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I am very confused about what the developer has done here. This is inside a class..a filter has been added and then removed, added again. This code results in many errors as well.

add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), -1, 4);

and the callback function

public static function get_post_metadata($meta_value, $post_id, $meta_key, $single){
        if (is_admin()){
            return null;
        }

        remove_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), 0);
        $child_meta = get_metadata('post', $post_id, $meta_key, false);
        $meta=null;
        if (WPKWP::get_parent()){
            $parent_meta = get_metadata('post', WPKWP::get_parent(), $meta_key, false);

            if (is_array($parent_meta) && is_array($child_meta)) {
                $meta = array_merge($parent_meta, $child_meta);
            } else {
                $meta = $parent_meta;
            }
        } else {
            $meta = $child_meta;
        }
        add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), 0, 4);
        return $meta;
    }

Closed. This question needs details or clarity. It is not currently accepting answers.

---

Want to improve this question? Add details and clarify the problem by editing this post.

Closed 5 years ago.

Improve this question

I am very confused about what the developer has done here. This is inside a class..a filter has been added and then removed, added again. This code results in many errors as well.

add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), -1, 4);

and the callback function

public static function get_post_metadata($meta_value, $post_id, $meta_key, $single){
        if (is_admin()){
            return null;
        }

        remove_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), 0);
        $child_meta = get_metadata('post', $post_id, $meta_key, false);
        $meta=null;
        if (WPKWP::get_parent()){
            $parent_meta = get_metadata('post', WPKWP::get_parent(), $meta_key, false);

            if (is_array($parent_meta) && is_array($child_meta)) {
                $meta = array_merge($parent_meta, $child_meta);
            } else {
                $meta = $parent_meta;
            }
        } else {
            $meta = $child_meta;
        }
        add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), 0, 4);
        return $meta;
    }

• filters
• post-meta
• actions

Share Improve this question asked Dec 10, 2019 at 2:28 user145078user145078

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1 Answer 1

Sorted by: Reset to default 1

The remove_filter() is necessary to avoid the callback from being called recursively, which would result in a memory issue due to a never-ending function execution.

And that recursion could happen because the callback is hooked to get_post_metadata which is invoked via the get_metadata() function which the callback calls.

So you need to first "unhook"/unregister the callback from the hook and only then you may call get_metadata().

Otherwise, you'd end up with something like:

WP applies the filter (calls apply_filters( 'get_post_metadata', ... ))
- your callback is called
- the callback calls get_metadata()
- WP applies the filter
  - your callback is called
  - the callback calls get_metadata()
  - WP applies the filter
      - your callback is called
      - the callback calls get_metadata()
      - WP applies the filter.....

Or:

your_callback() {
  your_callback() {
    your_callback() {
      your_callback() {
        ..... it never ends ...

So your code is good in that it's avoiding the unwanted recursion.

However, I noticed that you're not properly removing the filter:

// You added the filter like so:
add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), -1, 4);

// Then in the callback, you're removing it like this:
remove_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), 0);

// And later in that callback, the filter is added back:
add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), 0, 4);

And the problem is, when removing a filter/action, the priority and number of accepted parameters must match the same values you used when adding the filter:

// If you added the filter like so:
add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), -1, 4);

// Then use the same parameters when removing the filter:
remove_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), -1, 4);

// And not like this — priority should be -1 and this is missing the fourth parameter:
remove_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), 0);

And if you initially added the filter like so, then you should also add it back inside the callback the same way — using the same parameters:

// Like this:
add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), -1, 4);

// Not this: (priority doesn't match)
add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), 0, 4);