Proper way to Handle Exceptions in Flutter

Today we will see how we can properly handle errors and exceptions in Flutter.

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For this example we will be doing a service call and handle exceptions related to that.

We will create a sample service here

 static const String url = 'https://jsonplaceholder.typicode.com/users';

 static Future<List<User>> getUsers() async {
    try {
      final response = await http.get(url);
      if (200 == response.statusCode) {
        List<User> users = parseUsers(response.body);
        return users;
      } else {
        return List<User>();
      }
    } catch (e) {
      throw Exception(e.message);
    }
  }

  static List<User> parseUsers(String responseBody) {
    final parsed = json.decode(responseBody).cast<Map<String, dynamic>>();
    return parsed.map<User>((json) => User.fromJson(json)).toList();
  }

// User.dart
class User {
  int id;
  String name;
  String email;

  User({this.id, this.name, this.email});

  factory User.fromJson(Map<String, dynamic> json) {
    return User(
      id: json['id'] as int,
      name: json['name'] as String,
      email: json['email'] as String,
    );
  }
}

In the above example we are catching all exceptions using a simple try catch block which is not suitable since there can be a variety of Exceptions in this scenario like a SocketException, HttpException or a FormatException.

So in that case how do we catch those exceptions separately and provide appropriate messages to the user.

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Now the beauty of dart is you can throw any custom object as Exception. So lets create some custom classes to throw for each exception above.

class NoInternetException {
  String message;
  NoInternetException(this.message);
}

class NoServiceFoundException {
  String message;
  NoServiceFoundException(this.message);
}

class InvalidFormatException {
  String message;
  InvalidFormatException(this.message);
}

class UnknownException {
  String message;
  UnknownException(this.message);
}

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Now we will modify our function to get the users above like below

import 'dart:io';

import 'package:http/http.dart' as http;
import 'dart:convert';
import 'User.dart';
import 'Exceptions.dart';

class Services {
  static const String url = 'https://jsonplaceholder.typicode.com/users';

  static Future<List<User>> getUsers() async {
    try {
      final response = await http.get(url);
      if (200 == response.statusCode) {
        List<User> users = parseUsers(response.body);
        return users;
        //throw Exception('Unknown Error');
      } else {
        return List<User>();
      }
    } on SocketException catch (e) {
      throw NoInternetException('No Internet');
    } on HttpException {
      throw NoServiceFoundException('No Service Found');
    } on FormatException {
      throw InvalidFormatException('Invalid Data Format');
    } catch (e) {
      throw UnknownException(e.message);
    }
  }

  static List<User> parseUsers(String responseBody) {
    final parsed = json.decode(responseBody).cast<Map<String, dynamic>>();
    return parsed.map<User>((json) => User.fromJson(json)).toList();
  }
}

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In the UI, we will catch the exception like this

 list() {
    return FutureBuilder(
      future: users,
      builder: (context, snapshot) {
        if (snapshot.hasData) {
          List<User> users = snapshot.data;
          if (users.isEmpty) {
            return showError('No Users');
          }
          return Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: users
                .map(
                  (user) => Padding(
                    padding: EdgeInsets.all(10.0),
                    child: Text(
                      user.name,
                      style: TextStyle(
                        fontSize: 20.0,
                      ),
                    ),
                  ),
                )
                .toList(),
          );
        }
        if (snapshot.hasError) {
          if (snapshot.error is NoInternetException) {
            NoInternetException noInternetException =
                snapshot.error as NoInternetException;
            return showError(noInternetException.message);
          }
          if (snapshot.error is NoServiceFoundException) {
            NoServiceFoundException noServiceFoundException =
                snapshot.error as NoServiceFoundException;
            return showError(noServiceFoundException.message);
          }
          if (snapshot.error is InvalidFormatException) {
            InvalidFormatException invalidFormatException =
                snapshot.error as InvalidFormatException;
            return showError(invalidFormatException.message);
          }
          UnknownException unknownException =
              snapshot.error as UnknownException;
          return showError(unknownException.message);
        }
        return CircularProgressIndicator();
      },
    );
  }

In the above code, we catch each exception accordingly and show the correct error.

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