Working project

frontend/lib/main.dart

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+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);
+    }
+  }
+}

frontend/lib/shorten_service.dart

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+import 'dart:convert';
+
+import 'package:dio/dio.dart';
+import 'package:flutter/foundation.dart';
+
+BaseOptions dioOptions = BaseOptions(baseUrl: 'http://localhost:8080');
+
+Future<String?> shortenUrl(String url) async {
+  try {
+    assert(url.isNotEmpty);
+    final Response<String> res = await Dio(dioOptions).post('/', data: <String, String>{'url': url});
+    final Map<String, dynamic> json = jsonDecode(res.data ?? '') as Map<String, dynamic>;
+    print(json);
+    return json['url'] as String?;
+  } catch (err, st) {
+    if (kDebugMode) {
+      print('Encountered an error:');
+      print(st);
+      print(err);
+    }
+    return null;
+  }
+}

frontend/lib/shrink_ray.dart

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+class ShrinkRay {}