Torch/Torch.Server/ViewModels/DynamicViewModel.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Reflection.Emit;
using HarmonyLib;
using JetBrains.Annotations;

namespace Torch.Server.ViewModels;

public class DynamicViewModel<T>(T obj) : ViewModel
{
    private readonly T _obj = obj;
    
    public ViewModel Wrapper { get; } = CreateProxy(obj);
    
    public static implicit operator T(DynamicViewModel<T> obj) => obj._obj;

    private static ViewModel CreateProxy(T obj)
    {
        if (DynamicViewModel.Proxies.TryGetValue(typeof(T), out var proxy))
            return (ViewModel)Activator.CreateInstance(proxy, obj);

        var viewModelSetMethod = AccessTools.GetDeclaredMethods(typeof(ViewModel))
            .First(b => b.Name == "SetValue" && b.GetParameters()[0].ParameterType.IsByRef);

        DynamicViewModel.ModuleBuilder ??=
            AssemblyBuilder.DefineDynamicAssembly(new("Torch.Server.ViewModels.Generated"), AssemblyBuilderAccess.Run)
                .DefineDynamicModule("Torch.Server.ViewModels.Generated");

        var type = DynamicViewModel.ModuleBuilder.DefineType($"{typeof(T).FullName}ViewModel",
            TypeAttributes.Public | TypeAttributes.Sealed | TypeAttributes.Class, typeof(ViewModel));
        
        var instanceField = type.DefineField("_instance", typeof(T), FieldAttributes.Private | FieldAttributes.InitOnly);

        var ctor = type.DefineConstructor(MethodAttributes.Public, CallingConventions.HasThis, [typeof(T)]);
        {
            var il = ctor.GetILGenerator();
            
            il.Emit(OpCodes.Ldarg_0);
            il.Emit(OpCodes.Ldarg_1);
            il.Emit(OpCodes.Stfld, instanceField);
            il.Emit(OpCodes.Ret);
        }
        
        foreach (var field in typeof(T).GetFields(BindingFlags.Instance | BindingFlags.Public))
        {
            var prop = type.DefineProperty(field.Name, PropertyAttributes.None, field.FieldType, null);
            
            foreach (var customAttribute in field.GetCustomAttributesData())
            {
                var hasCustomArgs = customAttribute.NamedArguments?.Any() ?? false;
                var customArgsField = hasCustomArgs && customAttribute.NamedArguments[0].IsField;

                var constructorArgs = customAttribute.ConstructorArguments.Select(b => b.Value).ToArray();
                
                CustomAttributeBuilder attributeBuilder;
                if (!hasCustomArgs) 
                    attributeBuilder = new CustomAttributeBuilder(customAttribute.Constructor, constructorArgs);
                else if (customArgsField)
                    attributeBuilder = new CustomAttributeBuilder(customAttribute.Constructor, constructorArgs, 
                        customAttribute.NamedArguments.Select(b => (FieldInfo)b.MemberInfo).ToArray(),
                        customAttribute.NamedArguments.Select(b => b.TypedValue.Value).ToArray());
                else
                    attributeBuilder = new CustomAttributeBuilder(customAttribute.Constructor, constructorArgs, 
                        customAttribute.NamedArguments.Select(b => (PropertyInfo)b.MemberInfo).ToArray(),
                        customAttribute.NamedArguments.Select(b => b.TypedValue.Value).ToArray());
                
                prop.SetCustomAttribute(attributeBuilder);
            }
            
            var getMethod = type.DefineMethod($"get_{field.Name}", MethodAttributes.Public | MethodAttributes.HideBySig, field.FieldType, Type.EmptyTypes);
            {
                var il = getMethod.GetILGenerator();
                
                il.Emit(OpCodes.Ldarg_0);
                il.Emit(OpCodes.Ldfld, instanceField);
                il.Emit(OpCodes.Ldfld, field);
                il.Emit(OpCodes.Ret);
            }
            prop.SetGetMethod(getMethod);

            if (field.IsInitOnly) continue;
            
            var setMethod = type.DefineMethod($"set_{field.Name}", MethodAttributes.Public | MethodAttributes.HideBySig, typeof(void),
                    [field.FieldType]);
            {
                var il = setMethod.GetILGenerator();

                il.Emit(OpCodes.Ldarg_0);
                il.Emit(OpCodes.Dup);
                il.Emit(OpCodes.Ldfld, instanceField);
                il.Emit(OpCodes.Ldflda, field);
                il.Emit(OpCodes.Ldarg_1);
                il.Emit(OpCodes.Ldstr, field.Name);
                il.Emit(OpCodes.Callvirt, viewModelSetMethod.MakeGenericMethod(field.FieldType));
                il.Emit(OpCodes.Ret);
            }
                
            prop.SetSetMethod(setMethod);
        }
        
        proxy = type.CreateType();
        
        DynamicViewModel.Proxies[typeof(T)] = proxy;
        
        return (ViewModel)Activator.CreateInstance(proxy, obj);
    }
}

file static class DynamicViewModel
{
    public static readonly Dictionary<Type, Type> Proxies = [];
    [CanBeNull] public static ModuleBuilder ModuleBuilder;
}