恶心感性的代码，可以精确到一个像素，

    self.bgImageView.frame = CGRectMake(self.bgImageView.frame.origin.x, self.bgImageView.frame.origin.y, self.bgImageView.frame.size.width, cSize.height + 90);

    self.bgImageView.frame = CGRectMake(self.bgImageView.frame.origin.x, self.bgImageView.frame.origin.y, self.bgImageView.frame.size.width, self.commentChatView.frame.origin.y + self.commentChatView.frame.size.height + 3);