一些必须通过异步处理，又需要赋值的情况下，可以用使用action委托来操作。一个简单的例子：

public Texture2D texture2D;

void Start()
{
    StartCoroutine(LoadBundleTexAsync((value) => texture2D = value, filepath, filename));
}

IEnumerator LoadTextureAsync(Action<Texture2D> result, string filepath, string filename)
{
    string bundleName = string.Format("{0}/{1}.unity3d", filepath, filename);
    string path = Path.Combine(dir, bundleName);

    byte[] bytes = File.ReadAllBytes(path);
    AssetBundleCreateRequest request = AssetBundle.LoadFromMemoryAsync(bytes);
    yield return request;
    AssetBundle bundle = request.assetBundle;

    Texture2D texture2D = bundle.LoadAsset<Texture2D>(bundle.AllAssetNames()[0]);
    result(texture2D);

    //延迟到下一帧执行卸载
    yield return new WaitForEndOfFrame();
    bundle.Unload(false);
    bundle = null;
}