/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * 
 */
import { GraphQLError } from '../../error/GraphQLError';
export function singleFieldOnlyMessage(name) {
  return (name ? "Subscription \"".concat(name, "\" ") : 'Anonymous Subscription ') + 'must select only one top level field.';
}
/**
 * Subscriptions must only include one field.
 *
 * A GraphQL subscription is valid only if it contains a single root field.
 */

export function SingleFieldSubscriptions(context) {
  return {
    OperationDefinition: function OperationDefinition(node) {
      if (node.operation === 'subscription') {
        if (node.selectionSet.selections.length !== 1) {
          context.reportError(new GraphQLError(singleFieldOnlyMessage(node.name && node.name.value), node.selectionSet.selections.slice(1)));
        }
      }
    }
  };
}