main.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  var _discountController = new TextEditingController();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: ClearableTextFormField(
            controller: _discountController,
            keyboardType: TextInputType.number,
            decoration: InputDecoration(
                icon: Icon(Icons.money_off),
                floatingLabelBehavior: FloatingLabelBehavior.auto,
                labelText: 'Скидка'),
          ),
        ),
      ),
    );
  }
}

class ClearableTextFormField extends TextFormField {
  ClearableTextFormField({
    TextEditingController controller,
    FormFieldValidator validator,
    TextInputType keyboardType,
    InputDecoration decoration,
  }) : super(
          validator: validator,
          controller: controller,
          keyboardType: keyboardType,
          decoration: InputDecoration(
              icon: decoration.icon,
              labelText: decoration.labelText,
              floatingLabelBehavior: FloatingLabelBehavior.auto,
              suffixIcon: ValueListenableBuilder(
                valueListenable: controller,
                builder: (context, value, child) => Visibility(
                  visible: value.text.isNotEmpty,
                  child: IconButton(
                    icon: Icon(Icons.close),
                    onPressed: () => controller.clear(),
                  ),
                ),
              )),
        );
}