原因

因为 SDWebImage 的缓存机制, 相同的 URL 它就认为可以去到沙盒里面的缓存路片

解决方法

NSString *testImageUrl = 
[_IMG sd_setImageWithURL:[NSURL URLWithString:testImageUrl] placeholderImage:[UIImage imageNamed:@"1.png"] options:SDWebImageRefreshCached];

SDWebImageManager.m文件中，大概176行左右吧，把之前的代码：（如下）

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;
}

更换成如下代码：

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 if cached but force refreshing
    downloaderOptions |= SDWebImageDownloaderIgnoreCachedResponse;
}