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MVC

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captainkuro edited this page Dec 20, 2012 · 14 revisions

Category:Help::TipsAndTricks

Purpose

This tutorial will step through what MVC is, why it exists, and how CI supports it. Once this is all done, I'll have built a very very simple app (an application to show some links) - but more importantly will have explained the MVC model.

MVC

The model-view-controller (MVC) programming method is a way to split up the 3 major functions of a modern database-driven website:

  1. Database functions (CRUD)
  2. Application Logic (i.e. passwords must be 8 characters long)
  3. Presentation (HTML)

Each of these elements are represented by the MVC model

  1. Model - database
  2. Controller - application logic
  3. View - HTML pages

Some Rules

When it comes to MVC programming, there are some rules that need to be considered. Here are a few to help get started: Don’t do these: Views:

  1. Never access the database
  2. Never use anything more complicated than loops and if statements Controllers:
  3. Never contain HTML code
  4. Never access the database Models:
  5. Never ever ever spit out HTML

DO these: Views:

  1. Are modular - code fragments
  2. Contain loops and simple if logic Controllers:
  3. Scrub all information between the view and the database
  4. Provide all data needed by the view
  5. Applies business rules to data
  6. Calls database models to store/retrieve data
  7. Handles all errors/error messages

Models:

  1. Apply limited business logic - if any - to database calls
  2. Captures and sends any error to the controller
  3. Performs minimal data sanity checks

Creating the application

Database

Let's start with the 'links' table in your site database. It will need the following fields:

id - autoincrement
link_title - varchar(50)
link_url - varchar(255)

Feel free to add a few sample data into the database.

Create a user for the database. Try avoiding using the 'root' (sa) account to ensure a level of security is maintained.

Code Igniter Database Configuration

Using a default CI installation, open the system/application/config/database.php file:

$db['default']['hostname'] = "localhost";
$db['default']['username'] = "username"; //don't use root
$db['default']['password'] = "password";
$db['default']['database'] = "database";
$db['default']['dbdriver'] = "mysql"; //there are other drivers available!

I will assume you can configure the rest of CI, so I'm not going to discuss that. I prefer to autoload the database, so go ahead and do that.

Where do I go now?

The Model expects input from the controller the view needs a controller to be displayed the controller needs input from the view the model needs to put its data somewhere ...ugh

Here's the easiest approach that I've found:

  1. Build your desired HTML/CSS layout.
  2. De-construct the interface into the following 3 categories:
  • layout - just the structure
  • common data across all (most) pages:
      • footer
      • affiliate links/ad space
  • stuff that changes on each page:
      • navigation
      • page title
      • meta tags (keywords, description)
  1. Build your first controller that puts all the pieces back together again
  2. "Get dynamic" with the model, forms, and stuff...

The view(s)

I've gone ahead with an ultra-super-simple view. It's almost insulting; but bear with me. application/views/layout/main.php

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title><?= $title ?></title>
</head>
<body>
<h1><?= $current_page ?></h1>
<div><?= $navigation ?></div>
<div><?= $content ?></div>
</body>
</html>

You can see that there are three variables:

title - the page title
current_page - a header tag indicating the page title
navigation - a block for page navigation
content - a block for code fragments

application/views/navigation/nav_main.php

<ul id="navigation">
  <li><?= anchor("","Home") ?></li>
  <li><?= anchor("links","Links") ></li>
</ul>

I'm using the URL helper in that view to make my links

Finally, some content: application/views/content/link_list.php

<?php foreach($links as $link): ?>
<p><?= anchor($link->link_url,$link->link_title) ?><?= anchor('edit/'.$link->id,"Edit") ?></p>
<?php endforeach; ?>

Notice the similarity between this view and the database?

The controller

Let's use the controller to put some of it together: application/controllers/welcome.php

//function Welcome() (php4, php5 below)
function __construct()
{
  parent::Controller();
  $this->load->helper('url');    
}

function index() 
{
  $data = array(
   'title' => 'Test Application',
   'current_page' => 'Welcome',
   'navigation' => $this->load->view('navigation/nav_main','',true),
   'content' => ''
  );
  $this->load->view('layout/main',$data);
}

Notice that I'm setting the keys of the $data array to be the same as the variable names I used in the view pages. You'll also notice that I did something special with the navigation variable - I loaded a view into it. Loading a view with "true" as the third argument returns a processed string rather than loading the view to the page. This enables multiple view pages placed into a common layout! :)

You'll also notice that I left the 'content' blank. (I'll get to that in a minute) If you were to load your page now, you will see the basic layout with the "Welcome" h1 tag and a "Test Application" web page title. There's also the navigation unordered list (one of the links won't work)

The model

To get the list of links from the database, we need to build a model: application/models/links.php (lowercase 'L' for the file name)

<?php
//capital L for class name
class Links extends Model {

  function Links() {
    parent::Model();
  }

  function get_links() {
   $query = $this->db->get('links'); //same as Select * from links
   return $query->result();
  }
}
?>

This model provides a simple function - get_links - to get all records from the links database we created earlier.

Now, let's update the controller: application/controllers/welcome.php

function index() 
{
  $this->load->model('links');
  $links['links'] = $this->links->get_links();
  $data = array(
   'title' => 'Test Application',
   'current_page' => 'Welcome',
   'navigation' => $this->load->view('navigation/nav_main','',true),
   'content' => $this->load->view('content/link_list',$links,true)
  );
  $this->load->view('layout/main',$data);
}

Now I've gotten the data from the database and put it into the $links['links'] array. At the same time, I've also loaded another view snippet into the $data['content'] array - but the second argument is the $links array instead of an empty string.

If you reload the page, you should see a list of all the links you entered into your database!

Conclusion

Hopefully, this helps demonstrate a means to create a model, view, and controller to display information from your database.

Feel free to add to and edit this Wiki page as you see fit :)

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