[Material] Properly call onChangeStart and onChangeEnd in Range Slider (#34869)

Fixes a bug in the Range Sider where the onChangeStart and onChangeEnd were not being called if the initial thumb selection came from a drag (rather than a tap).

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

@@ -454,8 +454,8 @@ class _RangeSliderState extends State<RangeSlider> with TickerProviderStateMixin
   }
 
   // Finds closest thumb. If the thumbs are close to each other, no thumb is
-  // immediately selected while the drag velocity is zero. If the first
-  // non-zero velocity is negative, then the left thumb is selected, and if its
+  // immediately selected while the drag displacement is zero. If the first
+  // non-zero displacement is negative, then the left thumb is selected, and if its
   // positive, then the right thumb is selected.
   static final RangeThumbSelector _defaultRangeThumbSelector = (
       TextDirection textDirection,
@@ -463,16 +463,17 @@ class _RangeSliderState extends State<RangeSlider> with TickerProviderStateMixin
       double tapValue,
       Size thumbSize,
       Size trackSize,
-      double dx, // drag velocity
+      double dx, // The horizontal delta or displacement of the drag update.
     ) {
     final double touchRadius = math.max(thumbSize.width, RangeSlider._minTouchTargetWidth) / 2;
     final bool inStartTouchTarget = (tapValue - values.start).abs() * trackSize.width < touchRadius;
     final bool inEndTouchTarget = (tapValue - values.end).abs() * trackSize.width < touchRadius;
 
-    // Use the velocity if the thumb touch targets overlap. If dx is 0 and the
-    // the drag position is in both touch targets, no thumb is selected because
-    // it is ambiguous to which thumb should be selected. Once the drag
-    // velocity is non-zero, the thumb selection can be determined.
+    // Use dx if the thumb touch targets overlap. If dx is 0 and the drag
+    // position is in both touch targets, no thumb is selected because it is
+    // ambiguous to which thumb should be selected. If the dx is non-zero, the
+    // thumb selection is determined by the direction of the dx. The left thumb
+    // is chosen for negative dx, and the right thumb is chosen for positive dx.
     if (inStartTouchTarget && inEndTouchTarget) {
       bool towardsStart;
       bool towardsEnd;
@@ -729,6 +730,7 @@ class _RenderRangeSlider extends RenderBox {
   HorizontalDragGestureRecognizer _drag;
   TapGestureRecognizer _tap;
   bool _active = false;
+  RangeValues _newValues;
 
   bool get isEnabled => onChanged != null;
 
@@ -985,11 +987,10 @@ class _RenderRangeSlider extends RenderBox {
       // a tap, it consists of a call to onChangeStart with the previous value and
       // a call to onChangeEnd with the new value.
       final RangeValues currentValues = _discretizeRangeValues(values);
-      RangeValues newValues;
       if (_lastThumbSelection == Thumb.start) {
-        newValues = RangeValues(tapValue, currentValues.end);
+        _newValues = RangeValues(tapValue, currentValues.end);
       } else if (_lastThumbSelection == Thumb.end) {
-        newValues = RangeValues(currentValues.start, tapValue);
+        _newValues = RangeValues(currentValues.start, tapValue);
       }
       _updateLabelPainter(_lastThumbSelection);
 
@@ -997,7 +998,7 @@ class _RenderRangeSlider extends RenderBox {
         onChangeStart(currentValues);
       }
 
-      onChanged(_discretizeRangeValues(newValues));
+      onChanged(_discretizeRangeValues(_newValues));
 
       _state.overlayController.forward();
       if (showValueIndicator) {
@@ -1017,11 +1018,15 @@ class _RenderRangeSlider extends RenderBox {
   void _handleDragUpdate(DragUpdateDetails details) {
     final double dragValue = _getValueFromGlobalPosition(details.globalPosition);
 
-    // If no selection has been made yet, use the velocity of the drag to
-    // determine which thumb should be selected.
+    // If no selection has been made yet, test for thumb selection again now
+    // that the value of dx can be non-zero. If this is the first selection of
+    // the interaction, then onChangeStart must be called.
+    bool shouldCallOnChangeStart = false;
     if (_lastThumbSelection == null) {
       _lastThumbSelection = sliderTheme.thumbSelector(textDirection, values, dragValue, _thumbSize, size, details.delta.dx);
       if (_lastThumbSelection != null) {
+        shouldCallOnChangeStart = true;
+        _active = true;
         _state.overlayController.forward();
         if (showValueIndicator) {
           _state.valueIndicatorController.forward();
@@ -1031,16 +1036,18 @@ class _RenderRangeSlider extends RenderBox {
 
     if (isEnabled && _lastThumbSelection != null) {
       final RangeValues currentValues = _discretizeRangeValues(values);
+      if (onChangeStart != null && shouldCallOnChangeStart) {
+        onChangeStart(currentValues);
+      }
       final double currentDragValue = _discretize(dragValue);
 
-      RangeValues newValues;
       final double minThumbSeparationValue = isDiscrete ? 0 : sliderTheme.minThumbSeparation / _trackRect.width;
       if (_lastThumbSelection == Thumb.start) {
-        newValues = RangeValues(math.min(currentDragValue, currentValues.end - minThumbSeparationValue), currentValues.end);
+        _newValues = RangeValues(math.min(currentDragValue, currentValues.end - minThumbSeparationValue), currentValues.end);
       } else if (_lastThumbSelection == Thumb.end) {
-        newValues = RangeValues(currentValues.start, math.max(currentDragValue, currentValues.start + minThumbSeparationValue));
+        _newValues = RangeValues(currentValues.start, math.max(currentDragValue, currentValues.start + minThumbSeparationValue));
       }
-      onChanged(newValues);
+      onChanged(_newValues);
     }
   }
 
@@ -1050,10 +1057,8 @@ class _RenderRangeSlider extends RenderBox {
       _state.valueIndicatorController.reverse();
     }
 
-    if (_active && _state.mounted) {
-      if (_lastThumbSelection == null)
-        return;
-      final RangeValues discreteValues = _discretizeRangeValues(values);
+    if (_active && _state.mounted && _lastThumbSelection != null) {
+      final RangeValues discreteValues = _discretizeRangeValues(_newValues);
       if (onChangeEnd != null) {
         onChangeEnd(discreteValues);
       }