PHP - 我可以创建一个除了通过另一个类之外无法实例化的类吗？

Question

I'm trying to have a child class from the Exception class just to handle errors and emit the proper error message given the error code. I changed my original code and made it more simple just to illustrate my problem.

Maybe it's not possible, but I don't want the InvalidEmailException class to be instantiated by the script. I just want it to be used by the Subscribe class if it's necessary (errors are found). Why would I want to do this anyway? Doesn't matter I'm just trying to understand how classes work.

/* Child class from the parent Exception class to handle errors 
 * pertinent to users being subscribed
 */
class InvalidEmailException extends Exception{
   private $error_code;
   private $email;
   function __construct($error_code, $email){
      $this->error_code = $error_code;
      $this->email = $email;
      $this->notifyUser();
   }
   function notifyUser(){
        if($this->error_code == 2):
         echo "<p>Invalid email: <em>{$this->email}</em></p>";
      endif;
   }
}

// Initial class to subscribe a user with the try catch checks
class Subscribe{
   private $email;
   private $error_code = 0;
   function __construct($email){
      $this->email = $email;
      $this->validateEmail();
   }

   private function validateEmail(){
      try{
         if($this->email == ''):
            throw new Exception('<p>Error: empty email address.</p>');
         else:
            if($this->email == 'invalid test'){
                    $this->error_code = 2;
               throw new InvalidEmailException($this->error_code, $this->email);
            }elseif($this->error_code == 0){
               // Go to method to subscribe a user if the error code remains zero
               $this->subscribeUser();
            }
         endif;
      }catch(Exception $e){
         echo $e->getMessage();
      }
   }

   private function subscribeUser(){
      echo $this->email.' added to the database!';
   }
}

/* 
 * Script to use the Subscribe class, which would call 
 * the InvalidEmailException class if needed
 */
$email = 'invalid test'; // This could later on be used through the $_POST array to take an email from a form
$subscribe = new Subscribe($email); // Works well.
$test = new InvalidEmailException('2', 'a@b.c'); // Also works.  I want this to throw an error.

Answer 1

I think your best option would be to use dependency injection http://en.wikipedia.org/wiki/Dependency_injection. It's a bit advanced but I think it would help in the long run..

Essentially, you could create a common interface with methods getting the data you need, and make the constructor for your exception class have a parameter of type of the interface. Then have the subscribe class implement the interface, which would allow it to be passed in to the constructor of the exception.

You could also just pass in the subscribe object to the constructor of the exception class directly as the parameter and bypass the interface usage. However, the interface allows you to have multiple classes all use that interface and those common methods and they could all construct your exception class if you used that pattern. And you would also know how to interact with those classes as they use the same common design.

This doesn't completely prevent you from constructing the exception from outside the class and in the script, however you will have needed to construct the class that implements the interface first (or just the class itself, depending on which way you go), which is a good way to restrict the behavior.