Creating a view to draw on it

Before we start with the next view controller, we are going to create a custom view to draw the current picture's state and allow the user to add some features to it.

Create a new file called BoardView.swift and start a new class with the same name. This class must inherit from UIView (not UIViewController), therefore it needs to import UIKit:

import UIKit

class BoardView:UIView {

How is it going to work? It's very easy. We have to store the elements that should be drawn. Every time we store a new element, we will have to call setNeedsDisplay to report that there is something new. drawRect should be called as well. These elements are going to be implemented afterward; the only detail we need to know about them right now is that they inherit from a class that will be called Element, which will have a method called draw. Place the following code to add the elements to the array:

    private var elements = [Element]()

    func addElement(element:BoardView.Element...