回傳資料的封裝

http_result.dart

/// A generic class that holds a value or an exception.
class RequestResult<T> {
  const RequestResult._();

  const factory RequestResult.success(T? data) = Success<T>;

  const factory RequestResult.error(Exception exception) = Error<T>;

  const factory RequestResult.nothing() = Nothing<T>;
}

/// A successful result.
class Success<T> extends RequestResult<T> {
  const Success(this.data) : super._();
  final T? data;
}

/// An error result.
class Error<T> extends RequestResult<T> {
  const Error(this.exception) : super._();
  final Exception exception;
}

class Nothing<T> extends RequestResult<T> {
  const Nothing() : super._();
}

extension ResultExtensions<T> on RequestResult<T> {
  /// Returns the data if this is a successful result, or the given fallback otherwise.
  T successOr(T fallback) {
    if (this is Success<T>) {
      return (this as Success<T>).data ?? fallback;
    } else {
      return fallback;
    }
  }

  bool get isError => this is Error;

  bool get isSuccess => this is Success;
}