How to Use Wordpress's Custom Write Panels?

With the release of WordPress 3.0, a whole slew of features were introduced, including the merging of WordPress MU into the main codebase (allowing multiple sites to be managed from one WordPress installation), and the addition of custom post types. A feature that has been around for a while, but which is now extra-interesting as a result of the new custom post types, is Custom Write Panels.

A Custom Write Panel is a box of form fields (inputs, checkboxes, radio buttons, textareas, and so on) that's shown when you're writing or editing a post. The fields link up to custom fields that you've added to posts.

Although the Custom Field Panel displays in the Edit Post page by default, it can be tricky to use, especially when you want to enter a fair amount of data. That's where Custom Write Panels come in: they let you create slick-looking panels that allow users to enter all the additional information you want for a post.

For example, let's create a custom post type called Books. Books will have a title and description (the title and main blog content panels), but they also need some extra information in order to display properly: an author and an ISBN (a unique numeric commercial book identifier). So, in your theme'sfunctions.php, you need to add this code:

add_action( 'init', 'create_book_type' );
function create_book_type() {
  register_post_type( 'books',
    array(
      'labels' => array(
        'name' => __( 'Books' ),
        'singular_name' => __( 'Book' )
      ),
      'public' => true,
)
  );
}

What does this do?

add_action( 'init', 'create_book_type' ); tells WordPress to run our code on the init action (as soon as WordPress loads, in other words), and theregister_post_type() method adds the Book post type. Now, if you view your admin dashboard, you'll see a new Books panel in the left-hand navigation. We want to add the Custom Write Panel to this post type so, again in functions.php, we add the following code:

//We set up a few things, like where your 
//theme folder is, etc.
define(
  'MY_WORDPRESS_FOLDER',
  $_SERVER['DOCUMENT_ROOT']
);
define(
  'MY_THEME_FOLDER',
  str_replace("\\",'/',dirname(__FILE__))
);
define(
  'MY_THEME_PATH',
  '/' . substr(
    MY_THEME_FOLDER,
    stripos(MY_THEME_FOLDER,'wp-content')
  )
);
//This initializes the write panel.
add_action('admin_init','book_meta_init');
function book_meta_init() {
  //This adds our CSS file, 
  //so our write panels look pretty.
  wp_enqueue_style(
    'my_meta_css', 
    MY_THEME_PATH . '/custom/book_panel.css'
  );
  //This method is the one that actually adds the 
  //write panel, named 'Book Information' to the 
  //post type 'books'
  add_meta_box(
    'book_meta', 
    'Book Information', 
    'book_meta',
    'books', 
    'advanced', 
    'high'
  );
}
// The function below links the panel 
// to the custom fields
// ---------------------------------
function book_meta() {
  global $post;
  //The two variables.
  $author = get_post_meta($post->ID,'author',TRUE);
  $isbn = get_post_meta($post->ID,'isbn',TRUE);
  //Call the write panel HTML 
  include(MY_THEME_FOLDER . 
      '/custom/book_information.php');
  // create a custom nonce for submit 
  // verification later
  echo '';
}
//The function below checks the 
//authentication via the nonce, and saves 
//it to the database.
function my_meta_save($post_id) {
  // authentication checks
  // make sure data came from our meta box
  if(!wp_verify_nonce(
      $_POST['my_meta_noncename,__FILE__)
  ) return $post_id;
  if (!current_user_can('edit_post', $post_id)) {
    return $post_id;
  }
  // The array of accepted fields for Books
  $accepted_fields['books'] = array(
    'author',
    'isbn'
  );
  $post_type_id = $_POST['post_type'];
  //We loop through the list of fields, 
  //and save 'em!
  foreach($accepted_fields[$post_type_id] as $key){
    // Set it to a variable, so it's 
    // easier to deal with.
    $custom_field = $_POST[$key];
    //If no data is entered
    if(is_null($custom_field)) {
      //delete the field. No point saving it.
      delete_post_meta($post_id, $key);
      // If it is set (there was already data), 
      // and the new data isn't empty, update it.
    } 
    elseif(isset($custom_field) 
&& !is_null($custom_field)) 
    {
      // update
     update_post_meta($post_id,$key,$custom_field);
      //Just add the data.
    } else {
      // Add?
      add_post_meta($post_id, $key, 
        $custom_field, TRUE);
    }
  }
  return $post_id;
}

The code comments should make the workings of this snippet fairly evident, so onto the next step: adding the HTML for the Custom Write Panel. Inside your theme folder, create a new directory called custom and add two files: a CSS file for styling the panel's looks, and the HTML so it displays the fields properly. The CSS file should be named book_panel.css and contain this code :

.book_panel .description {
  display: none;
}
.book_panel label {
  display: block;
  font-weight: bold;
  margin: 6px;
  margin-bottom: 0;
  margin-top: 12px;
}
.book_panel label span {
  display: inline;
  font-weight: normal;
}
.book_panel span {
  color: #999;
  display: block;
}
.book_panel textarea, 
.book_panel input[type='text'] {
  margin-bottom: 3px;
  width: 100%;
}
.book_panel h4 {
  color: #999;
  font-size: 1em;
  margin: 15px 6px;
  text-transform:uppercase;
}

The HTML file should be named
book_information.php, and it should contain this code:

This panel contains information 
for displaying the book
  Author
  
  
  ISBN
  
  

If you've followed the steps, when you add a Book in the Books panel, a new box of fields (containing Author and the ISBN) will display. To display these fields in your theme, call the posts via the loop and grab the custom field information:

$loop = new WP_Query( array( 
  'post_type' => 'books', 
  'posts_per_page' => 10 ) 
);
while ($loop->have_posts()) : $loop->the_post();
  the_title();
  the_content();
  echo 'Author: ' . 
get_post_meta($post->ID, "author", true);
  echo 'ISBN: ' . 
get_post_meta($post->ID, "isbn", true);
endwhile;

That's it: your posts will display with the custom information. If you want to learn more about WordPress, check out our latest release Build Your Own Wicked WordPress Themes. Enjoy!