add prototype of mod-provided APIs (#409)

7 files changed, 145 insertions, 41 deletions

diff --git a/src/SMAPI/Framework/IModMetadata.cs b/src/SMAPI/Framework/IModMetadata.cs
index c21734a7..c4be7daf 100644
--- a/src/SMAPI/Framework/IModMetadata.cs
+++ b/src/SMAPI/Framework/IModMetadata.cs
@@ -30,6 +30,9 @@ namespace StardewModdingAPI.Framework
         /// <summary>The mod instance (if it was loaded).</summary>
         IMod Mod { get; }
 
+        /// <summary>The mod-provided API (if any).</summary>
+        IModProvidedApi Api { get; }
+
 
         /*********
         ** Public methods
@@ -42,6 +45,7 @@ namespace StardewModdingAPI.Framework
 
         /// <summary>Set the mod instance.</summary>
         /// <param name="mod">The mod instance to set.</param>
-        IModMetadata SetMod(IMod mod);
+        /// <param name="api">The mod-provided API (if any).</param>
+        IModMetadata SetMod(IMod mod, IModProvidedApi api);
     }
 }
diff --git a/src/SMAPI/Framework/ModHelpers/ModRegistryHelper.cs b/src/SMAPI/Framework/ModHelpers/ModRegistryHelper.cs
index 4e3f56de..340205f3 100644
--- a/src/SMAPI/Framework/ModHelpers/ModRegistryHelper.cs
+++ b/src/SMAPI/Framework/ModHelpers/ModRegistryHelper.cs
@@ -45,5 +45,11 @@ namespace StardewModdingAPI.Framework.ModHelpers
         {
             return this.Registry.Get(uniqueID) != null;
         }
+
+        /// <summary>Get the API provided by a mod, or <c>null</c> if it has none. This signature requires using the <see cref="IModHelper.Reflection"/> API to access the API's properties and methods.</summary>
+        public IModProvidedApi GetApi(string uniqueID)
+        {
+            return this.Registry.Get(uniqueID)?.Api;
+        }
     }
 }
diff --git a/src/SMAPI/Framework/ModLoading/ModMetadata.cs b/src/SMAPI/Framework/ModLoading/ModMetadata.cs
index 5055da75..2e5c27be 100644
--- a/src/SMAPI/Framework/ModLoading/ModMetadata.cs
+++ b/src/SMAPI/Framework/ModLoading/ModMetadata.cs
@@ -29,6 +29,9 @@ namespace StardewModdingAPI.Framework.ModLoading
         /// <summary>The mod instance (if it was loaded).</summary>
         public IMod Mod { get; private set; }
 
+        /// <summary>The mod-provided API (if any).</summary>
+        public IModProvidedApi Api { get; private set; }
+
 
         /*********
         ** Public methods
@@ -59,9 +62,11 @@ namespace StardewModdingAPI.Framework.ModLoading
 
         /// <summary>Set the mod instance.</summary>
         /// <param name="mod">The mod instance to set.</param>
-        public IModMetadata SetMod(IMod mod)
+        /// <param name="api">The mod-provided API (if any).</param>
+        public IModMetadata SetMod(IMod mod, IModProvidedApi api)
         {
             this.Mod = mod;
+            this.Api = api;
             return this;
         }
     }
diff --git a/src/SMAPI/IModProvidedApi.cs b/src/SMAPI/IModProvidedApi.cs
new file mode 100644
index 00000000..9884ca78
--- /dev/null
+++ b/src/SMAPI/IModProvidedApi.cs
@@ -0,0 +1,6 @@
+namespace StardewModdingAPI
+{
+    /// <summary>An API provided by a mod for other mods to use.</summary>
+    /// <remarks>This is a marker interface. Each mod can only have one implementation of <see cref="IModProvidedApi"/>.</remarks>
+    public interface IModProvidedApi { }
+}
diff --git a/src/SMAPI/IModRegistry.cs b/src/SMAPI/IModRegistry.cs
index 5ef3fd65..fd71d72a 100644
--- a/src/SMAPI/IModRegistry.cs
+++ b/src/SMAPI/IModRegistry.cs
@@ -1,4 +1,4 @@
-using System.Collections.Generic;
+using System.Collections.Generic;
 
 namespace StardewModdingAPI
 {
@@ -16,5 +16,9 @@ namespace StardewModdingAPI
         /// <summary>Get whether a mod has been loaded.</summary>
         /// <param name="uniqueID">The mod's unique ID.</param>
         bool IsLoaded(string uniqueID);
+
+        /// <summary>Get the API provided by a mod, or <c>null</c> if it has none. This signature requires using the <see cref="IModHelper.Reflection"/> API to access the API's properties and methods.</summary>
+        /// <param name="uniqueID">The mod's unique ID.</param>
+        IModProvidedApi GetApi(string uniqueID);
     }
-}
\ No newline at end of file
+}
diff --git a/src/SMAPI/Program.cs b/src/SMAPI/Program.cs
index bd4692e6..6330cc1a 100644
--- a/src/SMAPI/Program.cs
+++ b/src/SMAPI/Program.cs
@@ -696,55 +696,34 @@ namespace StardewModdingAPI
                         continue;
                     }
 
