CanvasRaycaster.cs

using UnityEngine;
using UnityEngine.EventSystems;
using UnityEngine.UI;
using System.Collections;
using System.Collections.Generic;

public class CanvasRaycaster : BaseRaycaster {

    Canvas m_Canvas = null;
    private Canvas canvas
    {
        get {
            if (m_Canvas != null)
                return m_Canvas;

            m_Canvas = GetComponent<Canvas> ();
            return m_Canvas;
        }
    }

    public override Camera eventCamera {
        get {
            return InputDeviceManager.instance.currentInputDevice.getEventCamera();
        }
    }

    public override void Raycast( PointerEventData data, List<RaycastResult> resultAppendList )
    {
        if (canvas == null)
            return;


        Camera cam = InputDeviceManager.instance.currentInputDevice.getEventCamera();
        Vector3 worldPos = canvas.transform.TransformPoint (new Vector3(data.position.x, data.position.y, 0 ));

        Vector3 screenPosition = cam.WorldToScreenPoint (worldPos);

        Rect fullCanvas = GetComponent<RectTransform> ().rect;

        if (data.position.x > fullCanvas.size.x || data.position.y > fullCanvas.size.y)
            return;

        List<Graphic> sortedGraphics = new List<Graphic> ();

        var foundGraphics = GraphicRegistry.GetGraphicsForCanvas (canvas);
        for (int i = 0; i < foundGraphics.Count; ++i) {
            Graphic graphic = foundGraphics [i];

            if (graphic.depth == -1 || !graphic.raycastTarget)
                continue;

            Vector2 localPos = positionFromCanvasSpaceToGraphicSpace (graphic, data.position);
            if (!graphic.rectTransform.rect.Contains (localPos))
                continue;

            sortedGraphics.Add (graphic);
        }

        sortedGraphics.Sort ((g1, g2) => g2.depth.CompareTo (g1.depth));


        for (int i = 0; i < sortedGraphics.Count; ++i) {
            Graphic graphic = sortedGraphics [i];
            var castResult = new RaycastResult {
                gameObject = graphic.gameObject,
                module = this,
                distance = i,
                screenPosition = new Vector2( screenPosition.x, screenPosition.y ),//positionFromCanvasSpaceToGraphicSpace (graphic, data.position),
                index = resultAppendList.Count,
                depth = graphic.depth,
                sortingLayer = canvas.sortingLayerID,
                sortingOrder = canvas.sortingOrder
            };
            resultAppendList.Add( castResult );
        }
    }

    public Vector2 positionFromCanvasSpaceToGraphicSpace( Graphic graphic, Vector2 pos )
    {
        // First, convert the position to world space:
        Vector3 worldPos = GetComponent<RectTransform> ().TransformPoint (pos.x, pos.y, 0);

        // Then inverse-transform it back to local space:
        Vector3 result = graphic.rectTransform.InverseTransformPoint( worldPos );
        return new Vector2 (result.x, result.y);
    }
}