is used.
   final Color dividerColor;
 
+  /// Defines elevation for the [ExpansionPanel] while it's expanded.
+  ///
+  /// This uses [kElevationToShadow] to simulate shadows, it does not use
+  /// [Material]'s arbitrary elevation feature.
+  ///
+  /// The following values can be used to define the elevation: 0, 1, 2, 3, 4, 6,
+  /// 8, 9, 12, 16, 24.
+  ///
+  /// By default, the value of elevation is 2.
+  final int elevation;
+
   @override
   State<StatefulWidget> createState() => _ExpansionPanelListState();
 }
@@ -456,6 +470,11 @@ class _ExpansionPanelListState extends State<ExpansionPanelList> {
 
   @override
   Widget build(BuildContext context) {
+    assert(kElevationToShadow.containsKey(widget.elevation),
+      'Invalid value for elevation. See the kElevationToShadow constant for'
+      ' possible elevation values.'
+    );
+
     final List<MergeableMaterialItem> items = <MergeableMaterialItem>[];
 
     for (int index = 0; index < widget.children.length; index += 1) {
@@ -537,6 +556,7 @@ class _ExpansionPanelListState extends State<ExpansionPanelList> {
     return MergeableMaterial(
       hasDividers: true,
       dividerColor: widget.dividerColor,
+      elevation: widget.elevation,
       children: items,
     );
   }

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

@@ -33,6 +33,9 @@ const Color _kKeyUmbraOpacity = Color(0x33000000); // alpha = 0.2
 const Color _kKeyPenumbraOpacity = Color(0x24000000); // alpha = 0.14
 const Color _kAmbientShadowOpacity = Color(0x1F000000); // alpha = 0.12
 const Map<int, List<BoxShadow>> _elevationToShadow = <int, List<BoxShadow>>{
+  // The empty list depicts no elevation.
+  0: <BoxShadow>[],
+
   1: <BoxShadow>[
     BoxShadow(offset: Offset(0.0, 2.0), blurRadius: 1.0, spreadRadius: -1.0, color: _kKeyUmbraOpacity),
     BoxShadow(offset: Offset(0.0, 1.0), blurRadius: 1.0, spreadRadius: 0.0, color: _kKeyPenumbraOpacity),

packages/flutter/test/material/expansion_panel_test.dart

@@ -15,12 +15,14 @@ class SimpleExpansionPanelListTestWidget extends StatefulWidget {
     this.canTapOnHeader = false,
     this.expandedHeaderPadding,
     this.dividerColor,
+    this.elevation = 2,
   }) : super(key: key);
 
   final Key firstPanelKey;
   final Key secondPanelKey;
   final bool canTapOnHeader;
   final Color dividerColor;
+  final int elevation;
 
   /// If null, the default [ExpansionPanelList]'s expanded header padding value is applied via [defaultExpandedHeaderPadding]
   final EdgeInsets expandedHeaderPadding;
@@ -48,6 +50,7 @@ class _SimpleExpansionPanelListTestWidgetState extends State<SimpleExpansionPane
         });
       },
       dividerColor: widget.dividerColor,
+      elevation: widget.elevation,
       children: <ExpansionPanel>[
         ExpansionPanel(
           headerBuilder: (BuildContext context, bool isExpanded) {
@@ -1396,4 +1399,65 @@ void main() {
     // For the last DecoratedBox, we will have a Border.top with the provided dividerColor.
     expect(boxDecoration.border.top.color, dividerColor);
   });
+
+  testWidgets('elevation is propagated properly to MergeableMaterial', (WidgetTester tester) async {
+    const int _elevation = 8;
+
+    // Test for ExpansionPanelList.
+    await tester.pumpWidget(const MaterialApp(
+      home: SingleChildScrollView(
+        child: SimpleExpansionPanelListTestWidget(
+          elevation: _elevation,
+        ),
+      ),
+    ));
+
+    expect(tester.widget<MergeableMaterial>(find.byType(MergeableMaterial)).elevation, _elevation);
+
+    // Test for ExpansionPanelList.radio.
+    await tester.pumpWidget(MaterialApp(
+      home: SingleChildScrollView(
+        child: ExpansionPanelList.radio(
+          elevation: _elevation,
+          children: <ExpansionPanelRadio>[
+            ExpansionPanelRadio(
+              headerBuilder: (BuildContext context, bool isExpanded) {
+                return Text(isExpanded ? 'B' : 'A', key: const Key('firstKey'));
+              },
+              body: const SizedBox(height: 100.0),
+              value: 0,
+            ),
+            ExpansionPanelRadio(
+              headerBuilder: (BuildContext context, bool isExpanded) {
+                return Text(isExpanded ? 'D' : 'C', key: const Key('secondKey'));
+              },
+              body: const SizedBox(height: 100.0),
+              value: 1,
+            ),
+          ],
+        ),
+      ),
+    ));
+
+    expect(tester.widget<MergeableMaterial>(find.byType(MergeableMaterial)).elevation, _elevation);
+  });
+
+  testWidgets('Using a value non defined value throws assertion error', (WidgetTester tester) async {
+
+    // It should throw an AssertionError since, 19 is not defined in kElevationToShadow.
+    await tester.pumpWidget(const MaterialApp(
+      home: SingleChildScrollView(
+        child: SimpleExpansionPanelListTestWidget(
+          elevation: 19,
+        ),
+      ),
+    ));
+
+    final dynamic exception = tester.takeException();
+    expect(exception, isAssertionError);
+    expect((exception as AssertionError).toString(), contains(
+      'Invalid value for elevation. See the kElevationToShadow constant for'
+      ' possible elevation values.'
+    ));
+  });
 }