An easy-reuse UIViewController extension to support making screen space for the keyboard, with minimal boilerplate code.

To use in any UIViewController, include this extension (KeyboardFriendly.swift) in your project, then in each controller where you want this behavior, do the following:

1. Make sure the view is wrapped in a top-level UIScrollView, to allow the view to be moved.

2. Override the computed property:

   var keyboardFriendlyScrollView: UIScrollView?

   to return that scroll view. If this var returns nil (the default), keyboard-friendliness will not be activated.

3. In viewDidLoad, call self.activateKeyboardFriendliness().

4. In deinit, call self.deactivateKeyboardFriendliness().

   Alternatively, depending on the needs of the controller, put the activate and deactivate calls in viewDidAppear and viewDidDisappear, respectively. That may be a better choice if the controller is still responding to keyboard events inappropriately while it's offscreen.

5. Optional — To auto-scroll the view to show the field being edited: Override the computed property:

   var keyboardFriendlyKeepOnscreen: CGRect

   to return the rect area of the current field, or any other rect that you want to keep onscreen. Return CGRect.zero (a.k.a. CGRectZero) to suppress auto-scrolling.¹

   Note that the rect must be in the scroll view's coordinate system! Use UIView.convertRect:toView: or UIView.convertRect:fromView: if necessary.

   A simple, basic implementation will track the bounds of the first responder in a concrete instance variable, and have keyboardFriendlyKeepOnscreen return that variable's value:

   var firstResponderBounds: CGRect?
   override var keyboardFriendlyKeepOnscreen: CGRect {
       return self.firstResponderBounds ?? CGRect.zero
   	}

   Use UITextFieldDelegate to keep track of the first responder.

¹Why use CGRect.zero to mean "no scrolling" instead of returning an optional? Current Swift limitations (2.2): declarations in an extension can only be overridden in the main class if they're marked @objc, but the CGRect? type can't be represented in Objective-C code.