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

//給空間添加監聽事件要實現的一些接口
public class MyDrag : MonoBehaviour, IPointerDownHandler, IDragHandler, IPointerUpHandler, 
 IEndDragHandler, IPointerEnterHandler, IPointerExitHandler {

 public RectTransform canvas; //得到canvas的ugui坐標
 private RectTransform imgRect; //得到圖片的ugui坐標
 Vector2 offset = new Vector3(); //用來得到鼠標和圖片的差值
 Vector3 imgReduceScale = new Vector3(0.8f, 0.8f, 1); //設置圖片縮放
 Vector3 imgNormalScale = new Vector3(1, 1, 1); //正常大小

 // Use this for initialization
 void Start () {
 imgRect = GetComponent<RectTransform>();

 }

 //當鼠標按下時調用 接口對應 IPointerDownHandler
 public void OnPointerDown(PointerEventData eventData)
 {
 Vector2 mouseDown = eventData.position; //記錄鼠標按下時的屏幕坐標
 Vector2 mouseUguiPos = new Vector2(); //定義一個接收返回的ugui坐標
 //RectTransformUtility.ScreenPointToLocalPointInRectangle()：把屏幕坐標轉化成ugui坐標
 //canvas：坐標要轉換到哪一個物體上，這里img父類是Canvas，我們就用Canvas
 //eventData.enterEventCamera：這個事件是由哪個攝像機執行的
 //out mouseUguiPos：返回轉換后的ugui坐標
 //isRect：方法返回一個bool值，判斷鼠標按下的點是否在要轉換的物體上
 bool isRect = RectTransformUtility.ScreenPointToLocalPointInRectangle(canvas, mouseDown, eventData.enterEventCamera, out mouseUguiPos);
 if (isRect) //如果在
 {
 //計算圖片中心和鼠標點的差值
 offset = imgRect.anchoredPosition - mouseUguiPos;
 }
 }

 //當鼠標拖動時調用 對應接口 IDragHandler
 public void OnDrag(PointerEventData eventData)
 {
 Vector2 mouseDrag = eventData.position; //當鼠標拖動時的屏幕坐標
 Vector2 uguiPos = new Vector2(); //用來接收轉換后的拖動坐標
 //和上面類似
 bool isRect = RectTransformUtility.ScreenPointToLocalPointInRectangle(canvas, mouseDrag, eventData.enterEventCamera, out uguiPos);

 if (isRect)
 {
 //設置圖片的ugui坐標與鼠標的ugui坐標保持不變
 imgRect.anchoredPosition = offset + uguiPos;
 }
 }

 //當鼠標抬起時調用 對應接口 IPointerUpHandler
 public void OnPointerUp(PointerEventData eventData)
 {
 offset = Vector2.zero;
 }

 //當鼠標結束拖動時調用 對應接口 IEndDragHandler
 public void OnEndDrag(PointerEventData eventData)
 {
 offset = Vector2.zero;
 }

 //當鼠標進入圖片時調用 對應接口 IPointerEnterHandler
 public void OnPointerEnter(PointerEventData eventData)
 {
 imgRect.localScale = imgReduceScale; //縮小圖片
 }

 //當鼠標退出圖片時調用 對應接口 IPointerExitHandler
 public void OnPointerExit(PointerEventData eventData)
 {
 imgRect.localScale = imgNormalScale; //回復圖片
 }
}