确定何时在屏幕上显示UITableView TableHeaderView或TableFooterView

I'm trying to determine when a tableHeaderView or tableFooterView is actually rendered on the screen. The problem I'm facing is I can only have 1 ad view rendered at a time. If I put it in the header and then the footer, it removes it from the header. So what I want to do is, when the footer shows up on the screen, move the ad view into the footer and when the header shows up on the screen, move the ad view into the header.I'm trying to determine when a tableHeaderView