Add RTL support to Scrollbar (#11843)

When scrolling vertically, the scrollbar is on the left in RTL.

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

@@ -49,7 +49,9 @@ class _ScrollbarState extends State<Scrollbar> with TickerProviderStateMixin {
   void didChangeDependencies() {
     super.didChangeDependencies();
     _painter ??= new _ScrollbarPainter(this);
-    _painter.color = Theme.of(context).highlightColor;
+    _painter
+      ..color = Theme.of(context).highlightColor
+      ..textDirection = Directionality.of(context);
   }
 
   bool _handleScrollNotification(ScrollNotification notification) {
@@ -102,12 +104,22 @@ class _ScrollbarPainter extends ChangeNotifier implements CustomPainter {
   Color _color;
   set color(Color value) {
     assert(value != null);
-    if (color == value)
+    if (_color == value)
       return;
     _color = value;
     notifyListeners();
   }
 
+  TextDirection get textDirection => _textDirection;
+  TextDirection _textDirection;
+  set textDirection(TextDirection value) {
+    assert(value != null);
+    if (_textDirection == value)
+      return;
+    _textDirection = value;
+    notifyListeners();
+  }
+
   @override
   void dispose() {
     _fadeOut?.cancel();
@@ -142,8 +154,19 @@ class _ScrollbarPainter extends ChangeNotifier implements CustomPainter {
 
   Paint get _paint => new Paint()..color = color.withOpacity(_opacity.value);
 
+  double _getThumbX(Size size) {
+    assert(textDirection != null);
+    switch (textDirection) {
+      case TextDirection.rtl:
+        return 0.0;
+      case TextDirection.ltr:
+        return size.width - _kThumbGirth;
+    }
+    return null;
+  }
+
   void _paintVerticalThumb(Canvas canvas, Size size, double thumbOffset, double thumbExtent) {
-    final Offset thumbOrigin = new Offset(size.width - _kThumbGirth, thumbOffset);
+    final Offset thumbOrigin = new Offset(_getThumbX(size), thumbOffset);
     final Size thumbSize = new Size(_kThumbGirth, thumbExtent);
     canvas.drawRect(thumbOrigin & thumbSize, _paint);
   }

packages/flutter/test/material/scrollbar_paint_test.dart

@@ -8,14 +8,31 @@ import 'package:flutter/material.dart';
 import '../rendering/mock_canvas.dart';
 
 void main() {
-  testWidgets('Viewport basic test', (WidgetTester tester) async {
-    await tester.pumpWidget(new Scrollbar(
+  testWidgets('Viewport basic test (LTR)', (WidgetTester tester) async {
+    await tester.pumpWidget(new Directionality(
+      textDirection: TextDirection.ltr,
+      child: new Scrollbar(
         child: new SingleChildScrollView(
           child: const SizedBox(width: 4000.0, height: 4000.0),
         ),
+      ),
+    ));
+    expect(find.byType(Scrollbar), isNot(paints..rect()));
+    await tester.fling(find.byType(SingleChildScrollView), const Offset(0.0, -10.0), 10.0);
+    expect(find.byType(Scrollbar), paints..rect(rect: new Rect.fromLTRB(800.0 - 6.0, 1.5, 800.0, 91.5)));
+  });
+
+  testWidgets('Viewport basic test (RTL)', (WidgetTester tester) async {
+    await tester.pumpWidget(new Directionality(
+      textDirection: TextDirection.rtl,
+      child: new Scrollbar(
+        child: new SingleChildScrollView(
+          child: const SizedBox(width: 4000.0, height: 4000.0),
+        ),
+      ),
     ));
     expect(find.byType(Scrollbar), isNot(paints..rect()));
     await tester.fling(find.byType(SingleChildScrollView), const Offset(0.0, -10.0), 10.0);
-    expect(find.byType(Scrollbar), paints..rect());
+    expect(find.byType(Scrollbar), paints..rect(rect: new Rect.fromLTRB(0.0, 1.5, 6.0, 91.5)));
   });
 }

packages/flutter/test/material/scrollbar_test.dart

@@ -19,8 +19,9 @@ class TestCanvas implements Canvas {
 
 void main() {
   testWidgets('Scrollbar doesn\'t show when tapping list', (WidgetTester tester) async {
-    await tester.pumpWidget(
-      new Center(
+    await tester.pumpWidget(new Directionality(
+      textDirection: TextDirection.ltr,
+      child: new Center(
         child: new Container(
           decoration: new BoxDecoration(
             border: new Border.all(color: const Color(0xFFFFFF00))
@@ -38,12 +39,12 @@ void main() {
                 new Container(height: 40.0, child: const Text('5')),
                 new Container(height: 40.0, child: const Text('6')),
                 new Container(height: 40.0, child: const Text('7')),
-              ]
-            )
-          )
-        )
-      )
-    );
+              ],
+            ),
+          ),
+        ),
+      ),
+    ));
 
     SchedulerBinding.instance.debugAssertNoTransientCallbacks('Building a list with a scrollbar triggered an animation.');
     await tester.tap(find.byType(ListView));
@@ -61,19 +62,20 @@ void main() {
   });
 
   testWidgets('ScrollbarPainter does not divide by zero', (WidgetTester tester) async {
-    await tester.pumpWidget(
-      new Container(
+    await tester.pumpWidget(new Directionality(
+      textDirection: TextDirection.ltr,
+      child: new Container(
         height: 200.0,
         width: 300.0,
         child: new Scrollbar(
           child: new ListView(
             children: <Widget>[
               new Container(height: 40.0, child: const Text('0')),
-            ]
-          )
-        )
-      )
-    );
+            ],
+          ),
+        ),
+      ),
+    ));
 
     final CustomPaint custom = tester.widget(find.descendant(of: find.byType(Scrollbar), matching: find.byType(CustomPaint)).first);
     final dynamic scrollPainter = custom.foregroundPainter;

packages/flutter/test/widgets/slivers_evil_test.dart

@@ -73,8 +73,9 @@ class TestViewportScrollPosition extends ScrollPositionWithSingleContext {
 void main() {
   testWidgets('Evil test of sliver features - 1', (WidgetTester tester) async {
     final GlobalKey centerKey = new GlobalKey();
-    await tester.pumpWidget(
-      new ScrollConfiguration(
+    await tester.pumpWidget(new Directionality(
+      textDirection: TextDirection.ltr,
+      child: new ScrollConfiguration(
         behavior: new TestBehavior(),
         child: new Scrollbar(
           child: new Scrollable(
@@ -159,7 +160,7 @@ void main() {
           ),
         ),
       ),
-    );
+    ));
     final ScrollPosition position = tester.state<ScrollableState>(find.byType(Scrollable)).position;
 
     position.animateTo(10000.0, curve: Curves.linear, duration: const Duration(minutes: 1));