Add `shape` property to SliverAppBar (#31662)

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

@@ -681,6 +681,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
     @required this.floating,
     @required this.pinned,
     @required this.snapConfiguration,
+    @required this.shape,
   }) : assert(primary || topPadding == 0.0),
        _bottomHeight = bottom?.preferredSize?.height ?? 0.0;
 
@@ -705,6 +706,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
   final double topPadding;
   final bool floating;
   final bool pinned;
+  final ShapeBorder shape;
 
   final double _bottomHeight;
 
@@ -759,6 +761,7 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
         primary: primary,
         centerTitle: centerTitle,
         titleSpacing: titleSpacing,
+        shape: shape,
         toolbarOpacity: toolbarOpacity,
         bottomOpacity: pinned ? 1.0 : (visibleMainHeight / _bottomHeight).clamp(0.0, 1.0),
       ),
@@ -902,6 +905,7 @@ class SliverAppBar extends StatefulWidget {
     this.floating = false,
     this.pinned = false,
     this.snap = false,
+    this.shape,
   }) : assert(automaticallyImplyLeading != null),
        assert(forceElevated != null),
        assert(primary != null),
@@ -1062,6 +1066,12 @@ class SliverAppBar extends StatefulWidget {
   /// Defaults to [NavigationToolbar.kMiddleSpacing].
   final double titleSpacing;
 
+  /// The material's shape as well its shadow.
+  ///
+  /// A shadow is only displayed if the [elevation] is greater than
+  /// zero.
+  final ShapeBorder shape;
+
   /// The size of the app bar when it is fully expanded.
   ///
   /// By default, the total height of the toolbar and the bottom widget (if
@@ -1215,6 +1225,7 @@ class _SliverAppBarState extends State<SliverAppBar> with TickerProviderStateMix
           topPadding: topPadding,
           floating: widget.floating,
           pinned: widget.pinned,
+          shape: widget.shape,
           snapConfiguration: _snapConfiguration,
         ),
       ),

packages/flutter/test/material/app_bar_test.dart

@@ -1514,4 +1514,39 @@ void main() {
 
     expect(getMaterialWidget().shape, roundedRectangleBorder);
   });
+
+  testWidgets('SliverAppBar with shape', (WidgetTester tester) async {
+    const RoundedRectangleBorder roundedRectangleBorder = RoundedRectangleBorder(
+      borderRadius: BorderRadius.all(Radius.circular(15.0)),
+    );
+    await tester.pumpWidget(
+      MaterialApp(
+        home: CustomScrollView(
+          slivers: const <Widget>[
+            SliverAppBar(
+              leading: Text('L'),
+              title: Text('No Scaffold'),
+              shape: roundedRectangleBorder,
+              actions: <Widget>[Text('A1'), Text('A2')],
+            ),
+          ],
+        ),
+      ),
+    );
+
+    final Finder sliverAppBarFinder = find.byType(SliverAppBar);
+
+    SliverAppBar getAppBarWidget() {
+      return tester.widget<SliverAppBar>(sliverAppBarFinder);
+    }
+
+    expect(getAppBarWidget().shape, roundedRectangleBorder);
+
+    final Finder materialFinder = find.byType(Material);
+    Material getMaterialWidget() {
+      return tester.widget<Material>(materialFinder);
+    }
+
+    expect(getMaterialWidget().shape, roundedRectangleBorder);
+  });
 }