IOS How to detect a change in focus between text fields on a user keyboard?

Question

I am creating a custom keyboard and I want to change the keyboard layout to adapt to the type UITextField(url, websearch, email) every time the user changes focus between text fields.

For example:

• Starting with user inputs in the url text box, a dot will be displayed on my keyboard.
• After editing in the url text box, the user will focus on the email text box, my keyboard will replace the dot button with the @ button.

The question is, how to catch this even in a custom keyboard?