Adding thumb color customisation functionality to CupertinoSlider (#42563)

packages/flutter/lib/src/cupertino/slider.dart

diff --git a/packages/flutter/lib/src/cupertino/slider.dart b/packages/flutter/lib/src/cupertino/slider.dart
index 770d0d5..5cae6ef 100644
--- a/packages/flutter/lib/src/cupertino/slider.dart
+++ b/packages/flutter/lib/src/cupertino/slider.dart
@@ -60,11 +60,13 @@
     this.max = 1.0,
     this.divisions,
     this.activeColor,
+    this.thumbColor = CupertinoColors.white,
   }) : assert(value != null),
        assert(min != null),
        assert(max != null),
        assert(value >= min && value <= max),
        assert(divisions == null || divisions > 0),
+       assert(thumbColor != null),
        super(key: key);
 
   /// The currently selected value for this slider.
@@ -193,6 +195,13 @@
   /// Defaults to the [CupertinoTheme]'s primary color if null.
   final Color activeColor;
 
+  /// The color to use for the thumb of the slider.
+  ///
+  /// Thumb color must not be null.
+  ///
+  /// Defaults to [CupertinoColors.white].
+  final Color thumbColor;
+
   @override
   _CupertinoSliderState createState() => _CupertinoSliderState();
 
@@ -233,6 +242,7 @@
         widget.activeColor ?? CupertinoTheme.of(context).primaryColor,
         context,
       ),
+      thumbColor: widget.thumbColor,
       onChanged: widget.onChanged != null ? _handleChanged : null,
       onChangeStart: widget.onChangeStart != null ? _handleDragStart : null,
       onChangeEnd: widget.onChangeEnd != null ? _handleDragEnd : null,
@@ -247,6 +257,7 @@
     this.value,
     this.divisions,
     this.activeColor,
+    this.thumbColor,
     this.onChanged,
     this.onChangeStart,
     this.onChangeEnd,
@@ -256,18 +267,19 @@
   final double value;
   final int divisions;
   final Color activeColor;
+  final Color thumbColor;
   final ValueChanged<double> onChanged;
   final ValueChanged<double> onChangeStart;
   final ValueChanged<double> onChangeEnd;
   final TickerProvider vsync;
 
-
   @override
   _RenderCupertinoSlider createRenderObject(BuildContext context) {
     return _RenderCupertinoSlider(
       value: value,
       divisions: divisions,
       activeColor: activeColor,
+      thumbColor: CupertinoDynamicColor.resolve(thumbColor, context),
       trackColor: CupertinoDynamicColor.resolve(CupertinoColors.systemFill, context),
       onChanged: onChanged,
       onChangeStart: onChangeStart,
@@ -283,6 +295,7 @@
       ..value = value
       ..divisions = divisions
       ..activeColor = activeColor
+      ..thumbColor = CupertinoDynamicColor.resolve(thumbColor, context)
       ..trackColor = CupertinoDynamicColor.resolve(CupertinoColors.systemFill, context)
       ..onChanged = onChanged
       ..onChangeStart = onChangeStart
@@ -305,6 +318,7 @@
     @required double value,
     int divisions,
     Color activeColor,
+    Color thumbColor,
     Color trackColor,
     ValueChanged<double> onChanged,
     this.onChangeStart,
@@ -316,6 +330,7 @@
        _value = value,
        _divisions = divisions,
        _activeColor = activeColor,
+       _thumbColor = thumbColor,
        _trackColor = trackColor,
        _onChanged = onChanged,
        _textDirection = textDirection,
@@ -363,6 +378,15 @@
     markNeedsPaint();
   }
 
+  Color get thumbColor => _thumbColor;
+  Color _thumbColor;
+  set thumbColor(Color value) {
+    if (value == _thumbColor)
+      return;
+    _thumbColor = value;
+    markNeedsPaint();
+  }
+
   Color get trackColor => _trackColor;
   Color _trackColor;
   set trackColor(Color value) {
@@ -512,7 +536,7 @@
     }
 
     final Offset thumbCenter = Offset(trackActive, trackCenter);
-    const CupertinoThumbPainter().paint(canvas, Rect.fromCircle(center: thumbCenter, radius: CupertinoThumbPainter.radius));
+    CupertinoThumbPainter(color: thumbColor).paint(canvas, Rect.fromCircle(center: thumbCenter, radius: CupertinoThumbPainter.radius));
   }
 
   @override

packages/flutter/test/cupertino/slider_test.dart

diff --git a/packages/flutter/test/cupertino/slider_test.dart b/packages/flutter/test/cupertino/slider_test.dart
index 3cb0342..ec62ca3 100644
--- a/packages/flutter/test/cupertino/slider_test.dart
+++ b/packages/flutter/test/cupertino/slider_test.dart
@@ -556,4 +556,52 @@
       isNot(paints..rrect(color: _kSystemFill.color)),
     );
   });
+
+  testWidgets('Thumb color can be overridden', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Center(
+          child: CupertinoSlider(
+            thumbColor: CupertinoColors.systemPurple,
+            onChanged: (double value) { },
+            value: 0,
+          ),
+        ),
+      ),
+    );
+
+    expect(
+      find.byType(CupertinoSlider),
+      paints
+      ..rrect()
+      ..rrect()
+      ..rrect()
+      ..rrect()
+      ..rrect()
+      ..rrect(color: CupertinoColors.systemPurple.color)
+    );
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: Center(
+          child: CupertinoSlider(
+            thumbColor: CupertinoColors.activeOrange,
+            onChanged: (double value) { },
+            value: 0,
+          ),
+        ),
+      ),
+    );
+
+    expect(
+        find.byType(CupertinoSlider),
+        paints
+          ..rrect()
+          ..rrect()
+          ..rrect()
+          ..rrect()
+          ..rrect()
+          ..rrect(color: CupertinoColors.activeOrange.color)
+    );
+  });
 }