The EventSystem is a powerful yet simple manager providing such capabilities as:
Cataloging all the input systems available
Monitoring the current input state
Maintaining the currently selected GameObject
Updating all the various input systems
Marshalling Raycast testing between input and screen objects (and not just UI GameObjects!)
We'll discuss what these mean in detail, shortly.
Note
It's worth noting, there can only ever be a single EventSystem manager in any scene, no matter how many you try to add. At the core of the EventSystem is a static instance that ensures there can be only one.
The EventSystem uses all the Event logic described in the following sections of this chapter and manages the state of the UI and even processes the events for selected objects (as described previously).
