EventSystem
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.
Overview
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:
- Manage which GameObject is considered selected
- Manage which InputModule is in use
- Manage Raycasting (if required)
- Updating all InputModules as required
Input Modules
An input module is where the main logic of how you want the EventSystem to behave lives, they are used for
- Handling Input
- Managing event state
- Sending events to scene objects.
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
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:
- GraphicRaycaster - Used for UI elements
- 2DPhysicsRaycaster - Used for 2D physics elements
- 3DPhysicsRaycaster - Used for 3D physics elements
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.
Supported Events
The Eventsystem supports a number of events, and they can be customised further in user custom user written InputModules.
The events that are supported by the StandaloneInputModule and TouchInputModule are provided by interface and can be implemented on a MonoBehaviour by implementing the interface. If you have a valid EventSystem configured the events will be called at the correct time.
- IPointerEnterHandler - OnPointerEnter - Called when a pointer enters the object
- IPointerExitHandler - OnPointerExit - Called when a pointer exits the object
- IPointerDownHandler - OnPointerDown - Called when a pointer is pressed on the object
- IPointerUpHandler - OnPointerUp - Called when a pointer is released (called on the original the pressed object)
- IPointerClickHandler - OnPointerClick - Called when a pointer is pressed and released on the same object
- IInitializePotentialDragHandler - OnInitializePotentialDrag - Called when a drag target is found, can be used to initialise values
- IBeginDragHandler - OnBeginDrag - Called on the drag object when dragging is about to begin
- IDragHandler - OnDrag - Called on the drag object when a drag is happening
- IEndDragHandler - OnEndDrag - Called on the drag object when a drag finishes
- IDropHandler - OnDrop - Called on the object where a drag finishes
- IScrollHandler - OnScroll - Called when a mouse wheel scrolls
- IUpdateSelectedHandler - OnUpdateSelected - Called on the selected object each tick
- ISelectHandler - OnSelect - Called when the object becomes the selected object
- IDeselectHandler - OnDeselect - Called on the selected object becomes deselected
- IMoveHandler - OnMove - Called when a move event occurs (left, right, up, down, ect)
- ISubmitHandler - OnSubmit - Called when the submit button is pressed
- ICancelHandler - OnCancel - Called when the cancel button is pressed