How a Wordpress metabox manages to find the save function

Question

I am making a wordpress metabox and i was wondering how the html part of the metabox manages to find the save function.Here is the entire code i am using which works

<?php
function true_add_a_metabox() {
    add_meta_box(
        'true_metabox', // metabox ID, it also will be it id HTML attribute
        'The Detailed Custom Meta Box', // title
        'true_display_metabox', // this is a callback functions, which will be print HTML of our metabox
        'post', // post type
        'normal', // position of the screen where metabox shoul be displayed (normal, side, advanced)
        'default' // priority over another metaboxes on this page (default, low, high, core)
    );
}

add_action( 'admin_menu', 'true_add_a_metabox' );

function true_display_metabox($post) {
    /*
     * needs for security checks
     */
    wp_nonce_field( basename( __FILE__ ), 'true_metabox_nonce' );
    /*
     * lets add a simple textarea field
     */
    $html .= '<p><label>SEO title <input type="text" name="seotitle" value="' . get_post_meta($post->ID, 'true_title',true) . '" /></label></p>';
    /*
     * add a checkbox
     */
    $html .= '<p><label><input type="checkbox" name="noindex"';
    $html .= (get_post_meta($post->ID, 'true_noindex',true) == 'on') ? ' checked="checked"' : '';
    $html .= ' /> Turn of page visibility for search engines</label></p>';
    /*
     * print all of this
     */
    echo $html;
}

function true_save_post_meta( $post_id, $post ) {
    /* 
     * Security checks
     */
    if ( !isset( $_POST['true_metabox_nonce'] ) || !wp_verify_nonce( $_POST['true_metabox_nonce'], basename( __FILE__ ) ) )
        return $post_id;
    /* 
     * Check current user permissions
     */
    $post_type = get_post_type_object( $post->post_type );
    if ( !current_user_can( $post_type->can->edit_post, $post_id ) )
        return $post_id;
    /*
     * Check if the autosave
     */
    if ( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE ) 
        return $post_id;

    if ($post->post_type == 'post') { // define your own post type here
        update_post_meta($post_id, 'true_title', esc_attr($_POST['seotitle']));
        update_post_meta($post_id, 'true_noindex', $_POST['noindex']);
    }
    return $post_id;
}

add_action( 'save_post', 'true_save_post_meta', 10, 2 );

?>

In the function that produces the html true_display_metabox there is no mention of true_save_post_meta which saves the options.Can anyone explain how this metabox manages to the save the data?.

Answer 1

You are calling true_save_post_meta on the save_post action (in the last line of your code). This means that every time the post is saved the true_save_post_meta function will run. The data from your meta box will be included in the $_POST object, which true_save_post_meta then uses to save those values in the database.