DragPanel

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

public class DragPanel : MonoBehaviour, IPointerDownHandler, IDragHandler {

private Vector2 originalLocalPointerPosition;
private Vector3 originalPanelLocalPosition;
private RectTransform panelRectTransform;
private RectTransform parentRectTransform;

void Awake () {
panelRectTransform = transform.parent as RectTransform;
parentRectTransform = panelRectTransform.parent as RectTransform;
}

public void OnPointerDown (PointerEventData data) {
originalPanelLocalPosition = panelRectTransform.localPosition;
RectTransformUtility.ScreenPointToLocalPointInRectangle (parentRectTransform, data.position, data.pressEventCamera, out originalLocalPointerPosition);
}

public void OnDrag (PointerEventData data) {
if (panelRectTransform == null || parentRectTransform == null)
return;

Vector2 localPointerPosition;
if (RectTransformUtility.ScreenPointToLocalPointInRectangle (parentRectTransform, data.position, data.pressEventCamera, out localPointerPosition)) {
Vector3 offsetToOriginal = localPointerPosition - originalLocalPointerPosition;
panelRectTransform.localPosition = originalPanelLocalPosition + offsetToOriginal;
}

ClampToWindow ();
}

// Clamp panel to area of parent
void ClampToWindow () {
Vector3 pos = panelRectTransform.localPosition;

Vector3 minPosition = parentRectTransform.rect.min - panelRectTransform.rect.min;
Vector3 maxPosition = parentRectTransform.rect.max - panelRectTransform.rect.max;

pos.x = Mathf.Clamp (panelRectTransform.localPosition.x, minPosition.x, maxPosition.x);
pos.y = Mathf.Clamp (panelRectTransform.localPosition.y, minPosition.y, maxPosition.y);

panelRectTransform.localPosition = pos;
}
}