Prior to any attempt to create a true observer object within the form validation application that you learned in the previous article, it would be very convenient to have close at hand the definition of all the data checking classes that I defined previously. In this way, you'll understand more easily how the Observer pattern can be properly implemented inside the application in question. Therefore, here's the complete listing of all the verification classes previously created:

  // define DataValidator class
  class DataValidator{
    protected $method;
    protected $formObserver;
    public function __construct(FormObserver $formObserver){
        $this->formObserver=$formObserver;
        $this->method=$_POST;
    }
    protected function notifyObserver($errorMessage){
        $this->formObserver->addNotification($errorMessage);
    }
  }
  // define StringValidator class
  class StringValidator extends DataValidator{
    public function __construct($formObserver){
        parent::__construct($formObserver);
    }
    // validate strings
    public function validate($field,$errorMessage,$min=4,$max=32)
{
        if(!isset($this->method[$field])||trim($this->method
[$field])==''||strlen($this->method[$field])<$min||strlen($this-
>method[$field])>$max){
            $this->notifyObserver($errorMessage);
        }
    }
  }
  // define IntegerValidator class
  class IntegerValidator extends DataValidator{
    public function __construct($formObserver){
        parent::__construct($formObserver);
    }
    // validate integers
    public function validate($field,$errorMessage){
        if(!isset($this->method[$field])||!is_numeric($this-
>method[$field])||intval($this->method[$field])!=$this->method
[$field]){
            $this->notifyObserver($errorMessage);
        }
    }
  }
  // define NumberValidator class
  class NumberValidator extends DataValidator{
    public function __construct($formObserver){
        parent::__construct($formObserver);
    }
    // validate numbers
    public function validate($field,$errorMessage){
        if(!isset($this->method[$field])||!is_numeric($this-
>method[$field])){
            $this->notifyObserver($errorMessage);
        }
    }
  }
  // define RangeValidator class
  class RangeValidator extends DataValidator{
    public function __construct($formObserver){
        parent::__construct($formObserver);
    }
    // validate ranges
    public function validate($field,$errorMessage,$min=1,$max=99){
        if(!isset($this->method[$field])||$this->method[$field]
<$min||$this->method[$field]>$max){
            $this->notifyObserver($errorMessage);
        }
    }
  }
  // define AlphaValidator class
  class AlphaValidator extends DataValidator{
    public function __construct($formObserver){
        parent::__construct($formObserver);
    }
    // validate alphabetic field
    public function validate($field,$errorMessage){
        if(!isset($this->method[$field])||!preg_match("/^[a-zA-Z]
+$/",$this->method[$field])){
            $this->notifyObserver($errorMessage);
        }
    }
  }
  // define AlphanumValidator class
  class AlphanumValidator extends DataValidator{
    public function __construct($formObserver){
        parent::__construct($formObserver);
    }
    // validate alphanumeric data
    public function validate($field,$errorMessage){
        if(!isset($this->method[$field])||!preg_match("/^[a-zA-
Z0-9]+$/",$this->method[$field])){
            $this->notifyObserver($errorMessage);
        }
    }
  }
  // define EmailValidator class
  class EmailValidator extends DataValidator{
    public function __construct($formObserver){
        parent::__construct($formObserver);
    }
    // validate email
    public function validate($field,$errorMessage){
        if(!isset($this->method[$field])||!preg_match
("/.+@.+..+/",$this->method[$field])||!checkdnsrr(array_pop
(explode("@",$this->method[$field])),"MX")){
            $this->notifyObserver($errorMessage);
        }
    }
  }
  // define EmailValidatorWin class (Windows systems)
  class EmailValidatorWin extends DataValidator{
    public function __construct($formObserver){
        parent::__construct($formObserver);
    }
    public function validate($field,$errorMessage){
        if(!isset($this->method[$field])||!preg_match
("/.+@.+..+/",$this->method[$field])||!$this->windnsrr(array_pop
(explode("@",$this->method[$field])),"MX")){
            $this->notifyObserver($errorMessage);
        }
    }
    // private method 'windnsrr()' for Windows systems
    private function windnsrr($hostName,$recType=''){
        if(!empty($hostName)){
            if($recType=='')$recType="MX";
            exec("nslookup -type=$recType $hostName",$result);
            foreach($result as $line){
                if(preg_match("/^$hostName/",$line)){
                    return true;
                }
            }
            return false;
        }
        return false;
    }
  }

Here you have them. As you can see above, I listed the complete set of data checking classes that I plan to use with the form validation system, in such a way that they can notify an observer object of any potential errors via the corresponding "notifyObserver()" method. Obviously, this object will determine the best course of action to take, in accordance with the programming logic implemented inside the application.

Since the above data validation classes should be familiar to you (remember that they were originally defined in the second part of this series), it's time to see how an observer class can be created, in order to decide what tasks to perform, based on the proper notifications sent by the pertinent checking classes.

Due to the important concepts that I'm going to deploy in the upcoming section, I strongly suggest you keep reading, in order learn how to include an observer class within the previous form validation application.