[Flutter] Provider 상태에 따른 추가 액션(without addListener) 오류

 

main.dart

 

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

import 'app_provider.dart';
import 'success_page.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider<AppProvider>(
      create: (_) => AppProvider(),
      child: MaterialApp(
        title: 'addListener of ChangeNotifier',
        debugShowCheckedModeBanner: false,
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: const MyHomePage(),
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key}) : super(key: key);

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final GlobalKey<FormState> formKey = GlobalKey<FormState>();
  AutovalidateMode autovalidateMode = AutovalidateMode.disabled;
  String? searchTerm;

  void submit() {
    setState(() {
      // 폼이 summit 된후, 모든 폼 입력에 대해서 항상 validation 확인
      autovalidateMode = AutovalidateMode.always;
    });

    final form = formKey.currentState;

    if (form == null || !form.validate()) return;

    form.save();

    var result = context.read<AppProvider>().getResult(searchTerm!);
  }

  @override
  Widget build(BuildContext context) {
    final appState = context.watch<AppProvider>().state;

// build 에서 처리하면 문제가 발생한다. re-build 할때마다 else 문으로 계속 분기됨.
    if (appState == AppState.success) {
      WidgetsBinding.instance!.addPostFrameCallback((_) {
        Navigator.push(context, MaterialPageRoute(
          builder: (context) {
            return const SuccessPage();
          },
        ));
      });
    } else if (appState == AppState.error) {
      WidgetsBinding.instance!.addPostFrameCallback((_) {
        showDialog(
          context: context,
          builder: (context) {
            return const AlertDialog(
              content: Text('Something went wrong'),
            );
          },
        );
      });
    }

    return GestureDetector(
      onTap: () => FocusScope.of(context).unfocus(),
      child: Scaffold(
        body: Center(
          child: Padding(
            padding: const EdgeInsets.symmetric(horizontal: 30.0),
            child: Form(
              key: formKey,
              autovalidateMode: autovalidateMode,
              child: ListView(
                shrinkWrap: true,
                children: [
                  TextFormField(
                    autofocus: true,
                    decoration: const InputDecoration(
                      border: OutlineInputBorder(),
                      label: Text('Search'),
                      prefixIcon: Icon(Icons.search),
                    ),
                    validator: (String? value) {
                      if (value == null || value.trim().isEmpty) {
                        return 'Search term required';
                      }
                      return null;
                    },
                    onSaved: (String? value) {
                      searchTerm = value;
                    },
                  ),
                  const SizedBox(height: 20.0),
                  ElevatedButton(
                    child: Text(
                      appState == AppState.loading
                          ? 'Loading...'
                          : 'Get Result',
                      style: const TextStyle(fontSize: 24.0),
                    ),
                    onPressed: appState == AppState.loading ? null : submit,
                  ),
                ],
              ),
            ),
          ),
        ),
      ),
    );
  }
}

 

app_provider.dart

 

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

enum AppState {
  initial,
  loading,
  success,
  error,
}

class AppProvider with ChangeNotifier {
  AppState _state = AppState.initial;
  AppState get state => _state;

  Future<void> getResult(String searchTerm) async {
    _state = AppState.loading;
    notifyListeners();

    await Future.delayed(const Duration(seconds: 1));

    try {
      if (searchTerm == 'fail') {
        throw 'Something went wrong';
      }

      _state = AppState.success;
      notifyListeners();
    } catch (e) {
      _state = AppState.error;
      notifyListeners();
// 이걸 안해도 build 에서 AppState.error 를 확인하니, 동작하는거 처럼 보인다. 하지만 rebuild 시 오류 창을 무한 반복한다.
      // rethrow; 
    }
  }
}

 

success_page.dart

 

import 'package:flutter/material.dart';

class SuccessPage extends StatelessWidget {
  const SuccessPage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Success'),
      ),
      body: const Center(
        child: Text(
          'Success',
          style: TextStyle(fontSize: 48.0),
        ),
      ),
    );
  }
}

 

 

 

 

[참고자료] udemy - Flutter Provider Essential 코스 (Korean)