Sitecore JSS GraphQL schema extension to output fields in JSS format

JssFieldsExtender.cs

using GraphQL.Types;
using Newtonsoft.Json;
using Sitecore.Configuration;
using Sitecore.Data.Items;
using Sitecore.Data.Fields;
using Sitecore.Diagnostics;
using Sitecore.LayoutService.Serialization.FieldSerializers;
using Sitecore.LayoutService.Serialization.ItemSerializers;
using Sitecore.LayoutService.Serialization.Pipelines.GetFieldSerializer;
using Sitecore.Services.GraphQL.Content.GraphTypes;
using Sitecore.Services.GraphQL.GraphTypes;
using Sitecore.Services.GraphQL.Schemas;
using System.Globalization;
using System.IO;
using System.Text;
using System.Linq;
using System.Collections.Generic;

public class JssFieldsExtender : SchemaExtender
  {
    private const string OwnFieldsAttributeName = "ownFields";

    public JssFieldsExtender(IGetFieldSerializerPipeline fieldSerializerPipeline)
    {
      ExtendTypes<ItemGraphType>(
        type => type.Field<JsonGraphType>(
          "jssFields", 
          "Gets the item fields in JSS format", 
          new QueryArguments(new QueryArgument<BooleanGraphType> { DefaultValue = false, Name = OwnFieldsAttributeName, Description = "Limits the fields to the fields on the template only or not" }), 
          context => Serialize(context, fieldSerializerPipeline), 
          null));

      ExtendTypes<ItemInterfaceGraphType>(
        type => type.Field<JsonGraphType>(
          "jssFields", 
          "Gets the item fields in JSS format", 
          null, 
          null, 
          null));
    }

    protected virtual string Serialize(
      ResolveFieldContext<Item> context,
      IGetFieldSerializerPipeline getFieldSerializerPipeline)
    {
      Item item = context.Source;
      bool ownFields = context.GetArgument(OwnFieldsAttributeName, false);
      StringBuilder result = new StringBuilder();
      IEnumerable<Field> fields;
      if(ownFields)
      {
        fields = item.Template.OwnFields.Select(f => item.Fields[f.ID]);
      }
      else
      {
        item.Fields.ReadAll();
        fields = item.Fields;
      }

      foreach (Field field in fields)
      {
        result.Append($"{SerializeField(field, getFieldSerializerPipeline)},");
      }

      // Strip last , if needed
      if(result.Length > 0)
      {
        result = result.Remove(result.Length - 1, 1);
      }

      return $"{{{result}}}";
    }

    protected virtual string SerializeField(
      Field field,
      IGetFieldSerializerPipeline getFieldSerializerPipeline)
    {
      using (new SettingsSwitcher("Media.AlwaysIncludeServerUrl", bool.TrueString))
      {
        GetFieldSerializerPipelineArgs args = new GetFieldSerializerPipelineArgs()
        {
          Field = field,
          ItemSerializer = new DefaultItemSerializer(getFieldSerializerPipeline)
        };
        IFieldSerializer result = getFieldSerializerPipeline.GetResult(args);
        Assert.IsNotNull(result, "fieldSerializer != null");
        StringWriter stringWriter = new StringWriter(new StringBuilder(256), CultureInfo.InvariantCulture);
        using (JsonTextWriter writer = new JsonTextWriter(stringWriter))
        {
          writer.Formatting = Formatting.None;
          result.EnableRenderedValues = Sitecore.Context.PageMode.IsExperienceEditorEditing;
          result.Serialize(field, writer);
        }

        return stringWriter.ToString();
      }
    }
  }