extension UILabel {
    func applyGlobalTextStyle() {
        let font = self.font ?? UIFont.preferredFont(forTextStyle: .body)
        let fontMetrics = UIFontMetrics(forTextStyle: .body)
        let scaledFont = fontMetrics.scaledFont(for: font)
        self.font = scaledFont
        self.adjustsFontForContentSizeCategory = true
    }
}

extension UIButton {
    func applyGlobalTextStyle() {
        let font =  self.titleLabel?.font ?? UIFont.preferredFont(forTextStyle: .body)
        let fontMetrics = UIFontMetrics(forTextStyle: .body)
        let scaledFont = fontMetrics.scaledFont(for: font)
        self.titleLabel?.font = scaledFont
        self.titleLabel?.adjustsFontForContentSizeCategory = true
    }
}

extension UITextField {
    func applyGlobalTextStyle() {
        let font = self.font ?? UIFont.preferredFont(forTextStyle: .body)
        let fontMetrics = UIFontMetrics(forTextStyle: .body)
        let scaledFont = fontMetrics.scaledFont(for: font)
        self.font = scaledFont
        self.adjustsFontForContentSizeCategory = true
    }
}


extension UITextView {
    func applyGlobalTextStyle() {
        let font = self.font ?? UIFont.preferredFont(forTextStyle: .body)
        let fontMetrics = UIFontMetrics(forTextStyle: .body)
        let scaledFont = fontMetrics.scaledFont(for: font)
        self.font = scaledFont
        self.adjustsFontForContentSizeCategory = true
    }
}

上面的方法会基于自己设置的字体大小，然后根据系统设置的字体大小进行计算得到最终的字体大小。

使用方法：

   lazy var subTitleLabel:UILabel = {
        var label = UILabel()
        label.numberOfLines = 2
        label.font = UIFont.systemFont(ofSize: 14.0)
        label.textColor = UIColor.color999
        label.applyGlobalTextStyle()
        return label
    }()

 label.applyGlobalTextStyle()

  label.font = UIFont.systemFont(ofSize: 14.0)，为初始值，系统更改字体后回到App页面就会刷新字体大小