TechbloxModdingAPI/TechbloxModdingAPI/EcsObjectBase.cs

using System;
using System.Collections.Generic;
using System.Linq.Expressions;
using Svelto.DataStructures;
using Svelto.ECS;
using Svelto.ECS.Internal;
using TechbloxModdingAPI.Utility;

namespace TechbloxModdingAPI
{
    public abstract class EcsObjectBase
    {
        public abstract EGID Id { get; } //Abstract to support the 'place' Block constructor

        private static readonly Dictionary<Type, WeakDictionary<EGID, EcsObjectBase>> _instances =
            new Dictionary<Type, WeakDictionary<EGID, EcsObjectBase>>();

        private static readonly WeakDictionary<EGID, EcsObjectBase> _noInstance =
            new WeakDictionary<EGID, EcsObjectBase>();

        internal static WeakDictionary<EGID, EcsObjectBase> GetInstances(Type type)
        {
            return _instances.TryGetValue(type, out var dict) ? dict : null;
        }

        /// <summary>
        /// Returns a cached instance if there's an actively used instance of the object already.
        /// Objects still get garbage collected and then they will be removed from the cache.
        /// </summary>
        /// <param name="egid">The EGID of the entity</param>
        /// <param name="constructor">The constructor to construct the object</param>
        /// <typeparam name="T">The object type</typeparam>
        /// <returns></returns>
        internal static T GetInstance<T>(EGID egid, Func<EGID, T> constructor, Type type = null) where T : EcsObjectBase
        {
            var instances = GetInstances(type ?? typeof(T));
            if (instances == null || !instances.TryGetValue(egid, out var instance))
                return constructor(egid); // It will be added by the constructor
            return (T)instance;
        }

        protected EcsObjectBase()
        {
            if (!_instances.TryGetValue(GetType(), out var dict))
            {
                dict = new WeakDictionary<EGID, EcsObjectBase>();
                _instances.Add(GetType(), dict);
            }

            // ReSharper disable VirtualMemberCallInConstructor
            // The ID should not depend on the constructor
            if (!dict.ContainsKey(Id)) // Multiple instances may be created
                dict.Add(Id, this);
            // ReSharper enable VirtualMemberCallInConstructor
        }

        #region ECS initializer stuff

        protected internal EcsInitData InitData;

        /// <summary>
        /// Holds information needed to construct a component initializer
        /// </summary>
        protected internal struct EcsInitData
        {
            private FasterDictionary<RefWrapperType, ITypeSafeDictionary> group;
            private EntityReference reference;

            public static implicit operator EcsInitData(EntityInitializer initializer) => new EcsInitData
                { group = GetInitGroup(initializer), reference = initializer.reference };

            public EntityInitializer Initializer(EGID id) => new EntityInitializer(id, group, reference);
            public bool Valid => group != null;
        }

        private delegate FasterDictionary<RefWrapperType, ITypeSafeDictionary> GetInitGroupFunc(
            EntityInitializer initializer);

        /// <summary>
        /// Accesses the group field of the initializer
        /// </summary>
        private static GetInitGroupFunc GetInitGroup = CreateAccessor<GetInitGroupFunc>("_group");

        //https://stackoverflow.com/questions/55878525/unit-testing-ref-structs-with-private-fields-via-reflection
        private static TDelegate CreateAccessor<TDelegate>(string memberName) where TDelegate : Delegate
        {
            var invokeMethod = typeof(TDelegate).GetMethod("Invoke");
            if (invokeMethod == null)
                throw new InvalidOperationException($"{typeof(TDelegate)} signature could not be determined.");

            var delegateParameters = invokeMethod.GetParameters();
            if (delegateParameters.Length != 1)
                throw new InvalidOperationException("Delegate must have a single parameter.");

            var paramType = delegateParameters[0].ParameterType;

            var objParam = Expression.Parameter(paramType, "obj");
            var memberExpr = Expression.PropertyOrField(objParam, memberName);
            Expression returnExpr = memberExpr;
            if (invokeMethod.ReturnType != memberExpr.Type)
                returnExpr = Expression.ConvertChecked(memberExpr, invokeMethod.ReturnType);

            var lambda =
                Expression.Lambda<TDelegate>(returnExpr, $"Access{paramType.Name}_{memberName}", new[] { objParam });
            return lambda.Compile();
        }

        #endregion
    }
}