How to use add_action for multiple instances of the same class

Question

I'm writing a plugin which defines a class that is meant to be called multiple times. The class uses add_action to register it's own methods as callbacks.

class MyClass {

    __construct($foo) {

        add_action('hook1', array($this, 'method1') );
        add_action('hook2', array($this, 'method2') );

    }

    public function method1() {...}

    public function method2() {...}

}

This works just fine. But when I do it twice, only the second works.

new MyClass('foo');
new MyClass('bar');

In other words, the results of the code above are exactly what I'd expect from the following:

// new MyClass('foo');
new MyClass('bar');

It seems like the second instance overrides the first. What am I doing wrong?

I've checked to make sure that spl_object_hash() is different for both instances and it is.

I added var_dump($wp_filter); after my code and I see BOTH instances listed under the relevant action hook key. So it looks like WP knows they are both there...

Full code from my complete plugin:

Paste this into a file in /wp-content/mu-plugins.php and then add/edit a post. You should see two new custom meta boxes "Foo" and "Bar" but you will only see "Bar". Comment in/out the final lines to test with different combinations of instances of the class.

<?php
/*
Plugin Name: SDP Custom Meta Boxes
Description: Defines a custom meta box framework class.
*/

class SDP_Custom_Field {

  public $name = NULL;
  public $slug = NULL;
  public $prefix = NULL;
  public $post_type = 'post'; //The type of Write screen on which to show the meta: 'post', 'page', 'dashboard', 'link', 'attachment' or 'custom_post_type'
  public $meta_box_location = 'normal'; //'normal', 'advanced', or 'side'
  public $meta_box_priority = 'default'; //'high', 'core', 'default' or 'low'
  public $meta_box_field_type = NULL;

  public function __construct( $custom_field_name, $args=array() ) {
    #set the name
    $this->name = $custom_field_name;

    #set the slug
    if (isset($args['slug'])) {
        $this->slug = $args['slug'];
    } else {
        $slug = strtolower($custom_field_name);
        $slug = str_replace(' ', '_', $slug);
        $this->slug = $slug;      
    }

    #set the meta_box_field_type
    if (isset($args['field_type'])) {
        switch ($args['field_type']) {
            case 'text':
                $this->meta_box_field_type = 'text';
                break;
            case 'textarea':
                $this->meta_box_field_type = 'textarea';
                break;            
            default:
                $this->meta_box_field_type = 'text';
                break;
        }

    } else {
        $this->meta_box_field_type = 'text';
    }

    // add_action( 'add_meta_boxes', array($this, '_add_custom_metaboxes'));
    // add_action( 'save_post', array($this, '_save_postdata'));

    add_action( 'add_meta_boxes', array(&$this, '_add_custom_metaboxes'));
    add_action( 'save_post', array(&$this, '_save_postdata'));

    //var_dump( is_object( array($this, '_add_custom_metaboxes') ));

  }

  // public function go() {
  //   add_action( 'add_meta_boxes', array($this, '_add_custom_metaboxes'));
  //   add_action( 'save_post', array($this, '_save_postdata'));
  // }

  #Echo the HTML for this meta box... 
  public function _print_field_HTML( $post ) {

      $value = get_post_meta( $post->ID, $this->slug, true );

      // Add an nonce field so we can check for it later.
      wp_nonce_field( $this->slug.'_custom_box', $this->slug.'_custom_box_nonce' );


      echo '<label for="'.$this->slug.'">'.$this->name.'</label> ';
      echo '<input type="text" id="'.$this->slug.'" name="'.$this->slug.'" value="' . esc_attr( $value ) . '" size="25" />';

      //TODO: Add update button
      //TODO: Add delet button
  }

  public function _add_custom_metaboxes() {
      #http://codex.wordpress.org/Function_Reference/add_meta_box
      add_meta_box( NULL, $this->name, array(&$this, '_print_field_HTML'), $this->post_type, $this->meta_box_location, $this->meta_box_priority, NULL );
  }




  /**
   * When the post is saved, saves our custom data.
   * @param int $post_id The ID of the post being saved.
   */
  public function _save_postdata( $post_id ) {

    /*
     * We need to verify this came from the our screen and with proper authorization,
     * because save_post can be triggered at other times.
     */

    // Check if our nonce is set.
    if ( ! isset( $_POST[$this->slug.'_custom_box_nonce'] ) )
      return $post_id;

    #http://codex.wordpress.org/Function_Reference/wp_nonce_field
    $nonce = $_POST[$this->slug.'_custom_box_nonce'];

    // Verify that the nonce is valid.
    if ( ! wp_verify_nonce( $nonce, $this->slug.'_custom_box' ) )
        return $post_id;

    // If this is an autosave, our form has not been submitted, so we don't want to do anything.
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
        return $post_id;


      if ( ! current_user_can( 'edit_post', $post_id ) )
          return $post_id;

    /* OK, its safe for us to save the data now. */

    // Sanitize user input.
    $mydata = sanitize_text_field( $_POST[$this->slug] );

    // Update the meta field in the database.
    update_post_meta( $post_id, $this->slug, $mydata );
  }

}

new SDP_Custom_Field( 'Foo' );
new SDP_Custom_Field( 'Bar' );

Answer 1

When you call add_meta_box() you must provide a unique ID as the first parameter.

Consider this case:

add_meta_box( 'foobox', 'Foo Title', 'foo_callback' );
add_meta_box( 'foobox', 'Bar Title', 'bar_callback' );

You will just get the Bar Title metabox now, because it will overwrite the first one.

What you need is a new ID for each box:

add_meta_box( 'foobox', 'Foo Title', 'foo_callback' );
add_meta_box( 'barbox', 'Bar Title', 'bar_callback' );

Try to create an ID from $custom_field_name or use a static counter.

Besides that, you should change your code in two other areas:

1. Get rid of that god class, separate it into multiple classes:

   • one for the output, the metabox markup
   • one for the nonce. Example
   • one to get and to save the data
   • a controller to stick these classes together

2. Fix your nonce, or your plugin will break badly on multi-site installations.