Simplify drawer scrimColor defaults, update tests (#31947)

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

@@ -181,7 +181,7 @@ class DrawerController extends StatefulWidget {
     @required this.alignment,
     this.drawerCallback,
     this.dragStartBehavior = DragStartBehavior.start,
-    this.scrimColor = Colors.black54,
+    this.scrimColor,
   }) : assert(child != null),
        assert(dragStartBehavior != null),
        assert(alignment != null),
@@ -223,7 +223,7 @@ class DrawerController extends StatefulWidget {
 
   /// The color to use for the scrim that obscures primary content while a drawer is open.
   ///
-  /// By default, the color is [Colors.black54]
+  /// By default, the color used is [Colors.black54]
   final Color scrimColor;
 
   @override
@@ -237,7 +237,7 @@ class DrawerControllerState extends State<DrawerController> with SingleTickerPro
   @override
   void initState() {
     super.initState();
-    _color = ColorTween(begin: Colors.transparent, end: widget.scrimColor);
+    _scrimColorTween = _buildScrimColorTween();
     _controller = AnimationController(duration: _kBaseSettleDuration, vsync: this)
       ..addListener(_animationChanged)
       ..addStatusListener(_animationStatusChanged);
@@ -250,6 +250,13 @@ class DrawerControllerState extends State<DrawerController> with SingleTickerPro
     super.dispose();
   }
 
+  @override
+  void didUpdateWidget(DrawerController oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    if (widget.scrimColor != oldWidget.scrimColor)
+      _scrimColorTween = _buildScrimColorTween();
+  }
+
   void _animationChanged() {
     setState(() {
       // The animation controller's state is our build state, and it changed already.
@@ -386,9 +393,13 @@ class DrawerControllerState extends State<DrawerController> with SingleTickerPro
       widget.drawerCallback(false);
   }
 
-  ColorTween _color;
+  ColorTween _scrimColorTween;
   final GlobalKey _gestureDetectorKey = GlobalKey();
 
+  ColorTween _buildScrimColorTween() {
+    return ColorTween(begin: Colors.transparent, end: widget.scrimColor ?? Colors.black54);
+  }
+
   AlignmentDirectional get _drawerOuterAlignment {
     assert(widget.alignment != null);
     switch (widget.alignment) {
@@ -452,8 +463,8 @@ class DrawerControllerState extends State<DrawerController> with SingleTickerPro
                   onTap: close,
                   child: Semantics(
                     label: MaterialLocalizations.of(context)?.modalBarrierDismissLabel,
-                    child: Container(
-                      color: _color.evaluate(_controller),
+                    child: Container( // The drawer's "scrim"
+                      color: _scrimColorTween.evaluate(_controller),
                     ),
                   ),
                 ),

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

@@ -904,7 +904,7 @@ class Scaffold extends StatefulWidget {
     this.primary = true,
     this.drawerDragStartBehavior = DragStartBehavior.start,
     this.extendBody = false,
-    this.drawerScrimColor = Colors.black54,
+    this.drawerScrimColor,
   }) : assert(primary != null),
        assert(extendBody != null),
        assert(drawerDragStartBehavior != null),

packages/flutter/test/material/drawer_test.dart

@@ -107,31 +107,71 @@ void main() {
     semantics.dispose();
   });
 
-  testWidgets('Drawer scrimDrawerColor test', (WidgetTester tester) async {
+  testWidgets('Scaffold drawerScrimColor', (WidgetTester tester) async {
+    // The scrim is a Container within a Semantics node labeled "Dismiss",
+    // within a DrawerController. Sorry.
+    Container getScrim() {
+      return tester.widget<Container>(
+        find.descendant(
+          of: find.descendant(
+            of: find.byType(DrawerController),
+            matching: find.byWidgetPredicate((Widget widget) {
+              if (widget is! Semantics)
+                return false;
+              final Semantics semantics = widget;
+              return semantics.properties.label == 'Dismiss';
+            }),
+          ),
+          matching: find.byType(Container),
+        ),
+      );
+    }
 
-    await tester.pumpWidget(
-      const MaterialApp(
+    final GlobalKey<ScaffoldState> scaffoldKey = GlobalKey<ScaffoldState>();
+    Widget buildFrame({ Color drawerScrimColor }) {
+      return MaterialApp(
         home: Scaffold(
-          drawerScrimColor: Color(0xFF323232),
-          drawer: Drawer(),
+          key: scaffoldKey,
+          drawerScrimColor: drawerScrimColor,
+          drawer: Drawer(
+            child: Builder(
+              builder: (BuildContext context) {
+                return GestureDetector(
+                  onTap: () { Navigator.pop(context); }, // close drawer
+                );
+              },
+            ),
+          ),
         ),
-      ),
-    );
+      );
+    }
 
-    final ScaffoldState state = tester.firstState(find.byType(Scaffold));
-    state.openDrawer();
+    // Default drawerScrimColor
 
-    await tester.pump();
-    await tester.pump(const Duration(seconds: 1));
+    await tester.pumpWidget(buildFrame(drawerScrimColor: null));
+    scaffoldKey.currentState.openDrawer();
+    await tester.pumpAndSettle();
 
-    final Container container = tester.widget<Container>(find.descendant(
-      of: find.byType(Scaffold),
-      matching: find.byType(Container),
-      ).first,
-    );
+    BoxDecoration decoration = getScrim().decoration;
+    expect(decoration.color, Colors.black54);
+    expect(decoration.shape, BoxShape.rectangle);
 
-    final BoxDecoration decoration = container.decoration;
+    await tester.tap(find.byType(Drawer));
+    await tester.pumpAndSettle();
+    expect(find.byType(Drawer), findsNothing);
+
+    // Specific drawerScrimColor
+
+    await tester.pumpWidget(buildFrame(drawerScrimColor: const Color(0xFF323232)));
+    scaffoldKey.currentState.openDrawer();
+    await tester.pumpAndSettle();
+
+    decoration = getScrim().decoration;
     expect(decoration.color, const Color(0xFF323232));
-    expect(decoration.shape,  BoxShape.rectangle);
+    expect(decoration.shape, BoxShape.rectangle);
+
+    await tester.tap(find.byType(Drawer));
+    await tester.pumpAndSettle();
+    expect(find.byType(Drawer), findsNothing);
   });
 }