In the next two chapters, you will create TouchTracker, an app that lets the user draw by touching the screen. In this chapter, you will create a view that draws lines in response to the user dragging across it (Figure 18.1). Using multitouch, the user will be able to draw more than one line at a time.
Figure 18.1 TouchTracker
As a subclass of UIResponder, a UIView can override four methods to handle the four distinct touch events:
one or more fingers touch the screen
func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?)
one or more fingers ...