url_shortner/frontend/lib/main.dart

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:url_launcher/url_launcher.dart';

import 'shorten_service.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Home(),
    );
  }
}

class Home extends StatefulWidget {
  const Home({super.key});

  @override
  State<Home> createState() => _HomeState();
}

class _HomeState extends State<Home> {
  late final TextEditingController controller;
  final GlobalKey<FormState> formKey = GlobalKey<FormState>();
  String? shortenedUrl;
  bool loading = false;

  @override
  void initState() {
    super.initState();
    controller = TextEditingController();
  }

  @override
  void dispose() {
    super.dispose();
    controller.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Stack(
          children: <Widget>[
            Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                const Text(
                  'Url shortner',
                  style: TextStyle(fontSize: 24),
                ),
                Row(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: <Widget>[
                    const SizedBox(width: 30),
                    ConstrainedBox(
                      constraints: const BoxConstraints(maxWidth: 300),
                      child: Form(
                        key: formKey,
                        child: TextFormField(
                          controller: controller,
                          onFieldSubmitted: loading ? null : (_) => processUrl(),
                          validator: (String? url) {
                            try {
                              if (url == null || url.isEmpty) {
                                return 'No url entered.';
                              }
                              final Uri uri = Uri.parse(url);
                              if (!uri.hasScheme || (!uri.isScheme('HTTP') && !uri.isScheme('HTTPS'))) {
                                throw const FormatException();
                              }
                              return null;
                            } on FormatException {
                              return 'Bad URL';
                            }
                          },
                        ),
                      ),
                    ),
                    IconButton(
                      icon: const Icon(Icons.arrow_circle_right_rounded),
                      onPressed: loading ? null : () => processUrl(),
                    ),
                  ],
                ),
                AnimatedOpacity(
                  opacity: shortenedUrl == null ? 0.0 : 1.0,
                  duration: const Duration(milliseconds: 200),
                  child: Padding(
                    padding: const EdgeInsets.only(top: 14.0),
                    child: Column(
                      children: <Widget>[
                        const Text('Short URL:'),
                        InkWell(
                          onTap: shortenedUrl == null ? null : () => launchUrl(Uri.parse(shortenedUrl!)),
                          child: Text(
                            shortenedUrl ?? '',
                            style: const TextStyle(color: Colors.blue, fontSize: 24),
                          ),
                        ),
                      ],
                    ),
                  ),
                ),
              ],
            ),
            Center(
              child: AnimatedOpacity(
                opacity: loading ? 1.0 : 0.0,
                duration: const Duration(milliseconds: 200),
                child: const CircularProgressIndicator(),
              ),
            )
          ],
        ),
      ),
    );
  }

  Future<void> processUrl() async {
    try {
      assert(formKey.currentState != null, 'Could not validate form, formKey currentState is null');

      if (loading) {
        return;
      }
      setState(() => loading = true);
      final bool isValid = formKey.currentState!.validate();

      if (!isValid) {
        setState(() => loading = false);
        return;
      }

      final String? shortUrl = await shortenUrl(controller.text);
      if (shortUrl != null) {
        setState(() {
          shortenedUrl = shortUrl;
          loading = false;
        });
      } else {
        // show error
        setState(() {
          shortenedUrl = null;
          loading = false;
        });
      }
    } catch (err, st) {
      if (kDebugMode) {
        print(st);
        print('An error occurred trying to shorten the url: $err');
      }
      setState(() => loading = false);
    }
  }
}