I have a naviframe and  a  genlist ,When  naviframe navigated to the genlist ,check the  “realized”  and  “unrealized” callback function , I have  found  that first the list  realized the top 9 items(0-8), and then unrealized them (0-8) ,and then realized  the 9 items .   Why  there is additional once time  realized and  unrealized process , Why not realized the top 9 items directly ??