Enable a button in Swift only if all text fields have been filled out

Xcode 9 • Swift 4

You can addTarget to your text fields to monitor for the control event .editingChanged and use a single selector method for all of them:

override func viewDidLoad() {
    super.viewDidLoad()
    doneBarButton.isEnabled = false
    [habitNameField, goalField, frequencyField].forEach({ $0.addTarget(self, action: #selector(editingChanged), for: .editingChanged) })
}

Create the selector method and use guard combined with where clause (Swift 3/4 uses a comma) to make sure all text fields are not empty otherwise just return. Swift 3 does not require @objc, but Swift 4 does:

@objc func editingChanged(_ textField: UITextField) {
    if textField.text?.characters.count == 1 {
        if textField.text?.characters.first == " " {
            textField.text = ""
            return
        }
    }
    guard
        let habit = habitNameField.text, !habit.isEmpty,
        let goal = goalField.text, !goal.isEmpty,
        let frequency = frequencyField.text, !frequency.isEmpty
    else {
        doneBarButton.isEnabled = false
        return
    }
    doneBarButton.isEnabled = true
}

sample

Swift 5.1 /Xcode 11

override func viewDidLoad() {
    super.viewDidLoad()
    setupAddTargetIsNotEmptyTextFields()        
}

func setupAddTargetIsNotEmptyTextFields() {
    okButton.isHidden = true //hidden okButton
    nameUserTextField.addTarget(self, action: #selector(textFieldsIsNotEmpty), 
                                for: .editingChanged)
    emailUserTextField.addTarget(self, action: #selector(textFieldsIsNotEmpty), 
                                for: .editingChanged)
    passwordUserTextField.addTarget(self, action: #selector(textFieldsIsNotEmpty), 
                                for: .editingChanged)
    confimPasswordUserTextField.addTarget(self, action: #selector(textFieldsIsNotEmpty), 
                                for: .editingChanged)        
   }

and then create the selector method and use guard:

@objc func textFieldsIsNotEmpty(sender: UITextField) {

    sender.text = sender.text?.trimmingCharacters(in: .whitespaces)

    guard
      let name = nameUserTextField.text, !name.isEmpty,
      let email = emailUserTextField.text, !email.isEmpty,
      let password = passwordUserTextField.text, !password.isEmpty,
      let confirmPassword = confimPasswordUserTextField.text,
          password == confirmPassword          
      else
    {
      self.okButton.isHidden = true
      return
    }
    // enable okButton if all conditions are met
    okButton.isHidden = false
   }