riversong-code-showcase/Source/Riversong/Game/UI/Panels/WorldUI/WorldUITrackingSystem.cs

using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Pool;
using UnityEngine.UIElements;

namespace DanieleMarotta.RiversongCodeShowcase
{
    [Service(typeof(IWorldUIService))]
    [GameSystemGroup(typeof(UISystemGroup))]
    public class WorldUITrackingSystem : GameSystem, IUpdatable, IWorldUIService
    {
        [InjectService]
        private IScene _scene;

        private readonly Dictionary<VisualElement, TrackedElement> _trackedElements = new();

        public WorldUITrackingSystem(IServiceLocator serviceLocator) : base(serviceLocator)
        {
        }

        public void Register(VisualElement element, Transform target)
        {
            _trackedElements[element] = new TrackedElement { Target = target };
        }

        public void Unregister(VisualElement element)
        {
            _trackedElements.Remove(element);
        }

        public void SetWorldOffset(VisualElement element, Vector3 offset)
        {
            var trackedElement = _trackedElements[element];
            trackedElement.Offset = offset;
            _trackedElements[element] = trackedElement;
        }

        public void Update()
        {
            if (_trackedElements.Count == 0) return;

            using var invalidScope = ListPool<VisualElement>.Get(out var invalidElements);
            var camera = _scene.MainCamera;

            foreach (var (element, trackedElement) in _trackedElements)
            {
                var target = trackedElement.Target;
                if (element.panel == null || target == null)
                {
                    invalidElements.Add(element);
                    continue;
                }

                var screenPoint = camera.WorldToScreenPoint(target.position + trackedElement.Offset);
                var isVisible = screenPoint.x >= 0 && screenPoint.x <= Screen.width && screenPoint.y >= 0 && screenPoint.y <= Screen.height && screenPoint.z > 0;

                element.style.visibility = isVisible ? Visibility.Visible : Visibility.Hidden;
                if (isVisible) element.SetAbsoluteScreenPosition(screenPoint);
            }

            foreach (var invalidElement in invalidElements) _trackedElements.Remove(invalidElement);
        }

        private struct TrackedElement
        {
            public Transform Target;

            public Vector3 Offset;
        }
    }
}