利用WWW类获取图片并且在unityUGUI的Image中显示

using System.Collections;
using System.Collections.Generic;
using System.IO;
using UnityEngine;
using UnityEngine.UI;
public class AsyncImageDownload : MonoBehaviour {

    public Sprite placeholder;

    private static AsyncImageDownload _instance = null;
    public static AsyncImageDownload GetInstance() { return Instance; }
    public static AsyncImageDownload Instance
    {
        get
        {
            if (_instance == null)
            {
                GameObject obj = new GameObject("AsyncImageDownload");
                _instance = obj.AddComponent<AsyncImageDownload>();
                DontDestroyOnLoad(obj);
                _instance.Init();
            }
            return _instance;
        }
    }

    public bool Init()
    {
        if (!Directory.Exists(Application.persistentDataPath + "/ImageCache/"))
        {
            Directory.CreateDirectory(Application.persistentDataPath + "/ImageCache/");
        }
        if (placeholder == null)
        {
            placeholder = Resources.Load("placeholder") as Sprite;
        }
        return true;

    }

    public void SetAsyncImage(string url, Image image)
    {
        //开始下载图片前，将UITexture的主图片设置为占位图
        image.sprite = placeholder;

        //判断是否是第一次加载这张图片
        if (!File.Exists(path + url.GetHashCode()))
        {
            //如果之前不存在缓存文件
            StartCoroutine(DownloadImage(url, image));
        }
        else
        {
            StartCoroutine(LoadLocalImage(url, image));
        }
    }

    IEnumerator DownloadImage(string url, Image image)
    {
        Debug.Log("downloading new image:" + path + url.GetHashCode());//url转换HD5作为名字
        WWW www = new WWW(url);
        yield return www;

        Texture2D tex2d = www.texture;
        //将图片保存至缓存路径
        byte[] pngData = tex2d.EncodeToPNG();
        File.WriteAllBytes(path + url.GetHashCode(), pngData);

        Sprite m_sprite = Sprite.Create(tex2d, new Rect(0, 0, tex2d.width, tex2d.height), new Vector2(0, 0));
        image.sprite = m_sprite;
    }

    IEnumerator LoadLocalImage(string url, Image image)
    {
        string filePath = "file:///" + path + url.GetHashCode();

        Debug.Log("getting local image:" + filePath);
        WWW www = new WWW(filePath);
        yield return www;

        Texture2D texture = www.texture;
        Sprite m_sprite = Sprite.Create(texture, new Rect(0, 0, texture.width, texture.height), new Vector2(0, 0));
        image.sprite = m_sprite;
    }

    public string path
    {
        get
        {
            //pc,ios //android :jar:file//
            return Application.persistentDataPath + "/ImageCache/";

        }
    }
}

原理就是先用一张placeholder来显示图片，等待图片加载，等加载完了之后替换placeholder，第二次加载网络图片时，先判断本地时候已经加载过图片，如果加载过就从本地获取图片，如果没有就去网络上加载。
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using UnityEngine;
using System.Collections;
using UnityEngine.UI;
public class GetSprite : MonoBehaviour
{
    //利用WWW类获取图片并且在unityUGUI的Image中显示
    public Image image;
    void Start()
    {
        string path = "http://pic.nipic.com/2007-11-09/2007119122519868_2.jpg";
        //StartCoroutine(LoadImage(path));

        AsyncImageDownload.Instance.SetAsyncImage(path, image);
    }
    //IEnumerator LoadImage(string path)
    //{
    //    WWW www = new WWW(path);
    //    yield return www;
    //    Texture2D texture = www.texture;
    //    Sprite sprites = Sprite.Create(texture, new Rect(0, 0, texture.width, texture.height), new Vector2(0.5f, 0.5f));
    //    image.sprite = sprites;
    //}

}