examples/test/flutter_widget_test/test/flutter_widget_test.dart

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

void main() {
  testWidgets('my first widget test', (WidgetTester tester) async {
    // You can use keys to locate the widget you need to test
    var sliderKey = UniqueKey();
    var value = 0.0;

    // Tells the tester to build a UI based on the widget tree passed to it
    await tester.pumpWidget(
      StatefulBuilder(
        builder: (BuildContext context, StateSetter setState) {
          return MaterialApp(
            home: Material(
              child: Center(
                child: Slider(
                  key: sliderKey,
                  value: value,
                  onChanged: (double newValue) {
                    setState(() {
                      value = newValue;
                    });
                  },
                ),
              ),
            ),
          );
        },
      ),
    );
    expect(value, equals(0.0));

    // Taps on the widget found by key
    await tester.tap(find.byKey(sliderKey));

    // Verifies that the widget updated the value correctly
    expect(value, equals(0.5));
  });
}