[ExpansionPanelList] adds dividerColor property (#59641)

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

@@ -230,6 +230,7 @@ class ExpansionPanelList extends StatefulWidget {
     this.expansionCallback,
     this.animationDuration = kThemeAnimationDuration,
     this.expandedHeaderPadding = _kPanelHeaderExpandedDefaultPadding,
+    this.dividerColor,
   }) : assert(children != null),
        assert(animationDuration != null),
        _allowOnlyOnePanelOpen = false,
@@ -319,6 +320,7 @@ class ExpansionPanelList extends StatefulWidget {
     this.animationDuration = kThemeAnimationDuration,
     this.initialOpenPanelValue,
     this.expandedHeaderPadding = _kPanelHeaderExpandedDefaultPadding,
+    this.dividerColor,
   }) : assert(children != null),
        assert(animationDuration != null),
        _allowOnlyOnePanelOpen = true,
@@ -363,6 +365,12 @@ class ExpansionPanelList extends StatefulWidget {
   /// during expansion.
   final EdgeInsets expandedHeaderPadding;
 
+  /// Defines color for the divider when [ExpansionPanel.isExpanded] is false.
+  ///
+  /// If `dividerColor` is null, then [DividerThemeData.color] is used. If that
+  /// is null, then [ThemeData.dividerColor] is used.
+  final Color dividerColor;
+
   @override
   State<StatefulWidget> createState() => _ExpansionPanelListState();
 }
@@ -528,6 +536,7 @@ class _ExpansionPanelListState extends State<ExpansionPanelList> {
 
     return MergeableMaterial(
       hasDividers: true,
+      dividerColor: widget.dividerColor,
       children: items,
     );
   }

packages/flutter/test/material/expansion_panel_test.dart

@@ -13,12 +13,14 @@ class SimpleExpansionPanelListTestWidget extends StatefulWidget {
     this.firstPanelKey,
     this.secondPanelKey,
     this.canTapOnHeader = false,
-    this.expandedHeaderPadding
+    this.expandedHeaderPadding,
+    this.dividerColor,
   }) : super(key: key);
 
   final Key firstPanelKey;
   final Key secondPanelKey;
   final bool canTapOnHeader;
+  final Color dividerColor;
 
   /// If null, the default [ExpansionPanelList]'s expanded header padding value is applied via [defaultExpandedHeaderPadding]
   final EdgeInsets expandedHeaderPadding;
@@ -45,6 +47,7 @@ class _SimpleExpansionPanelListTestWidgetState extends State<SimpleExpansionPane
           extendedState[_index] = !extendedState[_index];
         });
       },
+      dividerColor: widget.dividerColor,
       children: <ExpansionPanel>[
         ExpansionPanel(
           headerBuilder: (BuildContext context, bool isExpanded) {
@@ -1343,4 +1346,54 @@ void main() {
     expect(box.size.height, equals(128.0)); // _kPanelHeaderCollapsedHeight + 80.0 (double padding)
     expect(box.size.width, equals(736.0));
   });
+
+  testWidgets('ExpansionPanelList respects dividerColor', (WidgetTester tester) async {
+    const Color dividerColor = Colors.red;
+    await tester.pumpWidget(const MaterialApp(
+      home: SingleChildScrollView(
+        child: SimpleExpansionPanelListTestWidget(
+          dividerColor: dividerColor,
+        ),
+      ),
+    ));
+
+    final DecoratedBox decoratedBox = tester.widget(find.byType(DecoratedBox).last);
+    final BoxDecoration decoration = decoratedBox.decoration as BoxDecoration;
+
+    // For the last DecoratedBox, we will have a Border.top with the provided dividerColor.
+    expect(decoration.border.top.color, dividerColor);
+  });
+
+  testWidgets('ExpansionPanelList.radio respects DividerColor', (WidgetTester tester) async {
+    const Color dividerColor = Colors.red;
+    await tester.pumpWidget(MaterialApp(
+      home: SingleChildScrollView(
+        child: ExpansionPanelList.radio(
+          dividerColor: dividerColor,
+          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,
+            ),
+          ],
+        ),
+      ),
+    ));
+
+    final DecoratedBox decoratedBox = tester.widget(find.byType(DecoratedBox).last);
+    final BoxDecoration boxDecoration = decoratedBox.decoration as BoxDecoration;
+
+    // For the last DecoratedBox, we will have a Border.top with the provided dividerColor.
+    expect(boxDecoration.border.top.color, dividerColor);
+  });
 }