NSAttributedString - Underline colors are not applied correctly

Question

I want to underline and color a text in an UILabel. The underline and the text have a complex pattern of changing colors, but my code sometimes fails.

The following minimal code reproduces the issue.

        let attributedTitle = NSMutableAttributedString(string: "ABC")

        attributedTitle.addAttributes([
            .foregroundColor: UIColor.black,
            .underlineColor: UIColor.black,
            .underlineStyle: NSUnderlineStyle.single.rawValue,
        ],
        range: NSRange(location: 0, length: 1))

        attributedTitle.addAttributes([
            .foregroundColor: UIColor.yellow,
            .underlineColor: UIColor.yellow,
            .underlineStyle: NSUnderlineStyle.single.rawValue,
        ],
        range: NSRange(location: 1, length: 1))

        attributedTitle.addAttributes([
            .foregroundColor: UIColor.black,
            .underlineColor: UIColor.black,
            .underlineStyle: NSUnderlineStyle.single.rawValue,
        ],
        range: NSRange(location: 2, length: 1))

        let label = UILabel()
        label.attributedText = attributedTitle

It shows this.

I expected this. I photoshopped it to show what I expected :(

How to underline and color the text that I expected?

Answer 1

Noticed that this happens when you have the same UIColor for the first and third characters. But not sure why this happens. As a workaround, you could create a new UIColor equal to UIColor.black for the third character as below

let attributedTitle = NSMutableAttributedString(string: "ABC")

attributedTitle.addAttributes([
    .foregroundColor: UIColor.black,
    .underlineColor: UIColor.black,
    .underlineStyle: NSUnderlineStyle.single.rawValue,
],
range: NSRange(location: 0, length: 1))

attributedTitle.addAttributes([
    .foregroundColor: UIColor.systemYellow,
    .underlineColor: UIColor.systemYellow,
    .underlineStyle: NSUnderlineStyle.single.rawValue,
],
range: NSRange(location: 1, length: 1))

attributedTitle.addAttributes([
    .foregroundColor: UIColor(red: 0.0, green: 0.0, blue: 0.0, alpha: 1.0),
    .underlineColor: UIColor(red: 0.0, green: 0.0, blue: 0.0, alpha: 1.0),
    .underlineStyle: NSUnderlineStyle.single.rawValue,
],
range: NSRange(location: 2, length: 1))