import from UniRx and some modified.

Assets/UniRx.Async/Triggers/AsyncStartTrigger.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 // Missing XML comment for publicly visible type or member
+
+using UnityEngine;
+
+namespace UniRx.Async.Triggers
+{
+    [DisallowMultipleComponent]
+    public class AsyncStartTrigger : MonoBehaviour
+    {
+        bool awakeCalled = false;
+        bool called = false;
+        UniTaskCompletionSource promise;
+
+        void Awake()
+        {
+            awakeCalled = true;
+        }
+
+        void Start()
+        {
+            called = true;
+            promise?.TrySetResult();
+        }
+
+        public UniTask StartAsync()
+        {
+            if (called) return UniTask.CompletedTask;
+            if (!awakeCalled)
+            {
+                PlayerLoopHelper.AddAction(PlayerLoopTiming.Update, new AwakeMonitor(this));
+            }
+            return new UniTask(promise ?? (promise = new UniTaskCompletionSource()));
+        }
+
+        private void OnDestroy()
+        {
+            promise?.TrySetCanceled();
+        }
+
+        class AwakeMonitor : IPlayerLoopItem
+        {
+            readonly AsyncStartTrigger trigger;
+
+            public AwakeMonitor(AsyncStartTrigger trigger)
+            {
+                this.trigger = trigger;
+            }
+
+            public bool MoveNext()
+            {
+                if (trigger.awakeCalled) return false;
+                if (trigger == null)
+                {
+                    trigger.OnDestroy();
+                    return false;
+                }
+                return true;
+            }
+        }
+    }
+}
+
+#endif