How to disable title color change when highlighted state for UIButton using UIButtonConfiguration is active?

This is how my UIButton is defined and initialized:

lazy var addButton: UIButton = {
    let button = UIButton(radius: 32, title: "+", font: .poppinsRegular.withSize(40), backgroundColor: .purple)
    button.menu = UIMenu(title: "")
    button.showsMenuAsPrimaryAction = true
    return button
}()


extension UIButton {
    convenience init(
        radius: CGFloat = 0,
        title: String? = nil,
        backgroundColor: UIColor? = nil,
        foregroundColor: UIColor = .white,
        font: UIFont? = nil
    ) {
        var configuration = UIButton.Configuration.filled()
        configuration.baseForegroundColor = foregroundColor
        configuration.title = title
        configuration.background.cornerRadius = radius
        configuration.cornerStyle = .fixed
        configuration.contentInsets = .zero
        configuration.titleTextAttributesTransformer = UIConfigurationTextAttributesTransformer { incoming in
            var outgoing = incoming
            outgoing.font = font
            return outgoing
        }
        configuration.baseBackgroundColor = backgroundColor
        configuration.background.backgroundColor = backgroundColor
        self.init(configuration: configuration)
    }
}

What to change to keep my title full white even when highlighted?

Give this a try...

In your .titleTextAttributesTransformer:

    configuration.titleTextAttributesTransformer = UIConfigurationTextAttributesTransformer { incoming in
        var outgoing = incoming
        outgoing.font = font
        
        // add this line
        outgoing.foregroundColor = foregroundColor
        
        return outgoing
    }

and add a ConfigurationUpdateHandler:

let handler: UIButton.ConfigurationUpdateHandler = { button in
    var background = UIButton.Configuration.plain().background
    background.cornerRadius = 32
    background.backgroundColor = .purple
    button.configuration?.background = background
}
    
addButton.configurationUpdateHandler = handler
    

Edit

Another (better, I think) approach...

Implement .backgroundColorTransformer in your convenience init:

extension UIButton {
    convenience init(
        radius: CGFloat = 0,
        title: String? = nil,
        backgroundColor: UIColor? = nil,
        foregroundColor: UIColor = .white,
        font: UIFont? = nil
    ) {
        // use plain instead of filled
        var configuration = UIButton.Configuration.plain()
        configuration.baseForegroundColor = foregroundColor
        configuration.title = title
        configuration.background.cornerRadius = radius
        configuration.cornerStyle = .fixed
        configuration.contentInsets = .zero

        configuration.titleTextAttributesTransformer = UIConfigurationTextAttributesTransformer { incoming in
            var outgoing = incoming
            outgoing.font = font
            
            // add this line
            outgoing.foregroundColor = foregroundColor
            
            return outgoing
        }

        configuration.background.backgroundColor = backgroundColor
        
        // prevent default background color/tint change
        configuration.background.backgroundColorTransformer = UIConfigurationColorTransformer { _ in
            return backgroundColor!
        }

        self.init(configuration: configuration)
    }
}

Now, we no longer need to assign a ConfigurationUpdateHandler.