LayoutBuilderBase.cs

    public abstract class LayoutBuilderBase<T> : ApiContentBuilderBase<T> where T : IApiContent
    {
        private const string LayoutPropertyKey = "layout";

        protected readonly IEnumerable<IComponentResolver> ComponentResolvers;
        protected readonly IApiPublishedContentCache ApiPublishedContentCache;

        protected LayoutBuilderBase(
            IApiContentNameProvider apiContentNameProvider,
            IApiContentRouteBuilder apiContentRouteBuilder,
            IOutputExpansionStrategyAccessor outputExpansionStrategyAccessor,
            IEnumerable<IComponentResolver> componentResolvers,
            IApiPublishedContentCache apiPublishedContentCache)
            : base(apiContentNameProvider, apiContentRouteBuilder, outputExpansionStrategyAccessor)
        {
            ComponentResolvers = componentResolvers;
            ApiPublishedContentCache = apiPublishedContentCache;
        }

        public override T? Build(IPublishedContent content)
        {
            var currentItem = base.Build(content);
            if (currentItem == null)
            {
                return currentItem;
            }

            if (LayoutPropertyKey == null || currentItem.Properties[LayoutPropertyKey] == null)
            {
                return currentItem;
            }

            var currentLayout = currentItem.Properties[LayoutPropertyKey] as ApiBlockGridModel;
            if (currentLayout?.Items == null || !currentLayout.Items.Any())
            {
                return currentItem;
            }

            var currentAreas = new List<ApiBlockGridArea>(currentLayout.Items.First().Areas ?? Array.Empty<ApiBlockGridArea>());

            ResolvePreComponents(content, LayoutPropertyKey, ref currentAreas);
            ResolvePostComponents(content, LayoutPropertyKey, ref currentAreas);

            var currentLayoutItems = new List<ApiBlockGridItem>(currentLayout.Items);
            currentLayoutItems[0] = new ApiBlockGridItem(
                currentLayoutItems[0].Content,
                currentLayoutItems[0].Settings,
                currentLayoutItems[0].RowSpan,
                currentLayoutItems[0].ColumnSpan,
                currentLayoutItems[0].AreaGridColumns,
                currentAreas);

            var composedLayout = new ApiBlockGridModel(currentLayout.GridColumns, currentLayoutItems);
            currentItem.Properties[LayoutPropertyKey] = composedLayout;

            return currentItem;
        }

        private void ResolvePreComponents(IPublishedContent content, string layoutPropertyKey, ref List<ApiBlockGridArea> currentAreas)
        {
            foreach (var componentResolver in ComponentResolvers.Where(x => x.Order < 0).OrderByDescending(x => x.Order))
            {
                var partialPageAreas = GetPartialPageAreas(componentResolver, content, layoutPropertyKey);
                if (partialPageAreas == null)
                {
                    continue;
                }

                foreach (var area in partialPageAreas)
                {
                    var foundArea = currentAreas.FirstOrDefault(a => a.Alias == area.Alias);
                    if (foundArea != null)
                    {
                        var index = currentAreas.IndexOf(foundArea);
                        var tempItems = foundArea.Items.ToList();
                        tempItems.InsertRange(0, area.Items);
                        currentAreas[index] = new ApiBlockGridArea(foundArea.Alias, foundArea.RowSpan, foundArea.ColumnSpan, tempItems);
                    }
                    else
                    {
                        currentAreas.Insert(0, area);
                    }
                }
            }
        }

        private void ResolvePostComponents(IPublishedContent content, string layoutPropertyKey, ref List<ApiBlockGridArea> currentAreas)
        {
            foreach (var componentResolver in ComponentResolvers.Where(x => x.Order >= 0).OrderBy(x => x.Order))
            {
                var partialPageAreas = GetPartialPageAreas(componentResolver, content, layoutPropertyKey);
                if (partialPageAreas == null)
                {
                    continue;
                }

                foreach (var area in partialPageAreas)
                {
                    var foundArea = currentAreas.FirstOrDefault(a => a.Alias == area.Alias);
                    if (foundArea != null)
                    {
                        var index = currentAreas.IndexOf(foundArea);
                        var tempItems = foundArea.Items.ToList();
                        tempItems.AddRange(area.Items);
                        currentAreas[index] = new ApiBlockGridArea(foundArea.Alias, foundArea.RowSpan, foundArea.ColumnSpan, tempItems);
                    }
                    else
                    {
                        currentAreas.Add(area);
                    }
                }
            }
        }

        private IEnumerable<ApiBlockGridArea>? GetPartialPageAreas(IComponentResolver componentResolver, IPublishedContent content, string layoutPropertyKey)
        {
            var partialPage = base.Build(componentResolver.ResolveContent(content));
            if (partialPage == null)
            {
                return null;
            }

            var partialPageLayout = partialPage.Properties[layoutPropertyKey] as ApiBlockGridModel;
            var partialPageAreas = partialPageLayout?.Items?.First().Areas;
            if (partialPageAreas == null)
            {
                return null;
            }

            return partialPageAreas;
        }
    }