What main difference textFieldDidChange and shouldChangeCharactersIn method, the delegate of UITextFieldDelegate?

Can someone explain about when to use and how difference of them ?.

In Simple words textDidChange occurs when any text is typed in Textfield and you want to get text typed on TextField to check for validation or anything...

func textDidChange(_ textField: UITextField) {
        guard let email = emailTextField.text, !email.isEmpty, email.isValidEmail() else {
            //Write your code here...
            return }
        guard let password = passwordTextField.text, !password.isEmpty else {
            //Write your code here..
            return }
        //Write your success code. 
    }

And shouldChangeCharactersIn occurs event before any typed key is about to print in Textfield. So that you can varify that key and allow or restrict that key. Eg: If user enter space in Password Textfield you can restrict that key and space will never is printed in Textfield.

In below code I am restricting space(" ") key in My Textfields.

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
           if range.location == 0 && string == " " {
               return false
           }
           return true
       }