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";
    private const string FieldsAttributeName = "fields";

    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"
                },
                new QueryArgument<ListGraphType<StringGraphType>>
                {
                    DefaultValue = new List<string>(),
                    Name = FieldsAttributeName,
                    Description = "List of fields to display"
                }),
            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);
        List<string> fieldNames = context.GetArgument(FieldsAttributeName, new List<string>());
        StringBuilder result = new StringBuilder();
        IEnumerable<Field> fields;
        if (ownFields)
        {
            fields = item.Template.OwnFields.Select(f => item.Fields[f.ID]).ToList();
        }
        else
        {
            item.Fields.ReadAll();
            fields = item.Fields;
        }

        var filteredFieldsByName = fieldNames.Any()
            ? fields.Where(x => fieldNames.Contains(x.Name))
            : fields;

        foreach (Field field in filteredFieldsByName)
        {
            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();
        }
    }
}