Add lazyLoadChildren to ExpansionTile in material_ui

packages/material_ui/lib/src/expansion_tile.dart

@@ -171,6 +171,7 @@ class ExpansionTile extends StatefulWidget {
     this.expansionAnimationStyle,
     this.internalAddSemanticForOnTap = false,
     this.statesController,
+    this.lazyLoadChildren = false,
   }) : assert(
          expandedCrossAxisAlignment != CrossAxisAlignment.baseline,
          'CrossAxisAlignment.baseline is not supported since the expanded children '
@@ -256,6 +257,13 @@ class ExpansionTile extends StatefulWidget {
   /// collapsed and recreated upon expansion.
   final bool maintainState;
 
+  /// If [maintainState] is true, whether to delay building the children until
+  /// the tile is expanded for the first time.
+  ///
+  /// Defaults to false, meaning the children are built at the same time as the
+  /// header.
+  final bool lazyLoadChildren;
+
   /// Specifies padding for the [ListTile].
   ///
   /// Analogous to [ListTile.contentPadding], this property defines the insets for
@@ -521,6 +529,15 @@ class ExpansionTile extends StatefulWidget {
 
   @override
   State<ExpansionTile> createState() => _ExpansionTileState();
+
+  @override
+  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
+    super.debugFillProperties(properties);
+    properties.add(FlagProperty('maintainState', value: maintainState, ifTrue: 'maintainState'));
+    properties.add(
+      FlagProperty('lazyLoadChildren', value: lazyLoadChildren, ifTrue: 'lazyLoadChildren'),
+    );
+  }
 }
 
 class _ExpansionTileState extends State<ExpansionTile> {
@@ -893,6 +910,7 @@ class _ExpansionTileState extends State<ExpansionTile> {
       duration: _duration,
       reverseCurve: _reverseCurve,
       maintainState: widget.maintainState,
+      lazyLoadChildren: widget.lazyLoadChildren,
       headerBuilder: _buildHeader,
       bodyBuilder: _buildBody,
       expansibleBuilder: _buildExpansible,

packages/material_ui/test/expansion_tile_test.dart

@@ -279,6 +279,92 @@ void main() {
     expect(find.text('Discarding State'), findsNothing);
   });
 
+  testWidgets('ExpansionTile lazyLoadChildren defers child build until first expansion', (
+    WidgetTester tester,
+  ) async {
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Material(
+          child: SingleChildScrollView(
+            child: ExpansionTile(
+              title: Text('Title'),
+              lazyLoadChildren: true,
+              maintainState: true,
+              children: <Widget>[Text('Lazy Child')],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    // Child is not in the tree at all before first expansion, even with
+    // maintainState: true.
+    expect(find.text('Lazy Child', skipOffstage: false), findsNothing);
+
+    await tester.tap(find.text('Title'));
+    await tester.pumpAndSettle();
+    expect(find.text('Lazy Child'), findsOneWidget);
+
+    // After collapsing, the child remains in the tree (offstage) because
+    // maintainState is true and the tile has been expanded once.
+    await tester.tap(find.text('Title'));
+    await tester.pumpAndSettle();
+    expect(find.text('Lazy Child'), findsNothing);
+    expect(find.text('Lazy Child', skipOffstage: false), findsOneWidget);
+  });
+
+  testWidgets('ExpansionTile lazyLoadChildren builds children when initiallyExpanded is true', (
+    WidgetTester tester,
+  ) async {
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Material(
+          child: SingleChildScrollView(
+            child: ExpansionTile(
+              title: Text('Title'),
+              lazyLoadChildren: true,
+              initiallyExpanded: true,
+              children: <Widget>[Text('Lazy Child')],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    // Initially expanded, so the children are built right away.
+    expect(find.text('Lazy Child'), findsOneWidget);
+  });
+
+  testWidgets(
+    'ExpansionTile lazyLoadChildren with maintainState false still removes children when collapsed',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const MaterialApp(
+          home: Material(
+            child: SingleChildScrollView(
+              child: ExpansionTile(
+                title: Text('Title'),
+                lazyLoadChildren: true,
+                maintainState: false,
+                children: <Widget>[Text('Lazy Child')],
+              ),
+            ),
+          ),
+        ),
+      );
+
+      expect(find.text('Lazy Child', skipOffstage: false), findsNothing);
+
+      await tester.tap(find.text('Title'));
+      await tester.pumpAndSettle();
+      expect(find.text('Lazy Child'), findsOneWidget);
+
+      await tester.tap(find.text('Title'));
+      await tester.pumpAndSettle();
+      expect(find.text('Lazy Child', skipOffstage: false), findsNothing);
+    },
+  );
+
   testWidgets('ExpansionTile padding test', (WidgetTester tester) async {
     await tester.pumpWidget(
       const MaterialApp(