Unity - Scripting API:

public static function Slider2D(id: int, handlePos: Vector3, offset: Vector3, handleDir: Vector3, slideDir1: Vector3, slideDir2: Vector3, handleSize: float, drawFunc: Handles.DrawCapFunction, snap: Vector2, drawHelper: bool = false): Vector3;

public static function Slider2D(handlePos: Vector3, handleDir: Vector3, slideDir1: Vector3, slideDir2: Vector3, handleSize: float, drawFunc: Handles.DrawCapFunction, snap: Vector2, drawHelper: bool = false): Vector3;

public static function Slider2D(id: int, handlePos: Vector3, handleDir: Vector3, slideDir1: Vector3, slideDir2: Vector3, handleSize: float, drawFunc: Handles.DrawCapFunction, snap: Vector2, drawHelper: bool = false): Vector3;

public static function Slider2D(handlePos: Vector3, handleDir: Vector3, slideDir1: Vector3, slideDir2: Vector3, handleSize: float, drawFunc: Handles.DrawCapFunction, snap: float, drawHelper: bool = false): Vector3;

Parameters

id	(optional) override the default ControlID for this Slider2D instance.
handlePos	The position of the current point.
offset	(optional) renders the Slider2D at handlePos, but treats the Slider2D's origin as handlePos + offset. Useful for Slider2D instances that are placed/rendered relative to another object or handle.
handleDir	The direction of the handle, only used for rendering of the handle.
slideDir1	The first direction of the sliding.
slideDir2	The second direction of the sliding.
handleSize	The size of the handle.
drawFunc	The function to call for doing the actual drawing - by default, it's Handles.ArrowCap, but any function that has the same signature can be used.
snap	(float or Vector2) set the snap increment (Pass a Vector2 to use separate snap increments in each dimension).
drawHelper	(default: false) render a rectangle around the handle when dragging.

Vector3 The new handle position

Note: Use HandleUtility.GetHandleSize where you might want to have constant screen-sized handles.

Slide a handle in a 2D plane.

This will draw a draggable handle on the screen. The handle is constrained to sliding on a plane.