Fix multiple calls to Slider's onChanged. (flutter#143680)

Fixes flutter#143524

Author: yiiim

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

@@ -612,6 +612,11 @@ class _SliderState extends State<Slider> with TickerProviderStateMixin {
 
   bool _dragging = false;
 
+  // For discrete sliders, _handleChanged might receive the same value
+  // multiple times. To avoid calling widget.onChanged repeatedly, the
+  // value from _handleChanged is temporarily saved here.
+  double? _currentChangedValue;
+
   FocusNode? _focusNode;
   FocusNode get focusNode => widget.focusNode ?? _focusNode!;
 
@@ -664,8 +669,11 @@ class _SliderState extends State<Slider> with TickerProviderStateMixin {
   void _handleChanged(double value) {
     assert(widget.onChanged != null);
     final double lerpValue = _lerp(value);
-    if (lerpValue != widget.value) {
-      widget.onChanged!(lerpValue);
+    if (_currentChangedValue != lerpValue) {
+      _currentChangedValue = lerpValue;
+      if (_currentChangedValue != widget.value) {
+        widget.onChanged!(_currentChangedValue!);
+      }
     }
   }
 
@@ -676,6 +684,7 @@ class _SliderState extends State<Slider> with TickerProviderStateMixin {
 
   void _handleDragEnd(double value) {
     _dragging = false;
+    _currentChangedValue = null;
     widget.onChangeEnd?.call(_lerp(value));
   }
 

packages/flutter/test/material/slider_test.dart

@@ -4253,4 +4253,32 @@ void main() {
       );
     });
   });
+
+  // This is a regression test for https://github.com/flutter/flutter/issues/143524.
+  testWidgets('Discrete Slider.onChanged is called only once', (WidgetTester tester) async {
+    int onChangeCallbackCount = 0;
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: Center(
+            child: Slider(
+              max: 5,
+              divisions: 5,
+              value: 0,
+              onChanged: (double newValue) {
+                onChangeCallbackCount++;
+              },
+            ),
+          ),
+        ),
+      ),
+    );
+
+    final TestGesture gesture = await tester.startGesture(tester.getTopLeft(find.byType(Slider)));
+    await tester.pump(kLongPressTimeout);
+    await gesture.moveBy(const Offset(160.0, 0.0));
+    await gesture.moveBy(const Offset(1.0, 0.0));
+    await gesture.moveBy(const Offset(1.0, 0.0));
+    expect(onChangeCallbackCount, 1);
+  });
 }