IPhone : How to do? Now Typing... in chat app

I am working on chat app. I want to show how if my friend is typing something then in my end the chat window will show.. Now typing...

i know a bit about code (to judge but not sure), but where to call api or its done by some sort of chat server's property ?

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    if (range.length > 0)
    {
         // Friend is deleting
    }
    else
    {
        // Friend is typing
    }
}