Add ExpansionTile.childrenPadding property (#57291)

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

@@ -45,6 +45,7 @@ class ExpansionTile extends StatefulWidget {
     this.tilePadding,
     this.expandedCrossAxisAlignment,
     this.expandedAlignment,
+    this.childrenPadding,
   }) : assert(initiallyExpanded != null),
        assert(maintainState != null),
        assert(
@@ -138,6 +139,11 @@ class ExpansionTile extends StatefulWidget {
   /// When the value is null, the value of `expandedCrossAxisAlignment` is [CrossAxisAlignment.center].
   final CrossAxisAlignment expandedCrossAxisAlignment;
 
+  /// Specifies padding for [children].
+  ///
+  /// When the value is null, the value of `childrenPadding` is [EdgeInsets.zero].
+  final EdgeInsetsGeometry childrenPadding;
+
   @override
   _ExpansionTileState createState() => _ExpansionTileState();
 }
@@ -266,10 +272,13 @@ class _ExpansionTileState extends State<ExpansionTile> with SingleTickerProvider
 
     final Widget result = Offstage(
       child: TickerMode(
+        child: Padding(
+          padding: widget.childrenPadding ?? EdgeInsets.zero,
           child: Column(
             crossAxisAlignment: widget.expandedCrossAxisAlignment ?? CrossAxisAlignment.center,
             children: widget.children,
           ),
+        ),
         enabled: !closed,
       ),
       offstage: closed
@@ -280,6 +289,5 @@ class _ExpansionTileState extends State<ExpansionTile> with SingleTickerProvider
       builder: _buildChildren,
       child: shouldRemoveChildren ? null : result,
     );
-
   }
 }

packages/flutter/test/material/expansion_tile_test.dart

@@ -424,4 +424,61 @@ void main() {
 
   });
 
+  testWidgets('childrenPadding default value', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: Material(
+        child: Center(
+          child: ExpansionTile(
+            title: const Text('title'),
+            children: <Widget>[
+              Container(height: 100, width: 100),
+            ],
+          ),
+        ),
+      ),
+    ));
+
+    await tester.tap(find.text('title'));
+    await tester.pumpAndSettle();
+
+    final Rect columnRect = tester.getRect(find.byType(Column).last);
+    final Rect paddingRect = tester.getRect(find.byType(Padding).last);
+
+    // By default, the value of childrenPadding is EdgeInsets.zero, hence offset
+    // of all the edges from x-axis and y-axis should be equal for Padding and Column.
+    expect(columnRect.top, paddingRect.top);
+    expect(columnRect.left, paddingRect.left);
+    expect(columnRect.right, paddingRect.right);
+    expect(columnRect.bottom, paddingRect.bottom);
+  });
+
+  testWidgets('ExpansionTile childrenPadding test', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: Material(
+        child: Center(
+          child: ExpansionTile(
+            title: const Text('title'),
+            childrenPadding: const EdgeInsets.fromLTRB(10, 8, 12, 4),
+            children: <Widget>[
+              Container(height: 100, width: 100),
+            ],
+          ),
+        ),
+      ),
+    ));
+
+    await tester.tap(find.text('title'));
+    await tester.pumpAndSettle();
+
+    final Rect columnRect = tester.getRect(find.byType(Column).last);
+    final Rect paddingRect = tester.getRect(find.byType(Padding).last);
+
+    // Check the offset of all the edges from x-axis and y-axis after childrenPadding
+    // is applied.
+    expect(columnRect.left, paddingRect.left + 10);
+    expect(columnRect.top, paddingRect.top + 8);
+    expect(columnRect.right, paddingRect.right - 12);
+    expect(columnRect.bottom, paddingRect.bottom - 4);
+  });
+
 }