ContentInstance.cs

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using System;
using System.Collections;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
 
namespace Terraria.ModLoader
{
    public static class ContentInstance
    {
        private class ContentEntry
        {
            private object instance;
            private List<object> instances;
            private Action<object, IEnumerable> staticUpdate;
 
            public void Register(object obj) {
                lock (this) {
                    if (instances != null) {
                        instances.Add(obj);
                    }
                    else if (instance != null) {
                        instances = new List<object> { instance, obj };
                        instance = null;
                    }
                    else {
                        instance = obj;
                    }
 
                    staticUpdate?.Invoke(instance, instances);
                }
            }
 
            public void Link(Action<object, IEnumerable> update) {
                lock (this) {
                    staticUpdate = update;
                    update(instance, instances);
                }
            }
 
            public void Clear() {
                lock (this) {
                    instance = null;
                    instances = null;
                    staticUpdate?.Invoke(instance, instances);
                }
            }
        }
 
        private static ConcurrentDictionary<Type, ContentEntry> contentByType = new ConcurrentDictionary<Type, ContentEntry>();
 
        private static ContentEntry Factory(Type t) => new ContentEntry();
 
        internal static void Link(Type t, Action<object, IEnumerable> update) => contentByType.GetOrAdd(t, Factory).Link(update);
 
        public static void Register(object obj) => contentByType.GetOrAdd(obj.GetType(), Factory).Register(obj);
 
        internal static void Clear() {
            foreach (var entry in contentByType)
                entry.Value.Clear();
        }
    }
 
    public static class ContentInstance<T> where T : class
    {
        public static T Instance { get; private set; }
        public static IReadOnlyList<T> Instances { get; private set; }
 
        static ContentInstance() {
            ContentInstance.Link(typeof(T), Update);
        }
 
        private static void Update(object instance, IEnumerable instances) {
            Instance = (T)instance;
            Instances = instances?.Cast<T>()?.ToArray();
        }
    }
}