Enabling done button after inserting one char in a textfield: textFieldDidEndEditing: or textFieldShouldBeginEditing: or?

I would like to enable the done button on the navbar (in a modal view) when the user writes at least a char in a uitextfield. I tried:

• textFieldDidEndEditing: enables the button when the previous uitextfield resigns first responder (so with the zero chars in the current uitextfield).
• textFieldShouldBeginEditing: is called when the textfield becomes the first responder. Is there another way to do this?

[EDIT]

The solution could be

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string

but neither

 [self.navigationItem.rightBarButtonItem setEnabled:YES];

or

[doneButton setEnabled:YES]; //doneButton is an IBOutlet tied to my Done UIBarButtonItem in IB

work.