Replace character while user is typing

Question

Trying to replace a space with the character _

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    if textField.tag == 0 {
        username = textField.text!
        username = username.replacingOccurrences(of: " ", with: "_", options: .literal, range: nil)
        textField.text! = username
    }
    return true
}

Trouble is, the space is replaced after the character following the space is typed.

Answer 1

The method textField(_, shouldChangeCharactersIn:, replacementString:) is a query, you should not be putting side effects there. Instead make an IBAction and attach it to the editingChanged UIControlEvent...

@IBAction func editingChanged(_ sender: UITextField) {
    sender.text = sender.text?.replacingOccurrences(of: " ", with: "_")
}