1 files changed, 101 insertions, 0 deletions
diff --git a/VNLib.Plugins.Extensions.Loading/AssemblyLoader.cs b/VNLib.Plugins.Extensions.Loading/AssemblyLoader.cs
new file mode 100644
index 0000000..9c09b3f
--- /dev/null
+++ b/VNLib.Plugins.Extensions.Loading/AssemblyLoader.cs
@@ -0,0 +1,101 @@
+using System;
+using System.Linq;
+using System.Threading;
+using System.Reflection;
+
+using McMaster.NETCore.Plugins;
+
+using VNLib.Utils;
+using System.Runtime.Loader;
+
+namespace VNLib.Plugins.Extensions.Loading
+{
+    /// <summary>
+    /// <para>
+    /// Represents a disposable assembly loader wrapper for 
+    /// exporting a signle type from a loaded assembly
+    /// </para>
+    /// <para>
+    /// If the loaded type implements <see cref="IDisposable"/> the 
+    /// dispose method is called when the loader is disposed
+    /// </para>
+    /// </summary>
+    /// <typeparam name="T">The exported type to manage</typeparam>
+    public class AssemblyLoader<T> : OpenResourceHandle<T>
+    {
+        private readonly PluginLoader _loader;
+        private readonly Type _typeInfo;
+        private readonly CancellationTokenRegistration _reg;
+        private readonly Lazy<T> _instance;
+
+        /// <summary>
+        /// The instance of the loaded type
+        /// </summary>
+        public override T Resource => _instance.Value;
+
+        private AssemblyLoader(PluginLoader loader, in CancellationToken unloadToken)
+        {
+            _typeInfo = typeof(T);
+            _loader = loader;
+            //Init lazy loader
+            _instance = new(LoadAndGetExportedType, LazyThreadSafetyMode.PublicationOnly);
+            //Register dispose
+            _reg = unloadToken.Register(Dispose);
+        }
+        /// <summary>
+        /// Loads the default assembly and gets the expected export type,
+        /// creates a new instance, and calls its parameterless constructor
+        /// </summary>
+        /// <returns>The desired type instance</returns>
+        /// <exception cref="EntryPointNotFoundException"></exception>
+        private T LoadAndGetExportedType()
+        {
+            //Load the assembly
+            Assembly asm = _loader.LoadDefaultAssembly();
+          
+            //See if the type is exported
+            Type exp = (from type in asm.GetExportedTypes()
+                        where _typeInfo.IsAssignableFrom(type)
+                        select type)
+                        .FirstOrDefault()
+                        ?? throw new EntryPointNotFoundException($"Imported assembly does not export type {_typeInfo.FullName}");
+            //Create instance
+            return (T)Activator.CreateInstance(exp)!;
+        }
+
+        ///<inheritdoc/>
+        protected override void Free()
+        {
+            //If the instance is disposable, call its dispose method on unload
+            if (_instance.IsValueCreated && _instance.Value is IDisposable)
+            {
+                (_instance.Value as IDisposable)?.Dispose();
+            }
+            _loader.Dispose();
+            _reg.Dispose();
+        }
+
+        /// <summary>
+        /// Creates a new assembly loader for the specified type and 
+        /// </summary>
+        /// <param name="assemblyName">The name of the assmbly within the current plugin directory</param>
+        /// <param name="unloadToken">The plugin unload token</param>
+        internal static AssemblyLoader<T> Load(string assemblyName, CancellationToken unloadToken)
+        {
+            PluginConfig conf = new(assemblyName)
+            {
+                IsUnloadable = true,
+                EnableHotReload = false,
+                PreferSharedTypes = true,
+                DefaultContext = AssemblyLoadContext.GetLoadContext(Assembly.GetExecutingAssembly()) ?? AssemblyLoadContext.Default
+            };
+            
+            PluginLoader loader = new(conf);
+            
+            //Add the assembly the type originaged from
+            conf.SharedAssemblies.Add(typeof(T).Assembly.GetName());
+
+            return new(loader, in unloadToken);
+        }
+    }
+}