iOS文件操作，是否存在，文件大小获取

// 已下载文件大小
- (long long)downloadedFileDataSizeWithSavePath:(NSString *)savePath {
    signed long long fileSize = 0;
    NSFileManager *fileManager = [NSFileManager defaultManager]; // default is not thread safe
    if ([fileManager fileExistsAtPath:savePath]) {
        NSError *error = nil;
        NSDictionary *fileDict = [fileManager attributesOfItemAtPath:savePath error:&error];
        if (!error && fileDict) {
            fileSize = [fileDict fileSize];
        }
    }
    return fileSize;
}