Raycasters

The Event SystemA way of sending events to objects in the application based on input, be it keyboard, mouse, touch, or custom input. The Event System consists of a few components that work together to send events. More info
See in Glossary needs a method for detecting where current input events need to be sent to, and this is provided by the Raycasters. Given a screen space position they will collect all potential targets, figure out if they are under the given position, and then return the object that is closest to the screen. There are a few types of Raycasters that are provided:

• Graphic Raycaster - Used for UI(User Interface) Allows a user to interact with your application. Unity currently supports three UI systems. More info
  See in Glossary elements, lives on a Canvas and searches within the canvas
• Physics 2D Raycaster - Used for 2D physics elements
• Physics Raycaster - Used for 3D physics elements

When a Raycaster is present and enabled in the sceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
See in Glossary it will be used by the Event System whenever a query is issued from an Input Module.

If multiple Raycasters are used then they will all have casting happen against them and the results will be sorted based on distance to the elements.