RadialSlider.cs

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

public class RadialSlider : MonoBehaviour, IPointerEnterHandler, IPointerExitHandler, IPointerDownHandler, IPointerUpHandler
{
    bool isPointerDown = false;

    // Called when the pointer enters our GUI component.
    // Start tracking the mouse
    public void OnPointerEnter( PointerEventData eventData )
    {
        StartCoroutine( "TrackPointer" );
    }

    // Called when the pointer exits our GUI component.
    // Stop tracking the mouse
    public void OnPointerExit( PointerEventData eventData )
    {
        StopCoroutine( "TrackPointer" );
    }

    public void OnPointerDown( PointerEventData eventData )
    {
        isPointerDown = true;
        //Debug.Log("mousedown");
    }

    public void OnPointerUp( PointerEventData eventData )
    {
        isPointerDown = false;
        //Debug.Log("mousedown");
    }

    // mainloop
    IEnumerator TrackPointer()
    {
        var ray = GetComponentInParent<GraphicRaycaster>();
        var input = FindObjectOfType<StandaloneInputModule>();

        var text = GetComponentInChildren<Text>();

        if ( ray != null && input != null )
        {
            while ( Application.isPlaying )
            {

                // TODO: if mousebutton down
                if ( isPointerDown )
                {

                    Vector2 localPos; // Mouse position
                    RectTransformUtility.ScreenPointToLocalPointInRectangle( transform as RectTransform, Input.mousePosition, ray.eventCamera, out localPos );

                    // local pos is the mouse position.
                    float angle = ( Mathf.Atan2( -localPos.y, localPos.x ) * 180f / Mathf.PI + 180f ) / 360f;

                    GetComponent<Image>().fillAmount = angle;

                    GetComponent<Image>().color = Color.Lerp( Color.green, Color.red, angle );

                    text.text = ( ( int )( angle * 360f ) ).ToString();

                    //Debug.Log(localPos+" : "+angle);
                }

                yield return 0;
            }
        }
        else
            UnityEngine.Debug.LogWarning( "Could not find GraphicRaycaster and/or StandaloneInputModule" );
    }
}