一、

MonoBehaviour基类的成员方法 StartCoroutine开启异步任务是不支持静态方法中调用的，所以我们需要一个开启协程的工具类
CoroutineHelper.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class CoroutineHelper : MonoBehaviour
{
    private static CoroutineHelper m_instance;

    public static CoroutineHelper GetInstance()
    {
        if (m_instance == null)
        {
            GameObject obj = new GameObject("CoroutineHelper");
            GameObject.DontDestroyOnLoad(obj);

            m_instance = obj.AddComponent<CoroutineHelper>();
        }

        return m_instance;
    }

    public void Dispose()
    {
        m_instance = null;
        GameObject.Destroy(gameObject);
    }
}

二、

http请求的工具类
Http.cs

using UnityEngine;
using System;
using System.Collections;
using LuaInterface;

public class Http
{
    static Http m_instance;
    static public Http Instance
    {
        get
        {
            if (m_instance == null)
                m_instance = new Http();
            return m_instance;
        }
    }

    public static void Get(string url, Action<WWW> onReceive)
    {
        CoroutineHelper.GetInstance().StartCoroutine(Instance.EHttpGet(url, onReceive));
    }
    public static void Post(string url, string data, Action<WWW> onReceive)
    {
        CoroutineHelper.GetInstance().StartCoroutine(Instance.EHttpPost(url, data, onReceive));
    }

    public static void Post(string url, byte[] bytes, Action<WWW> onReceive)
    {
        CoroutineHelper.GetInstance().StartCoroutine(Instance.EHttpPost(url, bytes, onReceive));
    }

    public static void Post(string url, WWWForm wwwForm, Action<WWW> onReceive)
    {
        CoroutineHelper.GetInstance().StartCoroutine(Instance.EHttpPost(url, wwwForm, onReceive));
    }

    IEnumerator EHttpGet(string url, Action<WWW> onReceive)
    {
        using (WWW www = new WWW(HttpUtils.URLAntiCacheRandomizer(url)))
        {
            yield return www;
            if (!string.IsNullOrEmpty(www.error))
            {
                Debug.LogWarning(": url: " + www.url + "  error: " + www.error);
                onReceive(null);
            }
            else
            {
                onReceive(www);
            }
        }
    }


    IEnumerator EHttpPost(string url, string data, Action<WWW> onReceive)
    {
        using (WWW www = new WWW(HttpUtils.URLAntiCacheRandomizer(url), System.Text.Encoding.UTF8.GetBytes(data)))
        {
            yield return www;
            if (!string.IsNullOrEmpty(www.error))
            {
                Debug.LogWarning(": url: " + www.url + "  error: " + www.error);
                onReceive(null);
            }
            else
            {
                onReceive(www);
            }
        }
    }
    IEnumerator EHttpPost(string url, byte[] bytes, Action<WWW> onReceive)
    {
        using (WWW www = new WWW(HttpUtils.URLAntiCacheRandomizer(url), bytes))
        {
            yield return www;
            if (!string.IsNullOrEmpty(www.error))
            {
                Debug.LogWarning(": url: " + www.url + "  error: " + www.error);
                onReceive(null);
            }
            else
            {
                onReceive(www);
            }
        }
    }
    IEnumerator EHttpPost(string url, WWWForm wwwForm, Action<WWW> onReceive)
    {
        using (WWW www = new WWW(HttpUtils.URLAntiCacheRandomizer(url), wwwForm))
        {
            yield return www;
            if (!string.IsNullOrEmpty(www.error))
            {
                Debug.LogWarning(": url: " + www.url + "  error: " + www.error);
                onReceive(null);
            }
            else
            {
                onReceive(www);
            }
        }
    }
}

三、

加载图片，这里我们需要一个url和图片的唯一id，我一般是用rul的hashcode或者md5
CacheImage.cs

using UnityEngine;
using System.Collections;
using System.IO;
using System;
using UnityEngine.Events;

/// <summary>
/// 图片缓存
/// </summary>
public class CacheImage
{
    private static CacheImage instences = null;
    private string path = //Application.persistentDataPath;
#if UNITY_EDITOR || UNITY_STANDALONE_WIN
        Application.dataPath + "/StreamingAssets/Cache";
#elif UNITY_IPHONE || UNITY_ANDROID
        Application.persistentDataPath;
#else
        string.Empty;
#endif
    private string name = "{0}.png";

    private static UnityAction<Texture2D> cacheEvent;
    public static CacheImage Cache(UnityAction<Texture2D> callback)
    {
        cacheEvent = callback;
        if (instences == null)
        {
            instences = new CacheImage();
        }
        return instences;
    }
    public void DownLoad(string url, string identifyId)
    {
        LoadTexture(url, identifyId);
    }
    /// <summary>
    /// 判断是否本地有缓存
    /// </summary>
    /// <param name="url"></param>
    /// <returns></returns>
    private void LoadTexture(string url, string identifyId)
    {
        if (!string.IsNullOrEmpty(url))
        {
            if (!File.Exists(Path.Combine(path, string.Format(name, identifyId))))
            {
                LoadNetWorkTexture(url, identifyId);
            }
            else
            {
                LoadLocalTexture(url, identifyId);
            }
        }
    }
    /// <summary>
    /// 本地已缓存
    /// </summary>
    /// <param name="url"></param>
    /// <returns></returns>
    private void LoadLocalTexture(string url, string identifyId)
    {
        string filePath = "file:///" + Path.Combine(path, string.Format(name, identifyId));
        Http.Get(url, delegate(WWW www)
        {
            cacheEvent.Invoke(www.texture);
        });
    }
    /// <summary>
    /// 本地未缓存
    /// </summary>
    /// <param name="url"></param>
    /// <returns></returns>
    private void LoadNetWorkTexture(string url, string identifyId)
    {
         Http.Get(new Uri(url).AbsoluteUri,delegate(WWW www){
             if (!string.IsNullOrEmpty(www.error))
             {
                 Debug.Log(string.Format("Failed to load image: {0}, {1}", url, www.error));
             }
#if UNITY_EDITOR || UNITY_STANDALONE_WIN
             if (!Directory.Exists(path))
             {
                 Directory.CreateDirectory(path);//创建新路径
             }
#endif
             Texture2D image = www.texture;
             //将图片保存至缓存路径  
             byte[] pngData = image.EncodeToPNG();
             File.WriteAllBytes(Path.Combine(path, string.Format(name, identifyId)), pngData);
             cacheEvent.Invoke(www.texture);     
         });
  }
}

四、

调用

        public static void SetUrlTexture(GameObject go, string url)
        {
            CacheImage.Cache(delegate(Texture2D texture)
            {
                go.GetComponent<UITexture>().mainTexture = texture;
            }).DownLoad(url, url.GetHashCode().ToString());
        }