import from UniRx and some modified.

Assets/UniRx.Async/IAwaiter.cs

@@ -0,0 +1,64 @@
+#if CSHARP_7_OR_LATER || (UNITY_2018_3_OR_NEWER && (NET_STANDARD_2_0 || NET_4_6))
+#pragma warning disable CS1591
+
+using System.Runtime.CompilerServices;
+
+namespace UniRx.Async
+{
+    public enum AwaiterStatus
+    {
+        /// <summary>The operation has not yet completed.</summary>
+        Pending = 0,
+        /// <summary>The operation completed successfully.</summary>
+        Succeeded = 1,
+        /// <summary>The operation completed with an error.</summary>
+        Faulted = 2,
+        /// <summary>The operation completed due to cancellation.</summary>
+        Canceled = 3
+    }
+
+    public interface IAwaiter : ICriticalNotifyCompletion
+    {
+        AwaiterStatus Status { get; }
+        bool IsCompleted { get; }
+        void GetResult();
+    }
+
+    public interface IAwaiter<out T> : IAwaiter
+    {
+        new T GetResult();
+    }
+
+    public static class AwaiterStatusExtensions
+    {
+        /// <summary>!= Pending.</summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static bool IsCompleted(this AwaiterStatus status)
+        {
+            return status != AwaiterStatus.Pending;
+        }
+
+        /// <summary>== Succeeded.</summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static bool IsCompletedSuccessfully(this AwaiterStatus status)
+        {
+            return status == AwaiterStatus.Succeeded;
+        }
+
+        /// <summary>== Canceled.</summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static bool IsCanceled(this AwaiterStatus status)
+        {
+            return status == AwaiterStatus.Canceled;
+        }
+
+        /// <summary>== Faulted.</summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static bool IsFaulted(this AwaiterStatus status)
+        {
+            return status == AwaiterStatus.Faulted;
+        }
+    }
+}
+
+#endif