Practical PHP Patterns: Application Controller

The Application Controller pattern is a subpattern used in Web implementations of the Model-View-Controller one. It prescribes to interpose an object between the HTTP-related controllers (like the Action Controllers and the Front Controller) and the rest of the MVC machine of an application, or to substitute them.

This pattern involves an additional layer of complexity in an MVC implementation, but it is useful for modelling interactions as a (finite) state machine. By the way, when your project transition from a set of pages to a full featured web application, you can benefit from an Application Controller over the old page-based controllers.

Dynamic pages are produced on the fly, and are not always available; they may have dependencies on other previous pages, or may not be accessible until other events have already happened. An Application Controller takes care of this business logic, while not intruding into the domain model responsibilities.

User experience

For example, in basic use cases the pages of an application are orthogonal and can be visited in any order. Other times the planned user experience requires a particular path to be followed from the user, and some pages can be visited and executed only upon particular events. They may be valid only in some cases, or incomplete until their dependencies are satisfied.
Consider for example forms compiled in multiple steps, or application wizards. You'll never visit them out of order, and you must have some sort of mechanism to organize the single responses in a smooth flow. Application controllers centralize the logic relative to passing from one of the screens to another. Further logic should be encapsulated in the underlying domain objects, which the Application Controller drives with composition.

State machine

The interaction of every user with your application is in a certain state, and every state has predefined events that once happened may transition the application (for this particular user) to another state. Every transition is associated with a response, which is presented to the end user, with further orthogonal embellishment like a layout.
Usually interactions and this ideal [finite] state machines have a session-like scope (related to the current user), but nothing prevents considering the whole application a state machine. Finite State Machines are the simplest model for these interactions, but they can be either more than one (one for each user, for each notification, for each form...) or a more complex, custom model can be used by the Application Controller.

Implementation

Application controllers live behind an Action Controller to analyze the requests further, or behind a Front Controller to substitute the classical Page Controllers it forwards to. They have references as collaborators both to domain classes (on which the events provoke method execution) and also to view classes, since they must render a response.

Domain objects are juggled around by the Application Controller, created, stored and deleted from persistence. Even when using frameworks, view are usually scripts instead, driven by a generic View object that accepts the path as a parameter.

Logic, and what to put in it

Domain logic may scatter into the Application Controller. I usually prefer to keep as much as possible in the domain layer, but Application Controller is ideal for what we call application logic instead of domain logic: single interactions with the domain model. The boundary is not clearly defined here.

That said, you can easily decide where to put a bit of business logic by knwoing that what you put in this application layer won't be reused: it is transaction-specific logic. What you push down in the domain layer will probably be reused throughout the application.

For instance, you should put in an Application Controller:

• controller or view specific code, which doesn't fit into the domain layer
• particular use cases
• form objects management
• handling of multiple views
• infrastructure aspects.

Multiple Application Controllers

Multiple application controllers are the norm, unless your web application is fairly small. For example they are useful for dealing with different clients (mobile ones and ordinary browsers), since the user experience is likely to change between different devices (with more noisy or more optimized interactions).

Another use case for multiple Application Controllers is obviously in assigning one to each member of a set of interactions (multiple forms, or wizards, or any kinds of processes to accomplish.)

Example

Here I'll present a small, high-level sample of an Application Controller that manages a form compilation in multiple steps.

In Zend Framework, it can be thought of as homogeneous to Action Controllers (so that it can be easily put in an existing application's infrastructure.) It usually does not take advantage of automated mechanisms like the ViewRenderer plugin to achieve a finer control. In fact, it does not map to a single domain action or a single view: the view is chosen on the fly and the domain model is seen through a Facade.

<?php

class ApplicationController extends Zend_Controller_Action
{
    const LAST = 4;

    public function init()
    {
        // ... set up the various resources, like forms or domain entities
    }

    public function executeAction()
    {
        // other view variables...

        // keep partial compilations
        $this->session->merge($this->request->getPost());

        // this is our View change
        $step = $this->_request->getParam('step', 1);
        if ($step == self::LAST) { 
            return $this->render('final.phtml');
        } else {
            $this->view->form = $this->forms[$step];
        }

        // the script is always the same but the form rendered changes
        $this->render('appcontroller.phtml');
    }
}

You may go further in generalization, and code a generic Application Controller for this use case which can be fed metadata about the interaction (number of steps, Zend_Form objects, etc.)