detect when window size changed

Question

I am making a web browser with gtk-rs. I am trying to do a check for fullscreen. I can get the screen size and compare that to the window size, if they are the same it is in fullscreen and I can hide the unnecessary widgets. Though this works, I can only get it to work when I bind it some key event (so when I press a key it will check if the window and screen sizes are the same again). I want to be able to get this to work by simply noticing the window size has changed.