Call add_action() in function wordpress

Question

I have a problem with a WordPress hook. I want to call an action in another actions callback, but it doesn't seem to work. I want to call add_meta_tag action only if the page is saved. This is what I have:

function saveCustomField($post_id)
{
    add_action( 'wp_head', 'add_meta_tag' );
}
add_action( 'save_post', 'saveCustomField' );

function add_meta_tag(){
    echo "TEST";
}

How can I get the above to work properly?

Answer 1

You're thinking about this entirely wrong. A meta tag isn't something you add when a post gets saved, it's something that gets added to the output when a post is viewed.

So instead of trying to hook the action inside save_post, you hook it on every page load, and inside the hook you check if your custom field exists on the post being viewed. If it is, you output the tag.

function wpse_283352_add_meta_tag() {
    if ( is_singular() {
        $post_id = get_queried_object_id();
        $meta = get_post_meta( $post_id, '_my_custom_field', true );

        if ( $meta ) {
            echo '<meta name="my_custom_field" content="' . esc_attr( $meta ) . '">';
        }
    }
}
add_action( 'wp_head', 'wpse_283352_add_meta_tag' );

That function just goes in your plugin file/functions file, not inside any other hook.

Answer 2

The save_post hook is fired after the data is saved to the database - whenever post or page is created or updated - only on admin pages.

The wp_head hook fires when wp_head() function is run and this do not happen on admin pages.

This won't work. Where and when you want to add the meta tag?

Answer 3

The short answer: you can't.

The code is fine, but there is a flaw in logic. wp_head action is set too late. Headers are already sent, therefore wp_head will never fire! This code below, will prove two points. First: save_post will fire on both, admin pages, and on front end, as well. Second: it's easy to prevent an infinite loop in callback function.

In functions.php:

function saveCustomField($post_id) {

    // to prevent an infinite loop
    remove_action('save_post', 'saveCustomField', 10);

    // to prove that function was called
    error_log('I am here to add action');

    add_action('wp_head', 'add_meta_tag');
}
add_action('save_post', 'saveCustomField');

function add_meta_tag(){
    error_log('TEST');
    //echo "TEST";
}

In my page template ( front end ):

wp_update_post(array('ID' => 79, 'post_title' => 'My Current Test',));

In error_log:

[19-Oct-2017 02:57:08 UTC] I am here to add action

My post's title had been updated, save_post was fired, and wp_head wasn't.

Answer 4

I want to add the meta tag when publish/update button is fired. I have a checkbox on the admin page that add/remove meta tag. For info, it's intended for a plugin.

function saveCustomField($post_id)
{
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
        return;
}

if (
    !isset($_POST['my_custom_nonce']) ||
    !wp_verify_nonce($_POST['my_custom_nonce'], 'my_custom_nonce_'.$post_id)
) {
    return;
}

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

if (isset($_POST['_my_custom_field'])) {
    update_post_meta($post_id, '_my_custom_field', $_POST['_my_custom_field']);
    // here
    // >>>>> add_action('wp_head', 'add_meta_tag');
} else {
    delete_post_meta($post_id, '_my_custom_field');
}

}

add_action( 'save_post', 'saveCustomField' );

function add_meta_tag(){
    echo "TEST";
}