一点SnapKit的equal理解

snakit中equal里面如果是组件，则是对齐组件。如果里面是数值，则是对齐数值虽然好像很容易，实际使用中，如果是对于右对齐的常常会弄错如下：

        let exchangeLabel = UILabel()
        exchangeLabel.text = "To Exchange"
        exchangeLabel.font = UIFont.systemFont(ofSize: 20)
        exchangeLabel.textColor = UIColor.lightGray
        exchangeLabel.textAlignment = .right
        self.view.addSubview(exchangeLabel)
        exchangeLabel.snp.makeConstraints { (make) in
            make.top.equalTo(sourceCnyLabel.snp.bottom)
            make.right.equalTo(-self.view.center.x-15)
        }

 需要价格负号才能右边对齐中间