UGUI学习：抓取和放入替换图片

效果：

抓取的图片

注：

PointerEventData.pointerEnter
当前触发OnPointerEnter的对象,一般鼠标移动到ui上的时候会触发这个事件,对应的是OnPointerExit,鼠标移出的时候

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

[RequireComponent(typeof(Image))]
public class DragMe : MonoBehaviour, IBeginDragHandler, IDragHandler, IEndDragHandler
{
    private GameObject m_DraggingIcon; // 抓取的图片
    private RectTransform m_DraggingPlane;  //图片位置信息

    public void OnBeginDrag(PointerEventData eventData)
    {
        var canvas = FindInParents<Canvas>(gameObject);
        if (canvas == null)
            return;

        m_DraggingIcon = new GameObject("icon");
        m_DraggingIcon.transform.SetParent(canvas.transform, false);
        m_DraggingIcon.transform.SetAsLastSibling();


        var image = m_DraggingIcon.AddComponent<Image>();
        image.sprite = GetComponent<Image>().sprite;
        image.SetNativeSize();

        CanvasGroup group = m_DraggingIcon.AddComponent<CanvasGroup>();
        group.blocksRaycasts = false;  //不交互

        m_DraggingPlane = transform as RectTransform;
    }

    public void OnDrag(PointerEventData eventData)
    {
        if (m_DraggingIcon != null)
        {
            SetDraggedPosition(eventData);
        }
    }

    public void OnEndDrag(PointerEventData eventData)
    {
        if (m_DraggingIcon != null)
        {
            Destroy(m_DraggingIcon);
        }
    }

    public void SetDraggedPosition(PointerEventData data)
    {
        if (data.pointerEnter != null && data.pointerEnter.transform as RectTransform != null)
            m_DraggingPlane = data.pointerEnter.transform as RectTransform;
       
        var rt = m_DraggingIcon.transform as RectTransform;
        Vector3 pos;
        if (RectTransformUtility.ScreenPointToWorldPointInRectangle(FindInParents<Canvas>(gameObject).transform as RectTransform, data.position, data.pressEventCamera, out pos))
        {
            rt.position = pos;
            rt.rotation = m_DraggingPlane.rotation;
        }
    }

    static public T FindInParents<T>(GameObject go) where T : Component
    {
        if (go == null) return null;
        var comp = go.GetComponent<T>();

        if (comp != null)
            return comp;

        Transform t = go.transform.parent;
        while (t != null && comp == null)
        {
            comp = t.gameObject.GetComponent<T>();
            t = t.parent;
        }
        return comp;
    }

}

放入的图片

注：

1.PointerEventData .pointerDrag
当前触发 OnDrag事件的物体,设置为空的时候不再触发OnDrag和 OnEndDrag .

2.Unity Image.sprite 与 Image.overrideSprite 的区别

overrideSprite 用于修改图片，但是不会把原来的图片给消除掉，直接的效果是如果 overrideSprite 是 null 的话，会变回原本的图片。可以理解为 overrdieSprite 是覆盖在 sprite 上的一层图片。所以，若使用 overrideSprite 更换图片，接下来要更换也只能用overrideSprite再覆盖掉，除非把它置为 null，不然用 sprite 更换是显示不出来的。

3.选择Image组件中-type-simple的Preserve Aspect 确保图像保留其现有尺寸

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

public class DropMe : MonoBehaviour, IDropHandler, IPointerEnterHandler, IPointerExitHandler
{
    public Image containerImage;
    public Image receivingImage;
    private Color _normalColor;
    public Color highlightColor = Color.yellow;

    private void OnEnable()
    {
        if (containerImage != null)
            _normalColor = containerImage.color;
    }

    public void OnDrop(PointerEventData eventData)
    {
        containerImage.color = _normalColor;

        if (eventData.pointerDrag != null && receivingImage != null)
        {
            Image image = eventData.pointerDrag.GetComponent<Image>();
            receivingImage.sprite = image.sprite;
        }
    }

    public void OnPointerEnter(PointerEventData eventData)
    {
        if(eventData.pointerDrag != null && containerImage != null)
        containerImage.color = highlightColor;
    }

    public void OnPointerExit(PointerEventData eventData)
    {
        if (containerImage != null)
            containerImage.color = _normalColor;
    }



}