Cocoa – Reading NSWindow resize event

I was wondering if anyone knows, how do I read when NSWindow is being resized?
Let's imagine I have a button in an empty window (other than the button), then user resizes the window from the bottom right corner, now I should make it so that the button also resizes when the window is being resized. I know how to resize button, and I know how to resize a window, and I know lots of stuff, but I don't know how to get notificated when ever the user resizes the window, any tips?