-                    // validate assembly
-                    try
-                    {
-                        int modEntries = modAssembly.DefinedTypes.Count(type => typeof(Mod).IsAssignableFrom(type) && !type.IsAbstract);
-                        if (modEntries == 0)
-                        {
-                            TrackSkip(metadata, $"its DLL has no '{nameof(Mod)}' subclass.");
-                            continue;
-                        }
-                        if (modEntries > 1)
-                        {
-                            TrackSkip(metadata, $"its DLL contains multiple '{nameof(Mod)}' subclasses.");
-                            continue;
-                        }
-                    }
-                    catch (Exception ex)
-                    {
-                        TrackSkip(metadata, $"its DLL couldn't be loaded:\n{ex.GetLogSummary()}");
-                        continue;
-                    }
-
                     // initialise mod
                     try
                     {
-                        // get implementation
-                        TypeInfo modEntryType = modAssembly.DefinedTypes.First(type => typeof(Mod).IsAssignableFrom(type) && !type.IsAbstract);
-                        Mod mod = (Mod)modAssembly.CreateInstance(modEntryType.ToString());
-                        if (mod == null)
+                        // init mod helpers
+                        IMonitor monitor = this.GetSecondaryMonitor(metadata.DisplayName);
+                        IModHelper modHelper;
                         {
-                            TrackSkip(metadata, "its entry class couldn't be instantiated.");
-                            continue;
-                        }
-
-                        // inject data
-                        {
-                            IMonitor monitor = this.GetSecondaryMonitor(metadata.DisplayName);
                             ICommandHelper commandHelper = new CommandHelper(manifest.UniqueID, metadata.DisplayName, this.CommandManager);
                             IContentHelper contentHelper = new ContentHelper(contentManager, metadata.DirectoryPath, manifest.UniqueID, metadata.DisplayName, monitor);
                             IReflectionHelper reflectionHelper = new ReflectionHelper(manifest.UniqueID, metadata.DisplayName, this.Reflection, this.DeprecationManager);
                             IModRegistry modRegistryHelper = new ModRegistryHelper(manifest.UniqueID, this.ModRegistry);
                             ITranslationHelper translationHelper = new TranslationHelper(manifest.UniqueID, manifest.Name, contentManager.GetLocale(), contentManager.GetCurrentLanguage());
-
-                            mod.ModManifest = manifest;
-                            mod.Helper = new ModHelper(manifest.UniqueID, metadata.DirectoryPath, jsonHelper, contentHelper, commandHelper, modRegistryHelper, reflectionHelper, translationHelper);
-                            mod.Monitor = monitor;
+                            modHelper = new ModHelper(manifest.UniqueID, metadata.DirectoryPath, jsonHelper, contentHelper, commandHelper, modRegistryHelper, reflectionHelper, translationHelper);
                         }
 
+                        // get mod instances
+                        if (!this.TryLoadModEntry(modAssembly, error => TrackSkip(metadata, error), out Mod mod))
+                            continue;
+                        if (this.TryLoadModProvidedApi(modAssembly, modHelper, monitor, error => this.Monitor.Log($"Failed loading {metadata.DisplayName}'s mod-provided API. Integrations may not work correctly. Error: {error}", LogLevel.Warn), out IModProvidedApi api))
+                            this.Monitor.Log($"   Found mod-provided API ({api.GetType().FullName}).", LogLevel.Trace);
+
+                        // init mod
+                        mod.ModManifest = manifest;
+                        mod.Helper = modHelper;
+                        mod.Monitor = monitor;
+
                         // track mod
-                        metadata.SetMod(mod);
+                        metadata.SetMod(mod, api);
                         this.ModRegistry.Add(metadata);
                     }
                     catch (Exception ex)
@@ -854,6 +833,105 @@ namespace StardewModdingAPI
             }
         }
 
