Add AccessTools.AsyncMoveNext and the Async MethodType

Based on pardeike/Harmony#514

Harmony/Internal/PatchTools.cs

@@ -86,6 +86,11 @@ internal static MethodBase GetOriginalMethod(this HarmonyMethod attr)
 							return null;
 						return AccessTools.EnumeratorMoveNext(AccessTools.DeclaredMethod(attr.GetDeclaringType(),
 							attr.methodName, attr.argumentTypes));
+
+					case MethodType.Async:
+						if (attr.methodName is null)
+							return null;
+						return AccessTools.AsyncMoveNext(AccessTools.DeclaredMethod(attr.GetDeclaringType(), attr.methodName, attr.argumentTypes));
 				}
 			}
 			catch (AmbiguousMatchException ex)

Harmony/Public/Attributes.cs

@@ -21,7 +21,8 @@ public enum MethodType
 		StaticConstructor,
 		/// <summary>This is an enumerator (<see cref="IEnumerable{T}"/>, <see cref="IEnumerator{T}"/> or UniTask coroutine)</summary>
 		/// <remarks>This path will target the <see cref="IEnumerator.MoveNext"/> method that contains the actual enumerator code</remarks>
-		Enumerator
+		Enumerator,
+		Async
 	}
 
 	/// <summary>Specifies the type of argument</summary>

Harmony/Public/Harmony.cs

@@ -144,7 +144,7 @@ public void PatchAll(Assembly assembly)
 		{
 			AccessTools.GetTypesFromAssembly(assembly).Do(type => CreateClassProcessor(type).Patch());
 		}
-		
+
 		/// <summary>Searches the given type for Harmony annotation and uses them to create patches</summary>
 		/// <param name="type">The type to search</param>
 		///

Harmony/Tools/AccessTools.cs

@@ -542,6 +542,38 @@ public static MethodInfo EnumeratorMoveNext(MethodBase enumerator)
 			return moveNext;
 		}
 
+		private static readonly Type _stateMachineAttributeType = typeof(object).Assembly.GetType("System.Runtime.CompilerServices.AsyncStateMachineAttribute");
+		private static readonly MethodInfo _stateMachineTypeGetter = _stateMachineAttributeType?.GetProperty("StateMachineType").GetGetMethod();
+
+		/// <summary>Gets the <see cref="IAsyncStateMachine.MoveNext" /> method of an async method's state machine</summary>
+		/// <param name="method">Async method that creates the state machine internally</param>
+		/// <returns>The internal <see cref="IAsyncStateMachine.MoveNext" /> method of the async state machine or <b>null</b> if no valid async method is detected</returns>
+		public static MethodInfo AsyncMoveNext(MethodBase method)
+		{
+			if (method is null)
+			{
+				FileLog.Debug("AccessTools.AsyncMoveNext: method is null");
+				return null;
+			}
+
+			var asyncAttribute = method.GetCustomAttributes(false).FirstOrDefault(a => a.GetType() == _stateMachineAttributeType);
+			if (asyncAttribute == null)
+			{
+				FileLog.Debug($"AccessTools.AsyncMoveNext: Could not find AsyncStateMachine for {method.FullDescription()}");
+				return null;
+			}
+
+			var asyncStateMachineType = (Type)_stateMachineTypeGetter.Invoke(method, null);
+			var asyncMethodBody = DeclaredMethod(asyncStateMachineType, "MoveNext");
+			if (asyncMethodBody == null)
+			{
+				FileLog.Debug($"AccessTools.AsyncMoveNext: Could not find async method body for {method.FullDescription()}");
+				return null;
+			}
+
+			return asyncMethodBody;
+		}
+
 		/// <summary>Gets the names of all method that are declared in a type</summary>
 		/// <param name="type">The declaring class/type</param>
 		/// <returns>A list of method names</returns>