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The EventSystem is a way of sending events to objects in the application based on input, be it keyboard, mouse, touch, or custom input. The EventSystem consists of a few components that work together to send events.
When you add an EventSystem component to a GameObject you will notice that it does not have much functionality exposed, this is because the EventSystem itself is designed as a manager and facilitator of communication between EventSystem modules.
The primary roles of the EventSystem are as follows:
An input module is where the main logic of how you want the EventSystem to behave lives, they are used for
Only one InputModule can be active in the EventSystem at a time, and they must be components on the same GameObject as the EventSystem component.
If you wish to write a custom input module it is recommended that you send events supported by existing UI components in Unity, but you are also able to extend and write your own events as detailed in the Messaging System documentation.
Raycasters are used for sending figuring out what the pointer is over, it is common for InputModules to use the Raycasters configured in the scene to calculate what the pointing device is over.
There are 3 provided Raycasters that exist by default:
If you have a 2d / 3d Raycaster configured in your scene it is easily possible to have non UI elements receive messages from the InputModule. Simply attach a script that implements one of the event interfaces.