Demo code of building modifier by mutating function in SwiftUI

SwiftUI_mutatingFunc.swift

// buttonLabel modifier
struct TokenButtonLabel: View {
    // --snip--
    
    /// State properties
    var isHighlighted = false
    
    // --snip--    
}
extension TokenButtonLabel {
    /// Pass highlightSwitch argument bonded with @State variable in order to trigger updates
    mutating func highlight(_ highlightSwitch: TokenButton.StateSwitch) -> Self {
        self.isHighlighted = highlightSwitch == .on ? true : false
        return self
    }
}

// buttonStyle modifier
struct TokenButtonStyle: ButtonStyle {
    // --snip--
    
    /// State properties
    var isActive = true
    
    // --snip--    
}
extension TokenButtonStyle {
    // Pass highlightSwitch argument bonded with @State variable in order to trigger updates
    mutating func activate(_ activeSwitch: TokenButton.StateSwitch) -> Self {
        self.isActive = activeSwitch == .on ? true : false
        return self
    }
}