How to add and subtract user meta values after post meta update

Question

Basically, I'm trying to make a point exchange between current user and author when the post meta value is updated.

As well as add +1 to post meta.

Here's what I currently have:

    // Coin exchange function
    // 
    
    add_action( 'updated_post_meta', 'coin_exchange_after_post_meta_update', 10, 4 );
    function coin_exchange_after_post_meta_update( $meta_id, $post_id, $meta_key, $meta_value )
    {
        
        $current_user = wp_get_current_user();
        $current_user_id =  $current_user->ID;
        $author_id = get_post_field( 'post_author', $post_id );
    
         
                    if ( 'postcoins' == $meta_key ) {
                        
                    update_user_meta( $current_user_id, 'usercoins', ''); // subtract -1 from current user
                    update_user_meta( $author_id, 'usercoins', ''); // add +1 to post author    
                    update_post_meta( $post_id, 'postcoins', '' ); // add +1 to post meta
        }
     }

The only thing that's left is to figure out how to subtract and add values.

I know there's a way using sum=0, but I'm struggling with the logic.

I would really appreciate your help.

Answer 1

Basically, I have ditched the function, and now update meta data with a click on a hyperlink.

It fires this code, and does exactly what I wanted.

In case anyone interested:

$current_user = wp_get_current_user();
$current_user_id =  $current_user->ID;
$author_id = get_post_field( 'post_author', $post_id );

$user_coins = get_user_meta( $current_user_id, 'usercoins' , true );
$author_coins = get_user_meta( $author_id, 'usercoins' , true );
$post_coins = get_post_meta( $post_id, 'postcoins', true );

$user_coins_sum = $user_coins - 1;
$author_coins_sum = $author_coins + 1;
$post_coins_sum = $post_coins + 1;

         
                
                update_user_meta( $current_user_id, 'usercoins', $user_coins_sum); 
                update_user_meta( $author_id, 'usercoins', $author_coins_sum); 
                update_post_meta( $post_id, 'postcoins', $post_coins_sum );