iOS: sending messages across controllers

I have a menucell on a menuviewcontroller and a button in a mapviewcontroller.

When I click on the button in mapviewcontroller, I want to set the menucell contents to null.

How do I do this?

Read up on NSNotificationCenter. When the button is clicked in MapViewController you'll want to post a notification to MenuViewController. And don't forget to add a notification observer to MenuViewController to listen to the notification.