res/anim/fade_in_out.xml

<alpha xmlns:android="http://schemas.android.com/apk/res/android" 
       android:interpolator="@android:anim/accelerate_decelerate_interpolator"
       android:fromAlpha="1.0"
       android:toAlpha="0.0"
       android:duration="500"
	   android:repeatCount="-1"
       android:repeatMode="reverse"
       android:fillAfter="true"/>