let weights : [UIFont.Weight] = [
    .ultraLight, .thin, .light, .regular, 
    .medium, .semibold, .bold, .heavy, .black
]
var top : CGFloat = 25
for weight in weights {
    let desc = UIFont.systemFont(ofSize: 32).fontDescriptor.addingAttributes([
        UIFontDescriptor.AttributeName.traits: [
            UIFontDescriptor.TraitKey.weight: 
                weight.rawValue,
            UIFontDescriptor.TraitKey.symbolic: 
                UIFontDescriptor.SymbolicTraits.traitItalic.rawValue
        ]
    ])
    let lab = UILabel()
    lab.font = UIFont(descriptor: desc, size: 0)
    lab.text = "This is a test"
    lab.sizeToFit()
    lab.frame.origin = CGPoint(x: 20, y: top)
    self.view.addSubview(lab)
    top += lab.bounds.height + 8
}

查看效果

链接