I want to change bottom layer of text field color when i start typing to blue

func designTextField()
    {
        //Set the horizontal line in bottom of text field
        nameLayer.frame = CGRect(x: 0, y: self.tfName.bounds.size.height-1, width: self.tfName.bounds.size.width, height: 1)
        nameLayer.backgroundColor = UIColor.lightGray.cgColor
        tfName.layer.addSublayer(nameLayer)

        //Set the horizontal line in bottom of text field
        phoneLayer.frame = CGRect(x: 0, y: self.tfPhone.bounds.size.height-1, width: self.tfPhone.bounds.size.width, height: 1)
        phoneLayer.backgroundColor = UIColor.lightGray.cgColor
        tfPhone.layer.addSublayer(phoneLayer)

        //Set the horizontal line in bottom of text field
        emailLayer.frame = CGRect(x: 0, y: self.tfEmail.bounds.size.height-1, width: self.tfEmail.bounds.size.width, height: 1)
        emailLayer.backgroundColor = UIColor.lightGray.cgColor
        tfEmail.layer.addSublayer(emailLayer)
    }

When to use this function to show in the View :

override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

        //Text Field Design
        self.designTextField()

        self.view.layoutIfNeeded()
    }

Output :

Bottom layer background color will change to blue, when to hit the "Name" text field. But when i type some characters in the text field, then the text field bottom color will change to lightgray(but when i typing then the bottom layer will be same to blue.)

Output :

func textFieldDidBeginEditing(_ textField: UITextField)
    {
        if textField == self.tfName
        {
            nameLayer.backgroundColor = UIColor(red: 11/255, green: 174/255, blue: 250/255, alpha: 1).cgColor
            imgName = UIImage(named: "user2")!
            imgVwName.image = imgName
            tfName.leftView = imgVwName
        }
        else if textField == self.tfPhone
        {
            phoneLayer.backgroundColor = UIColor(red: 11/255, green: 174/255, blue: 250/255, alpha: 1).cgColor
            imgPhone = UIImage(named: "phone2")!
            imgVwPhone.image = imgPhone
            tfPhone.leftView = imgVwPhone
        }
        else if textField == self.tfEmail
        {
            emailLayer.backgroundColor = UIColor(red: 11/255, green: 174/255, blue: 250/255, alpha: 1).cgColor
            imgEmail = UIImage(named: "mail2")!
            imgVwEmail.image = imgEmail
            tfEmail.leftView = imgVwEmail
        }
    }

    func textFieldDidEndEditing(_ textField: UITextField)
    {
        if textField == self.tfName
        {
            nameLayer.backgroundColor = UIColor.lightGray.cgColor
            imgName = UIImage(named: "user1")!
            imgVwName.image = imgName
            tfName.leftView = imgVwName
        }
        else if textField == self.tfPhone
        {
            phoneLayer.backgroundColor = UIColor.lightGray.cgColor
            imgPhone = UIImage(named: "phone1")!
            imgVwPhone.image = imgPhone
            tfPhone.leftView = imgVwPhone
        }
        else if textField == self.tfEmail
        {
            emailLayer.backgroundColor = UIColor.lightGray.cgColor
            imgEmail = UIImage(named: "mail1")!
            imgVwEmail.image = imgEmail
            tfEmail.leftView = imgVwEmail
        }
    }

Can somebody plzz help ???

When you write in textfield viewDidLayoutSubviews is called every time and call designTextField (designTextField add a new sublayer with grayColor). You can check if is the first time with a flag, or call designTextField in viewWillAppear or viewDidAppear, or check if the layer was added.