1 files changed, 138 insertions, 0 deletions
diff --git a/src/SMAPI/Framework/Reflection/InterfaceProxyBuilder.cs b/src/SMAPI/Framework/Reflection/InterfaceProxyBuilder.cs
new file mode 100644
index 00000000..5abebc18
--- /dev/null
+++ b/src/SMAPI/Framework/Reflection/InterfaceProxyBuilder.cs
@@ -0,0 +1,138 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using System.Reflection.Emit;
+
+namespace StardewModdingAPI.Framework.Reflection
+{
+    /// <summary>Generates proxy classes to access mod APIs through an arbitrary interface.</summary>
+    internal class InterfaceProxyBuilder
+    {
+        /*********
+        ** Properties
+        *********/
+        /// <summary>The CLR module in which to create proxy classes.</summary>
+        private readonly ModuleBuilder ModuleBuilder;
+
+        /// <summary>The generated proxy types.</summary>
+        private readonly IDictionary<string, Type> GeneratedTypes = new Dictionary<string, Type>();
+
+
+        /*********
+        ** Public methods
+        *********/
+        /// <summary>Construct an instance.</summary>
+        public InterfaceProxyBuilder()
+        {
+            AssemblyBuilder assemblyBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(new AssemblyName($"StardewModdingAPI.Proxies, Version={this.GetType().Assembly.GetName().Version}, Culture=neutral"), AssemblyBuilderAccess.Run);
+            this.ModuleBuilder = assemblyBuilder.DefineDynamicModule("StardewModdingAPI.Proxies");
+        }
+
+        /// <summary>Create an API proxy.</summary>
+        /// <typeparam name="TInterface">The interface through which to access the API.</typeparam>
+        /// <param name="instance">The API instance to access.</param>
+        /// <param name="sourceModID">The unique ID of the mod consuming the API.</param>
+        /// <param name="targetModID">The unique ID of the mod providing the API.</param>
+        public TInterface CreateProxy<TInterface>(object instance, string sourceModID, string targetModID)
+            where TInterface : class
+        {
+            // validate
+            if (instance == null)
+                throw new InvalidOperationException("Can't proxy access to a null API.");
+            if (!typeof(TInterface).IsInterface)
+                throw new InvalidOperationException("The proxy type must be an interface, not a class.");
+
+            // get proxy type
+            Type targetType = instance.GetType();
+            string proxyTypeName = $"StardewModdingAPI.Proxies.From<{sourceModID}_{typeof(TInterface).FullName}>_To<{targetModID}_{targetType.FullName}>";
+            if (!this.GeneratedTypes.TryGetValue(proxyTypeName, out Type type))
+            {
+                type = this.CreateProxyType(proxyTypeName, typeof(TInterface), targetType);
+                this.GeneratedTypes[proxyTypeName] = type;
+            }
+
+            // create instance
+            ConstructorInfo constructor = type.GetConstructor(new[] { targetType });
+            if (constructor == null)
+                throw new InvalidOperationException($"Couldn't find the constructor for generated proxy type '{proxyTypeName}'."); // should never happen
+            return (TInterface)constructor.Invoke(new[] { instance });
+        }
+
+
+        /*********
+        ** Private methods
+        *********/
+        /// <summary>Define a class which proxies access to a target type through an interface.</summary>
+        /// <param name="proxyTypeName">The name of the proxy type to generate.</param>
+        /// <param name="interfaceType">The interface type through which to access the target.</param>
+        /// <param name="targetType">The target type to access.</param>
+        private Type CreateProxyType(string proxyTypeName, Type interfaceType, Type targetType)
+        {
+            // define proxy type
+            TypeBuilder proxyBuilder = this.ModuleBuilder.DefineType(proxyTypeName, TypeAttributes.Public | TypeAttributes.Class);
+            proxyBuilder.AddInterfaceImplementation(interfaceType);
+
+            // create field to store target instance
+            FieldBuilder field = proxyBuilder.DefineField("__Target", targetType, FieldAttributes.Private);
+
+            // create constructor which accepts target instance
+            {
+                ConstructorBuilder constructor = proxyBuilder.DefineConstructor(MethodAttributes.Public, CallingConventions.Standard | CallingConventions.HasThis, new[] { targetType });
+                ILGenerator il = constructor.GetILGenerator();
+
+                il.Emit(OpCodes.Ldarg_0); // this
+                // ReSharper disable once AssignNullToNotNullAttribute -- never null
+                il.Emit(OpCodes.Call, typeof(object).GetConstructor(new Type[0])); // call base constructor
+                il.Emit(OpCodes.Ldarg_0);      // this
+                il.Emit(OpCodes.Ldarg_1);      // load argument
+                il.Emit(OpCodes.Stfld, field); // set field to loaded argument
+                il.Emit(OpCodes.Ret);
+            }
+
+            // proxy methods
+            foreach (MethodInfo proxyMethod in interfaceType.GetMethods())
+            {
+                var targetMethod = targetType.GetMethod(proxyMethod.Name, proxyMethod.GetParameters().Select(a => a.ParameterType).ToArray());
+                if (targetMethod == null)
+                    throw new InvalidOperationException($"The {interfaceType.FullName} interface defines method {proxyMethod.Name} which doesn't exist in the API.");
+
+                this.ProxyMethod(proxyBuilder, targetMethod, field);
+            }
+
+            // create type
+            return proxyBuilder.CreateType();
+        }
+
+        /// <summary>Define a method which proxies access to a method on the target.</summary>
+        /// <param name="proxyBuilder">The proxy type being generated.</param>
+        /// <param name="target">The target method.</param>
+        /// <param name="instanceField">The proxy field containing the API instance.</param>
+        private void ProxyMethod(TypeBuilder proxyBuilder, MethodInfo target, FieldBuilder instanceField)
+        {
+            Type[] argTypes = target.GetParameters().Select(a => a.ParameterType).ToArray();
+
+            // create method
+            MethodBuilder methodBuilder = proxyBuilder.DefineMethod(target.Name, MethodAttributes.Public | MethodAttributes.Final | MethodAttributes.Virtual);
+            methodBuilder.SetParameters(argTypes);
+            methodBuilder.SetReturnType(target.ReturnType);
+
+            // create method body
+            {
+                ILGenerator il = methodBuilder.GetILGenerator();
+
+                // load target instance
+                il.Emit(OpCodes.Ldarg_0);
+                il.Emit(OpCodes.Ldfld, instanceField);
+
+                // invoke target method on instance
+                for (int i = 0; i < argTypes.Length; i++)
+                    il.Emit(OpCodes.Ldarg, i + 1);
+                il.Emit(OpCodes.Call, target);
+
+                // return result
+                il.Emit(OpCodes.Ret);
+            }
+        }
+    }
+}