[ExpansionTile] adds collapsedBackgroundColor property (flutter#69575)

Author: AyushBherwani1998

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

@@ -46,6 +46,7 @@ class ExpansionTile extends StatefulWidget {
     this.expandedCrossAxisAlignment,
     this.expandedAlignment,
     this.childrenPadding,
+    this.collapsedBackgroundColor,
   }) : assert(initiallyExpanded != null),
        assert(maintainState != null),
        assert(
@@ -85,6 +86,9 @@ class ExpansionTile extends StatefulWidget {
   /// The color to display behind the sublist when expanded.
   final Color? backgroundColor;
 
+  /// When not null, defines the background color of tile when the sublist is collapsed.
+  final Color? collapsedBackgroundColor;
+
   /// A widget to display instead of a rotating arrow icon.
   final Widget? trailing;
 
@@ -261,7 +265,9 @@ class _ExpansionTileState extends State<ExpansionTile> with SingleTickerProvider
     _iconColorTween
       ..begin = theme.unselectedWidgetColor
       ..end = theme.accentColor;
-    _backgroundColorTween.end = widget.backgroundColor;
+    _backgroundColorTween
+      ..begin = widget.collapsedBackgroundColor
+      ..end = widget.backgroundColor;
     super.didChangeDependencies();
   }
 

packages/flutter/test/material/expansion_tile_test.dart

@@ -481,4 +481,40 @@ void main() {
     expect(columnRect.bottom, paddingRect.bottom - 4);
   });
 
+  testWidgets('ExpansionTile.collapsedBackgroundColor', (WidgetTester tester) async {
+    const Key expansionTileKey = Key('expansionTileKey');
+    const Color backgroundColor = Colors.red;
+    const Color collapsedBackgroundColor = Colors.brown;
+
+    await tester.pumpWidget(const MaterialApp(
+      home: Material(
+        child: ExpansionTile(
+          key: expansionTileKey,
+          title: Text('Title'),
+          backgroundColor: backgroundColor,
+          collapsedBackgroundColor: collapsedBackgroundColor,
+          children: <Widget>[
+            SizedBox(height: 100, width: 100),
+          ],
+        ),
+      ),
+    ));
+
+    BoxDecoration boxDecoration =  tester.firstWidget<Container>(find.descendant(
+      of: find.byKey(expansionTileKey),
+      matching: find.byType(Container),
+    )).decoration! as BoxDecoration;
+
+    expect(boxDecoration.color, collapsedBackgroundColor);
+
+    await tester.tap(find.text('Title'));
+    await tester.pumpAndSettle();
+
+    boxDecoration =  tester.firstWidget<Container>(find.descendant(
+      of: find.byKey(expansionTileKey),
+      matching: find.byType(Container),
+    )).decoration! as BoxDecoration;
+
+    expect(boxDecoration.color, backgroundColor);
+  });
 }