HK_LongZhiMeng/Assets/Common/Interface/Scripts/DragPanel.cs

using UnityEngine;
using UnityEngine.EventSystems;

public class DragPanel : MonoBehaviour, IPointerDownHandler, IDragHandler {
	
	private Vector2 originalLocalPointerPosition;
	private Vector3 originalPanelLocalPosition;
	private RectTransform panelRectTransform;
	private RectTransform parentRectTransform;
	
	void Start () {
		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 (Input.GetMouseButton(0))
        {
            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;
	}
}