SDWebImage 不要缓存，及时刷新图片。

这个可能要用到客户端的多次登录密码不正确，会出现验证码，验证码输入不正确后验证码要及时刷新。

[ImageView sd_setImageWithURL:[NSURL URLWithString:model.vcode_url] placeholderImage:nil options:SDWebImageCacheMemoryOnly | SDWebImageRefreshCached];

用这个API需要修改内部的一些代码：

@sourcebits-mahesh @tbrknt

go to SDWebImageManager.m file inside the downloadImageWithURL method from line 176 and change this

if (image && options & SDWebImageRefreshCached) {

// force progressive off if image already cached but forced refreshing

downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;

// ignore image read from NSURLCache if image if cached but force refreshing

downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;

}

to this:

if (image && options & SDWebImageRefreshCached) {

// force progressive off if image already cached but forced refreshing

downloaderOptions &= ~SDWebImageDownloaderProgressiveDownload;

// remove SDWebImageDownloaderUseNSURLCache flag

downloaderOptions &= ~SDWebImageDownloaderUseNSURLCache;

// ignore image read from NSURLCache if image is cached but force refreshing

downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;

}

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over...........