Manage Network Request on Every Single Alphabet Typed on My TextField

I am creating a screen like Twitter set username screen where user types username and at each character the user types, it checks the username already exists @Server by making network requests. In my case I am using Alamofire.

Here is the snapshot of Twitter username textfield:

In my case I am making request everytime a user type something. How can I cancel the previous request when user types another character?