How to highlight text when textfield is tapped

Your question duplicates this. The best answer there is to use delegate functions textFieldShouldBeginEditing and textFieldShouldEndEditing to change and restore colors.

set delegate for textfield

TF.delegate = self

add code in delegate

extension viewcontroller: UITextFieldDelegate {
    func textFieldDidBeginEditing(_ textField: UITextField) {
        textField.layer.borderWidth = 2
        textField.layer.borderColor = global().defaultColor().cgColor
    }
    
    func textFieldDidEndEditing(_ textField: UITextField) {
        textField.layer.borderWidth = 0
    }
}

textFieldDidBeginEditing

This is not going to achieve what you exactly want because according to UITextField life-cycle, this method will be triggered exactly when editing started and about to be end. What you need is, highlight exact moment with tapping textfield.

First separate your highlighted and normal state appearences.

extension MainView {
func textFieldActiveDisplay() {

    self.isTextFieldOpen = true
    let color = COLOR
    let animation: CABasicAnimation = CABasicAnimation(keyPath: "borderColor")
    animation.fromValue = textField.layer.borderColor
    animation.toValue = color
    animation.duration = 0.3
    textField.layer.borderColor = color.cgColor
    
    let borderWidth: CABasicAnimation = CABasicAnimation(keyPath: "borderWidth")
    borderWidth.fromValue = 0
    borderWidth.toValue = 4
    borderWidth.duration = 0.2
    searchBar.layer.borderWidth = 4
    
    let group = CAAnimationGroup()
    group.animations = [animation, borderWidth]
    searchBar.layer.add(group, forKey: nil)
    
    textField.font = HIGHLIGHTED_FONT 
    textField.textColor = HIGHLIGHTED_COLOR
}

func textFieldInActiveDisplay() {
    guard isTextFieldOpen else { return }
    let borderWidth: CABasicAnimation = CABasicAnimation(keyPath: "borderWidth")
    borderWidth.fromValue = 4
    borderWidth.toValue = 0
    borderWidth.duration = 0.2
    textField.layer.borderWidth = 0
    textField.layer.add(borderWidth, forKey: nil)
    
    textField.font = NORMAL_FONT
    textField.textColor = NORMAL_COLOR
    self.isTextFieldOpen = false
    DispatchQueue.main.asyncAfter(deadline: .now() + 0.2) {
        self.textField.subviews.forEach({$0.layer.removeAllAnimations()})
        self.textField.layer.removeAllAnimations()
        self.textField.layoutIfNeeded()
    }
}

}

In your ViewController you need to link-up your textfield's delegate to itself.

override func viewDidLoad() {
  super.viewDidLoad() 
  view.textField.delegate = self
}


extension MainViewController: UITextFieldDelegate, UIGestureRecognizerDelegate {
    
func textFieldShouldBeginEditing(_ textField: UITextField) -> Bool {
    view.textFieldActiveDisplay()
    return true
}

func textFieldDidEndEditing(_ textField: UITextField) {
    view.textFieldInActiveDisplay()
}

}

Output: