TechbloxModdingAPI/TechbloxModdingAPI/EcsObjectBase.cs

using System;
using System.Linq.Expressions;
using Svelto.DataStructures;
using Svelto.ECS;
using Svelto.ECS.Internal;
using TechbloxModdingAPI.Blocks;

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