获取 iOS 14 中的 Font.Weight

Question

In iOS 13 and low I got Font.Weight using this extension for UIFont

var weight: UIFont.Weight {
    guard let traits = fontDescriptor.object(forKey: .traits) as? [UIFontDescriptor.TraitKey: Any], let weightValue = traits[.weight] as? CGFloat else { return .regular }
    let weight = UIFont.Weight(rawValue: weightValue)
    return weight
}

But since iOS 14 weightValue is wrong. For example:

let font = UIFont(name: "AvenirNext-Bold", size: 21)
print(font?.weight.rawValue)
print(font?.weight == .bold)

iOS 14 - 0.40000000000000002 false

iOS 13 - 0.40000000596046448 true

Anybody faced with it?

Answer 1

This is not a bug. It is in fact expected to be inaccurate after a high number of decimal places (16 in this case). Floats are calculated via the IEEE 754 standard for Single-Precision Floating-Point arithmetic. The original standard was only accurate up to 7 decimal places for integers between 4 and 6, but this has iteratively improved since the original, for example in 2019 and 2020 (see here for details).

What you see is probably the effect of conditional casting. The conversion from Float to CGFloat loses precision.