用 NSURLSessionDownloadTask 异步下载资源成功后，completionHandler 一般是在子线程回调，如果我们转到其它线程用 moveItemAtPath 方法移动文件的时候，偶尔会遇到错误：
Error Domain=NSCocoaErrorDomain Code=4 "“CFNetworkDownload_GAfbOO.tmp” couldn’t be moved to “某个文件夹名字” because either the former doesn't exist, or the folder containing the latter doesn't exist." UserInfo={NSSourceFilePathErrorKey=/private/var/mobile/Containers/Data/Application/DA1DFC03-1FD9-448C-BCE7-4B345FC2057D/tmp/CFNetworkDownload_GAfbOO.tmp, NSUserStringVariant=(
Move
), NSDestinationFilePath=/var/mobile/Containers/Data/Application/DA1DFC03-1FD9-448C-BCE7-4B345FC2057D/Library/Caches/某个文件夹名字/b60553f25575808c2d876c274de908a5, NSFilePath=/private/var/mobile/Containers/Data/Application/DA1DFC03-1FD9-448C-BCE7-4B345FC2057D/tmp/CFNetworkDownload_GAfbOO.tmp, NSUnderlyingError=0x17405
解决方法，不要转移到其它线程，直接在 completionHandler 线程 moveItemAtPath 转移文件。
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