[Slider]The initial value of discrete slider should respect the discrete values (#103966)

Fixes #103965

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

@@ -523,7 +523,7 @@ class _SliderState extends State<Slider> with TickerProviderStateMixin {
       vsync: this,
     );
     enableController.value = widget.onChanged != null ? 1.0 : 0.0;
-    positionController.value = _unlerp(widget.value);
+    positionController.value = _convert(widget.value);
     _actionMap = <Type, Action<Intent>>{
       _AdjustSliderIntent: CallbackAction<_AdjustSliderIntent>(
         onInvoke: _actionHandler,
@@ -623,6 +623,22 @@ class _SliderState extends State<Slider> with TickerProviderStateMixin {
     return value * (widget.max - widget.min) + widget.min;
   }
 
+  double _discretize(double value) {
+    assert(widget.divisions != null);
+    assert(value >= 0.0 && value <= 1.0);
+
+    final int divisions = widget.divisions!;
+    return (value * divisions).round() / divisions;
+  }
+
+  double _convert(double value) {
+    double ret = _unlerp(value);
+    if (widget.divisions != null) {
+      ret = _discretize(ret);
+    }
+    return ret;
+  }
+
   // Returns a number between 0.0 and 1.0, given a value between min and max.
   double _unlerp(double value) {
     assert(value <= widget.max);
@@ -771,7 +787,7 @@ class _SliderState extends State<Slider> with TickerProviderStateMixin {
           link: _layerLink,
           child: _SliderRenderObjectWidget(
             key: _renderObjectKey,
-            value: _unlerp(widget.value),
+            value: _convert(widget.value),
             divisions: widget.divisions,
             label: widget.label,
             sliderTheme: sliderTheme,

packages/flutter/test/material/slider_test.dart

@@ -101,6 +101,46 @@ class _StateDependentMouseCursor extends MaterialStateMouseCursor {
 }
 
 void main() {
+  testWidgets('The initial value should respect the discrete value', (WidgetTester tester) async {
+    final Key sliderKey = UniqueKey();
+    double value = 0.20;
+    final List<Offset> log = <Offset>[];
+    final LoggingThumbShape loggingThumb = LoggingThumbShape(log);
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Directionality(
+          textDirection: TextDirection.ltr,
+          child: StatefulBuilder(
+            builder: (BuildContext context, StateSetter setState) {
+              final SliderThemeData sliderTheme = SliderTheme.of(context).copyWith(thumbShape: loggingThumb);
+              return Material(
+                child: Center(
+                  child: SliderTheme(
+                    data: sliderTheme,
+                    child: Slider(
+                      key: sliderKey,
+                      value: value,
+                      divisions: 4,
+                      onChanged: (double newValue) {
+                        setState(() {
+                          value = newValue;
+                        });
+                      },
+                    ),
+                  ),
+                ),
+              );
+            },
+          ),
+        ),
+      ),
+    );
+
+    expect(value, equals(0.20));
+    expect(log.length, 1);
+    expect(log[0], const Offset(212.0, 300.0));
+  });
+
   testWidgets('Slider can move when tapped (LTR)', (WidgetTester tester) async {
     final Key sliderKey = UniqueKey();
     double value = 0.0;