UIGestureRecognizer
The base class is abstract, we only actually use concrete subclasses.
1. Need to add a gesture recognizer to a UI View. (this is usually done by controller —> when should we do this?, but sometimes we can do this in the storyboard)
2. Provide a method to “handle” the gesture. Callback given to gesture. (Provided by the UIView or the Controller, depends). If gesture modifies model, then controller. If gesture only modifies UI, then the callback is in the view.
Add this in didSet for an outlet of a view.
“action:” means we are invoking a method with an argument.












