把图片压缩到指定大小(kb)

UIImage *image=[UIImage imageNamed:@"xxoo.jpeg"];
    NSData  *imageData=UIImageJPEGRepresentation(image, 1.f);
    CGFloat size=40.f;// kb
    CGFloat scale=size/(imageData.length/1024);
    NSData *newData=UIImageJPEGRepresentation(image, scale);