Gemini AI en Flutter: ejercicio resuelto con chat multi-turno y google_generative_ai

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dependencies:
  flutter:
    sdk: flutter
  google_generative_ai: ^0.4.6

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flutter run --dart-define=GEMINI_API_KEY=tu_clave_aqui

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

// ── API key inyectada por --dart-define, nunca hardcodeada ────────────────────
const _apiKey = String.fromEnvironment('GEMINI_API_KEY');

Future<void> main() async {
  if (_apiKey.isEmpty) {
    throw Exception(
      'Ejecuta con: flutter run --dart-define=GEMINI_API_KEY=tu_clave',
    );
  }
  runApp(const MyApp());
}

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter + Gemini',
      theme: ThemeData(useMaterial3: true, colorSchemeSeed: Colors.indigo),
      home: const ChatPage(),
    );
  }
}

// ── Modelo de mensaje ─────────────────────────────────────────────────────────
class ChatMessage {
  final String text;
  final bool isUser;
  final bool isLoading;

  const ChatMessage({
    required this.text,
    required this.isUser,
    this.isLoading = false,
  });
}

// ── ChatPage ──────────────────────────────────────────────────────────────────
class ChatPage extends StatefulWidget {
  const ChatPage({super.key});

  @override
  State<ChatPage> createState() => _ChatPageState();
}

class _ChatPageState extends State<ChatPage> {
  final _scrollCtrl = ScrollController();
  final _inputCtrl = TextEditingController();
  final _messages = <ChatMessage>[];
  late final ChatSession _chat;
  bool _sending = false;

  @override
  void initState() {
    super.initState();
    final model = GenerativeModel(
      model: 'gemini-1.5-flash',
      apiKey: _apiKey,
      generationConfig: GenerationConfig(
        temperature: 0.7,
        maxOutputTokens: 1024,
      ),
      // Mensaje de sistema como primer turno del historial
      systemInstruction: Content.system(
        'Eres un asistente de programación Flutter. '
        'Responde de forma concisa, con ejemplos de código cuando sea útil.',
      ),
    );
    _chat = model.startChat();
  }

  @override
  void dispose() {
    _scrollCtrl.dispose();
    _inputCtrl.dispose();
    super.dispose();
  }

  void _scrollToBottom() {
    WidgetsBinding.instance.addPostFrameCallback((_) {
      if (_scrollCtrl.hasClients) {
        _scrollCtrl.animateTo(
          _scrollCtrl.position.maxScrollExtent,
          duration: const Duration(milliseconds: 300),
          curve: Curves.easeOut,
        );
      }
    });
  }

  Future<void> _sendMessage() async {
    final text = _inputCtrl.text.trim();
    if (text.isEmpty || _sending) return;

    _inputCtrl.clear();
    setState(() {
      _messages.add(ChatMessage(text: text, isUser: true));
      _messages.add(const ChatMessage(text: '', isUser: false, isLoading: true));
      _sending = true;
    });
    _scrollToBottom();

    try {
      final response = await _chat.sendMessage(Content.text(text));
      final replyText = response.text ?? '(sin respuesta)';
      setState(() {
        _messages.removeLast(); // quita el indicador de carga
        _messages.add(ChatMessage(text: replyText, isUser: false));
      });
    } on GenerativeAIException catch (e) {
      setState(() {
        _messages.removeLast();
        _messages.add(ChatMessage(text: 'Error de API: ${e.message}', isUser: false));
      });
    } catch (e) {
      setState(() {
        _messages.removeLast();
        _messages.add(ChatMessage(text: 'Error inesperado: $e', isUser: false));
      });
    } finally {
      if (mounted) setState(() => _sending = false);
      _scrollToBottom();
    }
  }

  @override
  Widget build(BuildContext context) {
    final colorScheme = Theme.of(context).colorScheme;
    return Scaffold(
      appBar: AppBar(
        title: const Text('Flutter + Gemini'),
        backgroundColor: colorScheme.primaryContainer,
        actions: [
          IconButton(
            icon: const Icon(Icons.delete_sweep_outlined),
            tooltip: 'Limpiar conversación',
            onPressed: () => setState(() => _messages.clear()),
          ),
        ],
      ),
      body: Column(
        children: [
          Expanded(
            child: _messages.isEmpty
                ? const Center(
                    child: Text(
                      'Pregunta algo sobre Flutter',
                      style: TextStyle(color: Colors.grey),
                    ),
                  )
                : ListView.builder(
                    controller: _scrollCtrl,
                    padding: const EdgeInsets.all(16),
                    itemCount: _messages.length,
                    itemBuilder: (context, index) {
                      return _ChatBubble(message: _messages[index]);
                    },
                  ),
          ),
          _InputBar(
            controller: _inputCtrl,
            sending: _sending,
            onSend: _sendMessage,
          ),
        ],
      ),
    );
  }
}

// ── Burbuja de chat ───────────────────────────────────────────────────────────
class _ChatBubble extends StatelessWidget {
  final ChatMessage message;

  const _ChatBubble({required this.message});

  @override
  Widget build(BuildContext context) {
    final colorScheme = Theme.of(context).colorScheme;
    final isUser = message.isUser;

    return Align(
      alignment: isUser ? Alignment.centerRight : Alignment.centerLeft,
      child: Container(
        margin: const EdgeInsets.only(bottom: 10),
        padding: const EdgeInsets.symmetric(horizontal: 14, vertical: 10),
        constraints: BoxConstraints(
          maxWidth: MediaQuery.of(context).size.width * 0.78,
        ),
        decoration: BoxDecoration(
          color: isUser
              ? colorScheme.primaryContainer
              : colorScheme.surfaceContainerHighest,
          borderRadius: BorderRadius.only(
            topLeft: const Radius.circular(16),
            topRight: const Radius.circular(16),
            bottomLeft: Radius.circular(isUser ? 16 : 4),
            bottomRight: Radius.circular(isUser ? 4 : 16),
          ),
        ),
        child: message.isLoading
            ? const SizedBox(
                width: 20,
                height: 20,
                child: CircularProgressIndicator(strokeWidth: 2),
              )
            : SelectableText(message.text),
      ),
    );
  }
}

// ── Barra de entrada ──────────────────────────────────────────────────────────
class _InputBar extends StatelessWidget {
  final TextEditingController controller;
  final bool sending;
  final VoidCallback onSend;

  const _InputBar({
    required this.controller,
    required this.sending,
    required this.onSend,
  });

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Padding(
        padding: const EdgeInsets.fromLTRB(12, 4, 12, 8),
        child: Row(
          children: [
            Expanded(
              child: TextField(
                controller: controller,
                decoration: const InputDecoration(
                  hintText: 'Escribe un mensaje...',
                  border: OutlineInputBorder(),
                  isDense: true,
                  contentPadding:
                      EdgeInsets.symmetric(horizontal: 12, vertical: 10),
                ),
                onSubmitted: (_) => onSend(),
                textInputAction: TextInputAction.send,
                maxLines: null,
              ),
            ),
            const SizedBox(width: 8),
            IconButton.filled(
              onPressed: sending ? null : onSend,
              icon: sending
                  ? const SizedBox(
                      width: 18,
                      height: 18,
                      child: CircularProgressIndicator(strokeWidth: 2),
                    )
                  : const Icon(Icons.send),
            ),
          ],
        ),
      ),
    );
  }
}