Use mixin

Author: jpnurmi

lib/src/base_spin_box.dart

@@ -44,7 +44,7 @@ abstract class BaseSpinBox extends StatefulWidget {
   FocusNode? get focusNode;
 }
 
-abstract class BaseSpinBoxState<T extends BaseSpinBox> extends State<T> {
+mixin SpinBoxMixin<T extends BaseSpinBox> on State<T> {
   late double _value;
   late double _cachedValue;
   late final FocusNode _focusNode;

lib/src/cupertino/spin_box.dart

@@ -252,7 +252,7 @@ class CupertinoSpinBox extends BaseSpinBox {
   State<CupertinoSpinBox> createState() => _CupertinoSpinBoxState();
 }
 
-class _CupertinoSpinBoxState extends BaseSpinBoxState<CupertinoSpinBox> {
+class _CupertinoSpinBoxState extends State<CupertinoSpinBox> with SpinBoxMixin {
   @override
   Widget build(BuildContext context) {
     final isHorizontal = widget.direction == Axis.horizontal;

lib/src/material/spin_box.dart

@@ -253,7 +253,7 @@ class SpinBox extends BaseSpinBox {
   State<SpinBox> createState() => _SpinBoxState();
 }
 
-class _SpinBoxState extends BaseSpinBoxState<SpinBox> {
+class _SpinBoxState extends State<SpinBox> with SpinBoxMixin {
   Color _activeColor(ThemeData theme) {
     if (hasFocus) {
       switch (theme.brightness) {