通常用于删除缓存的时，计算缓存大小    //单个文件的大小    -   (long    long)   fileSizeAtPath:(NSString*) filePath{        NSFileManager*   manager = [NSFileManager defaultManager];        if    ([manager fileExistsAtPath:filePath]){            return    [[manager attributesOfItemAtPath:filePath error:nil] fileSize];        }        return    0;    }    //遍历文件夹获得文件夹大小，返回多少M    -   (float    ) folderSizeAtPath:(NSString*) folderPath{        NSFileManager*   manager = [NSFileManager defaultManager];        if    (![manager fileExistsAtPath:folderPath]) return    0;        NSEnumerator   *childFilesEnumerator = [[manager subpathsAtPath:folderPath] objectEnumerator];        NSString*   fileName;        long    long   folderSize = 0;        while    ((fileName = [childFilesEnumerator nextObject]) != nil){            NSString*   fileAbsolutePath = [folderPath stringByAppendingPathComponent:fileName];            folderSize   += [self fileSizeAtPath:fileAbsolutePath];        }        return    folderSize/(1024.0*1024.0);    }