new actions:

ActionFadeIn.cs
ActionFadeOut.cs
ActionFadeTo.cs

src/ActionsInterval/ActionFadeBy.cs

@@ -22,11 +22,6 @@ class ActionFadeBy : ActionInterval
         return new ActionFadeBy(-delta, duration);
     }
 
-    public override void start()
-    {
-        base.start();
-    }
-
     public override void stepInterval(float dt)
     {
         float d = dt / duration;

src/ActionsInterval/ActionFadeIn.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+using UnityEngine;
+
+class ActionFadeIn : ActionFadeTo
+{
+
+    public ActionFadeIn(float tgtDuration)
+        : base(1, tgtDuration)
+    {
+    }
+
+    public override Action clone()
+    {
+        return new ActionFadeIn(duration);
+    }
+
+    public override Action reverse()
+    {
+        return new ActionFadeIn(duration);
+    }
+}

src/ActionsInterval/ActionFadeOut.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+using UnityEngine;
+
+class ActionFadeOut : ActionFadeTo
+{
+
+    public ActionFadeOut(float tgtDuration)
+        : base(0, tgtDuration)
+    {
+    }
+
+    public override Action clone()
+    {
+        return new ActionFadeOut(duration);
+    }
+
+    public override Action reverse()
+    {
+        return new ActionFadeOut(duration);
+    }
+}

src/ActionsInterval/ActionFadeTo.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Collections.Generic;
+using UnityEngine;
+
+class ActionFadeTo : ActionInterval
+{
+    public float value;
+    public float delta;
+
+    public ActionFadeTo(float tgtValue, float tgtDuration)
+        : base(tgtDuration)
+    {
+        value = tgtValue;
+    }
+
+    public override Action clone()
+    {
+        return new ActionFadeTo(value, duration);
+    }
+
+    public override void start()
+    {
+        base.start();
+        delta = value - target.gameObject.renderer.material.color.a;
+    }
+
+    public override void stepInterval(float dt)
+    {
+        float d = dt / duration;
+        Color tgtColor = target.gameObject.renderer.material.color;
+        tgtColor[3] += delta * d;
+        target.gameObject.renderer.material.color = tgtColor;
+    }
+}