StepCounter.dart

class StepCounter extends StatefulWidget {
  int value;
  final int minValue;
  final int maxValue;
  final Function(int) onValueChanged;

  StepCounter(
      {@required this.value,
      @required this.onValueChanged,
      this.minValue = 0,
      this.maxValue = 10}) {
    if (minValue > maxValue) {
      throw Exception(
          'Invalid range. Min value ($minValue) must be less than max value ($maxValue)');
    }
    if (value > maxValue || value < minValue) {
      throw Exception(
          'Invalid value. The value must be between $minValue and $maxValue');
    }
  }

  @override
  _StepCounterState createState() => _StepCounterState();
}

class _StepCounterState extends State<StepCounter> {
  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.start,
      children: <Widget>[
        StepButton(Icons.remove, () {
          if (widget.value > widget.minValue) {
            setState(() {
              widget.value--;
              widget.onValueChanged(widget.value);
            });
          }
        }),
        Container(
          width: 60,
          child: Container(
              padding: EdgeInsets.all(8),
              child: Text(
                widget.value.toString(),
                style: TextStyle(fontSize: 20),
                textAlign: TextAlign.center,
              )),
        ),
        StepButton(Icons.add, () {
          if (widget.value < widget.maxValue) {
            setState(() {
              widget.value++;
              widget.onValueChanged(widget.value);
            });
          }
        }),
      ],
    );
  }

  Widget StepButton(IconData icon, Function f) {
    return GestureDetector(
      child: Container(
          decoration: BoxDecoration(color: Colors.blue),
          padding: EdgeInsets.all(8),
          child: Icon(
            icon,
            color: Colors.white,
          )),
      onTap: () {
        f();
      },
    );
  }
}

StepCounterTest.dart

import 'package:first_app/stepper.dart';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  testWidgets('Test step counter', (WidgetTester tester) async {
    await tester.pumpWidget(MaterialApp(
      home: StepCounter(
        value: 1,
        onValueChanged: (_) {},
      ),
    ));

    // Verify that our counter starts at 0.
    expect(find.text('1'), findsOneWidget);
    expect(find.text('0'), findsNothing);

    // Tap the '+' icon and trigger a frame.
    await tester.tap(find.byIcon(Icons.add));
    await tester.pump();

    // Verify that our counter has incremented.
    expect(find.text('1'), findsNothing);
    expect(find.text('2'), findsOneWidget);
  });

  testWidgets('Test min value', (WidgetTester tester) async {
    await tester.pumpWidget(MaterialApp(
      home: StepCounter(
        value: 1,
        minValue: 1,
        onValueChanged: (_) {},
      ),
    ));

    // Verify that our counter starts at 1.
    expect(find.text('1'), findsOneWidget);
    expect(find.text('0'), findsNothing);

    // Tap the '-' icon and trigger a frame.
    await tester.tap(find.byIcon(Icons.remove));
    await tester.pump();

    // Verify that our counter has decremented.
    expect(find.text('0'), findsNothing);
    expect(find.text('1'), findsOneWidget);
  });

  testWidgets('Test max value', (WidgetTester tester) async {
    await tester.pumpWidget(MaterialApp(
      home: StepCounter(
        value: 10,
        maxValue: 10,
        onValueChanged: (_) {},
      ),
    ));

    // Verify that our counter starts at 10.
    expect(find.text('10'), findsOneWidget);

    // Tap the '+' icon and trigger a frame.
    await tester.tap(find.byIcon(Icons.add));
    await tester.pump();

    // Verify that our counter has not incremented.
    expect(find.text('11'), findsNothing);
    expect(find.text('10'), findsOneWidget);
  });
}