Enable UIAlertAction of UIAlertController only after input

You can add an observer to your UITextField:

[alertController addTextFieldWithConfigurationHandler:^(UITextField *textField) {
    [textField addTarget:self action:@selector(alertControllerTextFieldDidChange:) forControlEvents:UIControlEventEditingChanged];
}

but first disable your button:

okAction.enabled = NO;

Then validate it in the method you specified :

- (void)alertTextFieldDidChange:(UITextField *)sender {
  UIAlertController *alertController = (UIAlertController *)self.presentedViewController;
  if (alertController) {
    UITextField *someTextField = alertController.textFields.firstObject;
    UIAlertAction *okAction = alertController.actions.lastObject;
    okAction.enabled = someTextField.text.length > 2;
  }
}

Add following property in your header file

@property(nonatomic, strong)UIAlertAction *okAction;   

then copy the following code in your viewDidLoad method of your ViewController

self.okAction = [UIAlertAction actionWithTitle:@"OK"
                                         style:UIAlertActionStyleDefault
                                       handler:nil];
self.okAction.enabled = NO;

UIAlertController *controller = [UIAlertController alertControllerWithTitle:nil
                                                                    message:@"Enter your text"
                                                             preferredStyle:UIAlertControllerStyleAlert];

[controller addTextFieldWithConfigurationHandler:^(UITextField *textField) {

    textField.delegate = self;
}];

[controller addAction:self.okAction];
[self presentViewController:controller animated:YES completion:nil];

Also implement the following UITextField delegate method in your Class

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

    NSString *finalString = [textField.text stringByReplacingCharactersInRange:range withString:string];
   [self.okAction setEnabled:(finalString.length >= 5)];
   return YES;
}

This should work

Swift 3 implementation based on soulshined's answer:

var someAlert: UIAlertController {
    let alert = UIAlertController(title: "Some Alert", message: nil, preferredStyle: .alert)

    alert.addTextField {
        $0.placeholder = "Write something"
        $0.addTarget(self, action: #selector(self.textFieldTextDidChange(_:)), for: .editingChanged)
    }

    alert.addAction(UIAlertAction(title: "Cancel", style: .cancel))

    let submitAction = UIAlertAction(title: "Submit", style: .default) { _ in
        // Do something...
    }

    submitAction.isEnabled = false
    alert.addAction(submitAction)
    return alert
}

func textFieldTextDidChange(_ textField: UITextField) {
    if let alert = presentedViewController as? UIAlertController,
        let action = alert.actions.last,
        let text = textField.text {
        action.isEnabled = text.characters.count > 0
    }
}