avoid AnimatedBuilder rebuilding widget (#55832)

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

@@ -369,20 +369,6 @@ class _ModalBottomSheetState<T> extends State<_ModalBottomSheet<T>> {
 
     return AnimatedBuilder(
       animation: widget.route.animation,
-      builder: (BuildContext context, Widget child) {
-        // Disable the initial animation when accessible navigation is on so
-        // that the semantics are added to the tree at the correct time.
-        final double animationValue = animationCurve.transform(
-            mediaQuery.accessibleNavigation ? 1.0 : widget.route.animation.value
-        );
-        return Semantics(
-          scopesRoute: true,
-          namesRoute: true,
-          label: routeLabel,
-          explicitChildNodes: true,
-          child: ClipRect(
-            child: CustomSingleChildLayout(
-              delegate: _ModalBottomSheetLayout(animationValue, widget.isScrollControlled),
       child: BottomSheet(
         animationController: widget.route._animationController,
         onClosing: () {
@@ -399,6 +385,21 @@ class _ModalBottomSheetState<T> extends State<_ModalBottomSheet<T>> {
         onDragStart: handleDragStart,
         onDragEnd: handleDragEnd,
       ),
+      builder: (BuildContext context, Widget child) {
+        // Disable the initial animation when accessible navigation is on so
+        // that the semantics are added to the tree at the correct time.
+        final double animationValue = animationCurve.transform(
+            mediaQuery.accessibleNavigation ? 1.0 : widget.route.animation.value
+        );
+        return Semantics(
+          scopesRoute: true,
+          namesRoute: true,
+          label: routeLabel,
+          explicitChildNodes: true,
+          child: ClipRect(
+            child: CustomSingleChildLayout(
+              delegate: _ModalBottomSheetLayout(animationValue, widget.isScrollControlled),
+              child: child,
             ),
           ),
         );

packages/flutter/test/material/bottom_sheet_test.dart

@@ -306,6 +306,38 @@ void main() {
     expect(find.text('BottomSheet'), findsNothing);
   });
 
+  testWidgets('Modal BottomSheet builder should only be called once', (WidgetTester tester) async {
+    BuildContext savedContext;
+
+    await tester.pumpWidget(MaterialApp(
+      home: Builder(
+        builder: (BuildContext context) {
+          savedContext = context;
+          return Container();
+        },
+      ),
+    ));
+
+    int numBuilderCalls = 0;
+    showModalBottomSheet<void>(
+      context: savedContext,
+      isDismissible: false,
+      enableDrag: true,
+      builder: (BuildContext context) {
+        numBuilderCalls++;
+        return const Text('BottomSheet');
+      },
+    );
+
+    await tester.pumpAndSettle();
+    expect(numBuilderCalls, 1);
+
+    // Swipe the bottom sheet to dismiss it.
+    await tester.drag(find.text('BottomSheet'), const Offset(0.0, 150.0));
+    await tester.pumpAndSettle(); // Bottom sheet dismiss animation.
+    expect(numBuilderCalls, 1);
+  });
+
   testWidgets('Verify that a downwards fling dismisses a persistent BottomSheet', (WidgetTester tester) async {
     final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
     bool showBottomSheetThenCalled = false;