How can I get weight of a UILabel?

It seems that fontDescriptor.fontAttributes won't return all the attributes of a font. Luckily fontDescriptor.object(forKey: .traits) will, so we can hop onto that.

extension UIFont {
    var weight: UIFont.Weight {
        guard let weightNumber = traits[.weight] as? NSNumber else { return .regular }
        let weightRawValue = CGFloat(weightNumber.doubleValue)
        let weight = UIFont.Weight(rawValue: weightRawValue)
        return weight
    }

    private var traits: [UIFontDescriptor.TraitKey: Any] {
        return fontDescriptor.object(forKey: .traits) as? [UIFontDescriptor.TraitKey: Any]
            ?? [:]
    }
}

and then it's as simple as label.font.weight

(This is fundamentally equivalent to https://stackoverflow.com/a/48688000/1288097 but it uses UIKit APIs)

To get the font weight string name, use the font descriptor and pass in the face attribute.

Swift 4.2

let font = UIFont.systemFont(ofSize: 14, weight: UIFont.Weight.bold)
let face = font.fontDescriptor.object(forKey: UIFontDescriptorFaceAttribute) as! String
print("face: \(face)")

Swift 3

let font = UIFont.systemFont(ofSize: 14, weight: UIFontWeightBold)
let face = font.fontDescriptor.object(forKey: UIFontDescriptorFaceAttribute) as! String
print("face: \(face)")

Try following sample font extension with Swift 4. (It needs some improvement for all types of font weights)

extension UIFont {

    func getFontWeight() -> UIFont.Weight {
    
        let fontAttributeKey = UIFontDescriptor.AttributeName.init(rawValue: "NSCTFontUIUsageAttribute")
        if let fontWeight = self.fontDescriptor.fontAttributes[fontAttributeKey] as? String {
            switch fontWeight {

            case "CTFontBoldUsage":
                return UIFont.Weight.bold
        
            case "CTFontBlackUsage":
                return UIFont.Weight.black
        
            case "CTFontHeavyUsage":
                return UIFont.Weight.heavy
        
            case "CTFontUltraLightUsage":
                return UIFont.Weight.ultraLight
        
            case "CTFontThinUsage":
                return UIFont.Weight.thin
        
            case "CTFontLightUsage":
                return UIFont.Weight.light
        
            case "CTFontMediumUsage":
                return UIFont.Weight.medium
        
            case "CTFontDemiUsage":
                return UIFont.Weight.semibold
        
            case "CTFontRegularUsage":
                return UIFont.Weight.regular

            default:
                return UIFont.Weight.regular
            }
        }
        
    return UIFont.Weight.regular
}

Try with label:

let label = UILabel()
var fontWeight = label.font.getFontWeight()
print("fontWeight - \(fontWeight)")

label.font = UIFont.systemFont(ofSize: 14, weight: UIFont.Weight.bold)
fontWeight = label.font.getFontWeight()
print("fontWeight - \(fontWeight)")

label.font = UIFont.systemFont(ofSize: 14, weight: UIFont.Weight.black)
fontWeight = label.font.getFontWeight()
print("fontWeight - \(fontWeight)")

label.font = UIFont.systemFont(ofSize: 14, weight: UIFont.Weight.heavy)
fontWeight = label.font.getFontWeight()
print("fontWeight - \(fontWeight)")

label.font = UIFont.systemFont(ofSize: 14, weight: UIFont.Weight.ultraLight)
fontWeight = label.font.getFontWeight()
print("fontWeight - \(fontWeight)")

label.font = UIFont.systemFont(ofSize: 14, weight: UIFont.Weight.thin)
fontWeight = label.font.getFontWeight()
print("fontWeight - \(fontWeight)")

label.font = UIFont.systemFont(ofSize: 14, weight: UIFont.Weight.light)
fontWeight = label.font.getFontWeight()
print("fontWeight - \(fontWeight)")

label.font = UIFont.systemFont(ofSize: 14, weight: UIFont.Weight.medium)
fontWeight = label.font.getFontWeight()
print("fontWeight - \(fontWeight)")

label.font = UIFont.systemFont(ofSize: 14, weight: UIFont.Weight.semibold)
fontWeight = label.font.getFontWeight()
print("fontWeight - \(fontWeight)")

Here is Apple document for list of Font Weights

The value of this weight is an NSNumber object. The valid value range is from -1.0 to 1.0. The value of 0.0 corresponds to the regular or medium font weight. You can also use a font weight constant to specify a particular weight.