+        /// <summary>Load a mod's entry class.</summary>
+        /// <param name="modAssembly">The mod assembly.</param>
+        /// <param name="onError">A callback invoked when loading fails.</param>
+        /// <param name="mod">The loaded instance.</param>
+        private bool TryLoadModEntry(Assembly modAssembly, Action<string> onError, out Mod mod)
+        {
+            mod = null;
+
+            // find type
+            TypeInfo[] modEntries = modAssembly.DefinedTypes.Where(type => typeof(Mod).IsAssignableFrom(type) && !type.IsAbstract).Take(2).ToArray();
+            if (modEntries.Length == 0)
+            {
+                onError($"its DLL has no '{nameof(Mod)}' subclass.");
+                return false;
+            }
+            if (modEntries.Length > 1)
+            {
+                onError($"its DLL contains multiple '{nameof(Mod)}' subclasses.");
+                return false;
+            }
+
+            // get implementation
+            mod = (Mod)modAssembly.CreateInstance(modEntries[0].ToString());
+            if (mod == null)
+            {
+                onError("its entry class couldn't be instantiated.");
+                return false;
+            }
+
+            return true;
+        }
+
+        /// <summary>Load a mod's <see cref="IModProvidedApi"/> implementation.</summary>
+        /// <param name="modAssembly">The mod assembly.</param>
+        /// <param name="modHelper">The mod's helper instance.</param>
+        /// <param name="monitor">The mod's monitor instance.</param>
+        /// <param name="onError">A callback invoked when loading fails.</param>
+        /// <param name="api">The loaded instance.</param>
+        private bool TryLoadModProvidedApi(Assembly modAssembly, IModHelper modHelper, IMonitor monitor, Action<string> onError, out IModProvidedApi api)
+        {
+            api = null;
+
+            // find type
+            TypeInfo[] apis = modAssembly.DefinedTypes.Where(type => typeof(IModProvidedApi).IsAssignableFrom(type) && !type.IsAbstract).Take(2).ToArray();
+            if (apis.Length == 0)
+                return false;
+            if (apis.Length > 1)
+            {
+                onError($"its DLL contains multiple '{nameof(IModProvidedApi)}' implementations.");
+                return false;
+            }
+
+            // get constructor
+            ConstructorInfo constructor = (
+                from constr in apis[0].GetConstructors()
+                let args = constr.GetParameters()
+                where
+                    !args.Any()
+                    || args.All(arg => typeof(IModHelper).IsAssignableFrom(arg.ParameterType) || typeof(IMonitor).IsAssignableFrom(arg.ParameterType))
+                orderby args.Length descending
+                select constr
+            ).FirstOrDefault();
+            if (constructor == null)
+            {
+                onError($"its {nameof(IModProvidedApi)} must have a constructor with zero arguments, or only arguments of type {nameof(IModHelper)} or {nameof(IMonitor)}.");
+                return false;
+            }
+
+            // construct instance
+            try
+            {
+                // prepare constructor args
+                ParameterInfo[] args = constructor.GetParameters();
+                object[] values = new object[args.Length];
+                for (int i = 0; i < args.Length; i++)
+                {
+                    if (typeof(IModHelper).IsAssignableFrom(args[i].ParameterType))
+                        values[i] = modHelper;
+                    else if (typeof(IMonitor).IsAssignableFrom(args[i].ParameterType))
+                        values[i] = monitor;
+                    else
+                    {
+                        // shouldn't happen
+                        onError($"its {nameof(IModProvidedApi)} instance's constructor has unexpected argument type {args[i].ParameterType.FullName}.");
+                        return false;
+                    }
+                }
+
+                // instantiate
+                api = (IModProvidedApi)constructor.Invoke(values);
+                return true;
+            }
+            catch (Exception ex)
+            {
+                onError($"its {nameof(IModProvidedApi)} couldn't be constructed: {ex.GetLogSummary()}");
+                return false;
+            }
+        }
+
         /// <summary>Reload translations for all mods.</summary>
         private void ReloadTranslations()
         {
diff --git a/src/SMAPI/StardewModdingAPI.csproj b/src/SMAPI/StardewModdingAPI.csproj
index 0db94843..579ed487 100644
--- a/src/SMAPI/StardewModdingAPI.csproj
+++ b/src/SMAPI/StardewModdingAPI.csproj
@@ -114,6 +114,7 @@
     <Compile Include="IReflectedField.cs" />
     <Compile Include="IReflectedMethod.cs" />
     <Compile Include="IReflectedProperty.cs" />
+    <Compile Include="IModProvidedApi.cs" />
     <Compile Include="Metadata\CoreAssets.cs" />
     <Compile Include="ContentSource.cs" />
     <Compile Include="Events\ContentEvents.cs" />