How would I go about allowing inputting only alphanumeric characters in an iOS UITextField?
UITextField
You will have to use the textField delegate methods, and use methods textFieldDidBeginEditing, shouldChangeCharactersInRange and textFieldDidEndEditing to check for the characters.
textField delegate
textFieldDidBeginEditing
shouldChangeCharactersInRange
textFieldDidEndEditing
Please refer to this link for the documentation.