Sample IRenderingContentsResolver implementations.

ContextItemChildrenRenderingContentsResolver.cs

using Newtonsoft.Json.Linq;
using Sitecore;
using Sitecore.Data.Items;
using Sitecore.Diagnostics;
using Sitecore.LayoutService.Configuration;
using Sitecore.LayoutService.ItemRendering;
using Sitecore.Links;
using Sitecore.Mvc.Presentation;

namespace Nomad.Jss.CodeFirst.ItemRendering
{
    public class ContextItemChildrenRenderingContentsResolver : IRenderingContentsResolver
    {
        private UrlOptions _urlOptions;

        public virtual UrlOptions UrlOptions
        {
            get
            {
                if (_urlOptions != null)
                {
                    return _urlOptions;
                }

                _urlOptions = LinkManager.GetDefaultUrlOptions();
                _urlOptions.LanguageEmbedding = LanguageEmbedding.Never;
                _urlOptions.AlwaysIncludeServerUrl = false;
                _urlOptions.SiteResolving = false;
                return _urlOptions;
            }
            set => _urlOptions = value;
        }

        public virtual object ResolveContents(Rendering rendering, IRenderingConfiguration renderingConfig)
        {
            Assert.ArgumentNotNull(rendering, "rendering");
            Assert.ArgumentNotNull(renderingConfig, "renderingConfig");

            var contextItem = Context.Item;

            var itemArr = new JArray();
            using (new Sitecore.Configuration.SettingsSwitcher("Media.AlwaysIncludeServerUrl", "true"))
            {
                
                foreach (Item item in contextItem.GetChildren())
                {
                    var itemJson = JObject.Parse(renderingConfig.ItemSerializer.Serialize(item));
                    itemJson["ItemUrl"] = LinkManager.GetItemUrl(item, UrlOptions);
                    var resultJson = new JObject {["fields"] = itemJson};

                    itemArr.Add(resultJson);
                }
            }

            var contents = new JObject
            {
                ["items"] = itemArr
            };
            return contents;
        }
    }
}

DatasourceChildItemsRenderingContentsResolver.cs

using Newtonsoft.Json.Linq;
using Sitecore.Data.Items;
using Sitecore.Diagnostics;
using Sitecore.LayoutService.Configuration;
using Sitecore.LayoutService.ItemRendering;
using Sitecore.Mvc.Presentation;

namespace Nomad.Jss.CodeFirst.ItemRendering
{
    public class DatasourceChildItemsRenderingContentsResolver : IRenderingContentsResolver
    {
        public object ResolveContents(Rendering rendering, IRenderingConfiguration renderingConfig)
        {
            Assert.ArgumentNotNull(rendering, "rendering");
            Assert.ArgumentNotNull(renderingConfig, "renderingConfig");

            //don't use Rendering.Item, as we don't want the context item
            var datasource = !string.IsNullOrEmpty(rendering.DataSource)
                ? rendering.RenderingItem?.Database.GetItem(rendering.DataSource)
                : null;

            if (datasource == null || !datasource.HasChildren)
            {
                return null;
            }


            var itemArr = new JArray();
            using (new Sitecore.Configuration.SettingsSwitcher("Media.AlwaysIncludeServerUrl", "true"))
            {
                foreach (Item item in datasource.GetChildren())
                {
                    var itemJson = JObject.Parse(renderingConfig.ItemSerializer.Serialize(item));
                    var resultJson = new JObject { ["fields"] = itemJson };

                    itemArr.Add(resultJson);
                }
            }

            var contents = new JObject
            {
                ["items"] = itemArr
            };
            return contents;
        }
    }
}