The operation between viewWillAppear and viewDidAppear?

I change the navigationBar's height in the method viewWillAppear In first time, it works and the bar‘s height isn't changed. But when I push again, the bar's height change to 44

if I change the bar's height in viewWillAppear and viewDidAppear, it works and isn't changed

So, Is not a system between these two methods to do what action？