UIAppearance Swift 4

After updating to Swift 4, I am getting a compiler error:

Static member 'appearance' cannot be used on protocol metatype 'UIAppearance.Protocol'

Here is my viewWillAppear method in my custom Tab Bar Controller subclass, I am setting the font of the item text.

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)

    // compiler error on line below
    UIAppearance.appearance().setTitleTextAttributes([NSAttributedStringKey.font: font], for: UIControlState.normal)
}

I'm having trouble fixing this, any guidance would be appreciated, thanks!