Nerfed/Nerfed.Editor/Systems/EditorHierarchyWindow.cs

using ImGuiNET;
using MoonTools.ECS;
using Nerfed.Runtime.Components;

namespace Nerfed.Editor.Systems
{
    internal class EditorHierarchyWindow : MoonTools.ECS.DebugSystem
    {
        private readonly Filter rootEntitiesWithTransformFilter;
        private readonly Filter rootEntitiesFilterBroken;
        private readonly Filter rootEntitiesFilter;

        public EditorHierarchyWindow(World world) : base(world)
        {
            rootEntitiesWithTransformFilter = FilterBuilder.Include<LocalTransform>().Exclude<Child>().Build();

            // TODO: this doesn't work.
            rootEntitiesFilterBroken = FilterBuilder.Exclude<Child>().Build();

            // Maybe the parent/child functions should add a root component when not being a child.
            rootEntitiesFilter = FilterBuilder.Include<Root>().Build(); 

            // Maybe instead of a root, if we need a component that is always on an entity and has some use we could create something like a VersionComponent which only hold an int.
            // The version would update each time something changes on the entity.
            // 
        }

        public override void Update(TimeSpan delta)
        {
            ImGui.Begin("Hierarchy");

            foreach (Entity entity in rootEntitiesWithTransformFilter.Entities)
            {
                DrawEntityAndChildren(entity);
            }

            ImGui.NewLine();

            foreach (Entity entity in rootEntitiesFilter.Entities)
            {
                DrawEntityAndChildren(entity);
            }

            ImGui.End();
        }
        
        private void DrawEntityAndChildren(in Entity entity, int depth = 0)
        {
            string label = new string(' ', depth);
            ImGui.Text($"{label}{entity.ID} | {GetTag(entity)}");

            depth++;
            ReverseSpanEnumerator<Entity> childEntities = World.InRelations<ChildParentRelation>(entity);
            foreach (Entity childEntity in childEntities)
            {
                DrawEntityAndChildren(childEntity, depth);
            }
        }
    }
}