import from UniRx and some modified.

Assets/UniRx.Async/AsyncUnit.cs

@@ -0,0 +1,28 @@
+#if CSHARP_7_OR_LATER || (UNITY_2018_3_OR_NEWER && (NET_STANDARD_2_0 || NET_4_6))
+#pragma warning disable CS1591 // Missing XML comment for publicly visible type or 
+
+using System;
+
+namespace UniRx.Async
+{
+    public struct AsyncUnit : IEquatable<AsyncUnit>
+    {
+        public static readonly AsyncUnit Default = new AsyncUnit();
+
+        public override int GetHashCode()
+        {
+            return 0;
+        }
+
+        public bool Equals(AsyncUnit other)
+        {
+            return true;
+        }
+
+        public override string ToString()
+        {
+            return "()";
+        }
+    }
+}
+#endif