对swift中单个对象的NSNotification。

I have two text fields, topTextField and bottomTextField, at the top and bottom of the screen, respectively. bottomTextField hides behind the virtual keyboard when it comes up so to fix that I'm using NSNotification to listen for the virtual keyboard and sliding the view up when that happens. However, the view slides up whenever the keyboard comes onto the screen, including when topTextField becomes first responder, effectively moving it off the screen. Here is the code:I have two text fields, topTextField and bottom