I did this once using a "flipping" technique.
+>pv selected is equal to 2
->sprite set width to lerp(.width,.orginalwidth-.orginalwidth, 1-0.5^timedelta)
+>pv selected is equal to 1
->sprite set width to lerp(.width,.orginalwidth, 1-0.5^timedelta)
+>sprite compare width >0 sprite set animation frame to 1
+>sprite compare width <0 sprite set animation frame to 2