使用 Result 对象进行错误处理

dart

class ApiClientService {
  // ···
​
  Future<UserProfile> getUserProfile() async {
    try {
      final request = await client.get(_host, _port, '/user');
      final response = await request.close();
      if (response.statusCode == 200) {
        final stringData = await response.transform(utf8.decoder).join();
        return UserProfile.fromJson(jsonDecode(stringData));
      } else {
        throw const HttpException('Invalid response');
      }
    } finally {
      client.close();
    }
  }
}

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dart

class UserProfileRepository {
  // ···
​
  Future<UserProfile> getUserProfile() async {
    return await _apiClientService.getUserProfile();
  }
}

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dart

class UserProfileViewModel extends ChangeNotifier {
  // ···
​
  Future<void> load() async {
    try {
      _userProfile = await userProfileRepository.getUserProfile();
      notifyListeners();
    } on Exception catch (exception) {
      // handle exception
    }
  }
}

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dart

class UserProfileViewModel extends ChangeNotifier {
  // ···
​
  Future<void> load() async {
    _userProfile = await userProfileRepository.getUserProfile();
    notifyListeners();
  }
}

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dart

/// Utility class that simplifies handling errors.
///
/// Return a [Result] from a function to indicate success or failure.
///
/// A [Result] is either an [Ok] with a value of type [T]
/// or an [Error] with an [Exception].
///
/// Use [Result.ok] to create a successful result with a value of type [T].
/// Use [Result.error] to create an error result with an [Exception].
sealed class Result<T> {
  const Result();
​
  /// Creates an instance of Result containing a value
  factory Result.ok(T value) => Ok(value);
​
  /// Create an instance of Result containing an error
  factory Result.error(Exception error) => Error(error);
}
​
/// Subclass of Result for values
final class Ok<T> extends Result<T> {
  const Ok(this.value);
​
  /// Returned value in result
  final T value;
}
​
/// Subclass of Result for errors
final class Error<T> extends Result<T> {
  const Error(this.error);
​
  /// Returned error in result
  final Exception error;
}

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dart

class ApiClientService {
  // ···
​
  Future<Result<UserProfile>> getUserProfile() async {
    // ···
  }
}

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dart

class ApiClientService {
  // ···
​
  Future<Result<UserProfile>> getUserProfile() async {
    try {
      final request = await client.get(_host, _port, '/user');
      final response = await request.close();
      if (response.statusCode == 200) {
        final stringData = await response.transform(utf8.decoder).join();
        return Result.ok(UserProfile.fromJson(jsonDecode(stringData)));
      } else {
        return const Result.error(HttpException('Invalid response'));
      }
    } on Exception catch (exception) {
      return Result.error(exception);
    } finally {
      client.close();
    }
  }
}

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dart

Future<Result<UserProfile>> getUserProfile() async {
  return await _apiClientService.getUserProfile();
}

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dart

class UserProfileViewModel extends ChangeNotifier {
  // ···
​
  UserProfile? userProfile;
​
  Exception? error;
​
  Future<void> load() async {
    final result = await userProfileRepository.getUserProfile();
    switch (result) {
      case Ok<UserProfile>():
        userProfile = result.value;
      case Error<UserProfile>():
        error = result.error;
    }
    notifyListeners();
  }
}

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dart

class UserProfileRepository {
  // ···
​
  Future<UserProfile> getUserProfile() async {
    try {
      return await _apiClientService.getUserProfile();
    } catch (e) {
      try {
        return await _databaseService.createTemporaryUser();
      } catch (e) {
        throw Exception('Failed to get user profile');
      }
    }
  }
}

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dart

Future<Result<UserProfile>> getUserProfile() async {
  final apiResult = await _apiClientService.getUserProfile();
  if (apiResult is Ok) {
    return apiResult;
  }
​
  final databaseResult = await _databaseService.createTemporaryUser();
  if (databaseResult is Ok) {
    return databaseResult;
  }
​
  return Result.error(Exception('Failed to get user profile'));
}

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dart

/// Utility class that simplifies handling errors.
///
/// Return a [Result] from a function to indicate success or failure.
///
/// A [Result] is either an [Ok] with a value of type [T]
/// or an [Error] with an [Exception].
///
/// Use [Result.ok] to create a successful result with a value of type [T].
/// Use [Result.error] to create an error result with an [Exception].
///
/// Evaluate the result using a switch statement:
/// ```dart
/// switch (result) {
///   case Ok(): {
///     print(result.value);
///   }
///   case Error(): {
///     print(result.error);
///   }
/// }
/// ```
sealed class Result<T> {
  const Result();
​
  /// Creates a successful [Result], completed with the specified [value].
  const factory Result.ok(T value) = Ok._;
​
  /// Creates an error [Result], completed with the specified [error].
  const factory Result.error(Exception error) = Error._;
}
​
/// A successful [Result] with a returned [value].
final class Ok<T> extends Result<T> {
  const Ok._(this.value);
​
  /// The returned value of this result.
  final T value;
​
  @override
  String toString() => 'Result<$T>.ok($value)';
}
​
/// An error [Result] with a resulting [error].
final class Error<T> extends Result<T> {
  const Error._(this.error);
​
  /// The resulting error of this result.
  final Exception error;
​
  @override
  String toString() => 'Result<$T>.error($error)';
}

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