onValueChangeStart property Null safety

ValueChanged<double>? onValueChangeStart
final

Called when the user starts selecting a new value of pointer by dragging.

This callback shouldn't be used to update the pointer value (use onValueChanged for that), but rather to be notified when the user has started selecting a new value by starting a drag. The value passed will be the last value that the pointer had before the change began.

Widget build(BuildContext context) {
 return Container(
     child: SfRadialGauge(axes: <RadialAxis>[
   RadialAxis(
     pointers: <GaugePointer>[
       MarkerPointer(
           value: 50,
           onValueChangeStart: (double startValue) {
             setState(() {
               print('Started change at $startValue');
             });
           })
     ],
   )
 ]));
}

Implementation

/// The value passed will be the last value that the pointer had before
/// the change began.
///
/// ```dart
/// Widget build(BuildContext context) {
///  return Container(
///      child: SfRadialGauge(axes: <RadialAxis>[
///    RadialAxis(
///      pointers: <GaugePointer>[
///        MarkerPointer(
///            value: 50,
///            onValueChangeStart: (double startValue) {
///              setState(() {
///                print('Started change at $startValue');
///              });
///            })
///      ],
///    )
///  ]));
/// }
/// ```
final ValueChanged<double>? onValueChangeStart;