How to implement verifying existing username
<p>I am newbie in iOS with swift. What I need to do right now is that I have a textfield for user to enter his/er username. And once he clicks the other textfields, say password section, the application will automatically check whether this name exists or not and display a "V" or "X" image in a imageView. But I don't know how to do that or what method or action I should deal with. In Android, I could detect the focus of that textfield.Once the textfield loses the focus and if the text isn't empty, I can retrieve the text and request to my server to verify whether it exists or not. In iOS, I'm totally confused how to detect this, and is this related with first responder? Thx for advice in advance!</p>