PointerCallbackExtensions.cs

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

public static class PointerCallbackExtensions {

	static void AddPointerEventCallback(this GameObject obj,EventTriggerType triggerType,UnityAction<BaseEventData> ev) {

		var eventTriggerEntry = obj.GetEventTriggerEntry (triggerType);

		eventTriggerEntry.callback.AddListener (ev);
	}

	public static void AddPointerEventCallback(this Graphic graphic,EventTriggerType triggerType,UnityAction<BaseEventData> ev) {
		graphic.gameObject.AddPointerEventCallback (triggerType, ev);
	}

	public static void AddPointerEventCallback(this Renderer renderer,EventTriggerType triggerType,UnityAction<BaseEventData> ev) {
		renderer.gameObject.AddPointerEventCallback (triggerType, ev);
	}

	static EventTrigger.Entry GetEventTriggerEntry(this GameObject obj,EventTriggerType triggerType) {
		
		EventTrigger trigger = obj.GetComponent<EventTrigger> ();
		if (trigger == null)
			trigger = obj.AddComponent<EventTrigger> ();

		var eventTriggerEntry = trigger.triggers.Find (entry => entry.eventID ==triggerType);

		if (eventTriggerEntry == null) {
			eventTriggerEntry = new EventTrigger.Entry ();
			eventTriggerEntry.eventID = triggerType;
			trigger.triggers.Add (eventTriggerEntry);
		}

		return eventTriggerEntry;
	}

	public static void RemovePointerEventCallback(this Graphic graphic,EventTriggerType triggerType,UnityAction<BaseEventData> ev) {
		RemovePointerEventCallback(graphic.gameObject,triggerType,ev);
	}

	public static void RemovePointerEventCallback(this Renderer renderer,EventTriggerType triggerType,UnityAction<BaseEventData> ev) {
		RemovePointerEventCallback(renderer.gameObject,triggerType,ev);
	}

	static void RemovePointerEventCallback(this GameObject gameObject,EventTriggerType triggerType,UnityAction<BaseEventData> ev) {
		var eventTriggerEntry = gameObject.GetEventTriggerEntry (triggerType);

		eventTriggerEntry.callback.RemoveListener (ev);
	}

	public static void RemovePointerEventCallbacks(this Graphic graphic,EventTriggerType triggerType) {
		RemovePointerEventCallbacks (graphic.gameObject, triggerType);
	}

	public static void RemovePointerEventCallbacks(this Renderer renderer,EventTriggerType triggerType) {
		RemovePointerEventCallbacks (renderer.gameObject, triggerType);
	}

	static void RemovePointerEventCallbacks(this GameObject gameObject,EventTriggerType triggerType) {
		var eventTriggerEntry = gameObject.GetEventTriggerEntry (triggerType);

		eventTriggerEntry.callback.RemoveAllListeners ();
	}
}

Usage.cs

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

public class Usage : MonoBehaviour {

	public Image Draggable;
  
	void OnEnable() {
		Draggable.AddPointerEventCallback (EventTriggerType.Drag, OnDrag);
	}

	void OnDisable() {
		Draggable.RemovePointerEventCallback (EventTriggerType.Drag, OnDrag);
	}

	void OnDrag(BaseEventData data) {
		PointerEventData pointerData = (PointerEventData)data;
		var previousPosition = Draggable.transform.position;
		var newPosition = Camera.main.ScreenToWorldPoint(pointerData.position);
		newPosition.z = previousPosition.z;
    
		Draggable.transform.position = newPosition;
	}
  
}