Strategy pattern or Chain of Responsibility pattern for determining right action

Question

I have the following UML diagram. CurlRequestHandler and KernelRequestHandler are both an implementation of the RequestHandlerInterface. The request handlers are responsible to process a certain Request object, all of them will return the same Response object.

   +------------------------+               +-------------------------+
   | CurlRequestHandler     |               | KernelRequestHandler    |
   |------------------------|               |-------------------------|
   |                        |               |                         |
   | - handleRequest(Request)               | - handleRequest(Request)|
   |                        |               |                         |
   |                        |               |                         |
   |                        |               |                         |
   +------------------------+               +-------------------------+
             +
             |                                             +
             |                                             |
             |        +---------------------------+        |
             |        | RequestHandlerInterface   |        |              +---------------+
             +---->   |---------------------------|  <-----+              |               |
                      |                           |                       |               |
                      | - handleRequest(Request)  |                       |    CLIENT     |
                      |                           |                       |               |
                      |                           |                       +---------------+
                      |                           |
                      |                           |
                      +---------------------------+

Now, to determine which handler I need to use, I have the following if statement in my client:

if ($mode == "kernel") {
    $handler = new KernelRequestHandler();
} else {
    $handler = new CurlRequestHandler();
}

$response = $handler->handleRequest($request);

Now, the problem is, when I need to add a new handler, I need to alter the if statement. I looked into the Chain of Responsibility design pattern and this seems to do a better job at this, but I'm not sure.

Which design pattern would be the best approach for this?

Steffen

Answer 1

What you need is to implement a factory method design pattern to create the handlers.

class HandlerFactory {

    public function make($mode) {

        switch(strtolower($mode)) {

            case 'kernel': return new KernelRequestHandler();
            case 'curl': return new CurlRequestHandler();
        }

    }

}

And, yes, you need to add a case for every new handler you make.

PS: Why you shouldn't call your classes 'handler'

Answer 2

If your $mode equals to the beginning of your RequestHandler class, then you can just append it.

$mode = 'kernel';
$class = ucfirst($mode).'RequestHandler';
$handler = new $class;

Will produce new KernelRequestHandler

Answer 3

class RequestFactory {
    public static function getHandler($mode) {
        $className = ucfirst($mode).'RequestHandler';
        return new $className();
    }
}

You can use it like this:

$handler = RequestFactory::getHandler('kernel');
$handler->handleRequest($request);