import from UniRx and some modified.

Assets/UniRx.Async/Triggers/AsyncRectTransformTrigger.cs

@@ -0,0 +1,55 @@
+
+#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 System.Collections.Generic;
+using System.Threading;
+using UnityEngine;
+using UnityEngine.EventSystems;
+
+namespace UniRx.Async.Triggers
+{
+    [DisallowMultipleComponent]
+    public class AsyncRectTransformTrigger : AsyncTriggerBase
+    {
+        AsyncTriggerPromise<AsyncUnit> onRectTransformDimensionsChange;
+        AsyncTriggerPromiseDictionary<AsyncUnit> onRectTransformDimensionsChanges;
+        AsyncTriggerPromise<AsyncUnit> onRectTransformRemoved;
+        AsyncTriggerPromiseDictionary<AsyncUnit> onRectTransformRemoveds;
+
+
+        protected override IEnumerable<ICancelablePromise> GetPromises()
+        {
+            return Concat(onRectTransformDimensionsChange, onRectTransformDimensionsChanges, onRectTransformRemoved, onRectTransformRemoveds);
+        }
+
+
+        void OnRectTransformDimensionsChange()
+        {
+            TrySetResult(onRectTransformDimensionsChange, onRectTransformDimensionsChanges, AsyncUnit.Default);
+        }
+
+
+        public UniTask OnRectTransformDimensionsChangeAsync(CancellationToken cancellationToken = default(CancellationToken))
+        {
+            return GetOrAddPromise(ref onRectTransformDimensionsChange, ref onRectTransformDimensionsChanges, cancellationToken);
+        }
+
+
+        void OnRectTransformRemoved()
+        {
+            TrySetResult(onRectTransformRemoved, onRectTransformRemoveds, AsyncUnit.Default);
+        }
+
+
+        public UniTask OnRectTransformRemovedAsync(CancellationToken cancellationToken = default(CancellationToken))
+        {
+            return GetOrAddPromise(ref onRectTransformRemoved, ref onRectTransformRemoveds, cancellationToken);
+        }
+
+
+    }
+}
+
+#endif
+