Post is in descendant category not working in home.php

Question

I created a custom function in functions.php to check if post_is_in_descendant_category and I'm controlling it from a plugin which is returning true is a post is in descendant category, it is working well

But it's not working in my home.php, the strange thing is that when I check if post_is_in_descendant_category for any post in home.php result stuck to true

I want to know the reason for that and how to fix it

my code for post_is_in_descendant_category

if ( ! function_exists( 'post_is_in_descendant_category' ) ) {
    function post_is_in_descendant_category( $cats, $_post = null ) {
        foreach ( (array) $cats as $cat ) {
            // get_term_children() accepts integer ID only
            $descendants = get_term_children( (int) $cat, 'category' );
            if ( $descendants && in_category( $descendants, $_post ) )
                return true;
        }
        return false;
    }
}

my plugin code:

function check_category(){  
    global $rtlDir;
    if ( $category_to_check = get_term_by( 'slug', 'category-3', 'category' )){
        if(post_is_in_descendant_category($category_to_check->term_id)){
            $rtlDir = true;
        }else{
            $rtlDir = false;
        }
    }

}
add_filter("wp","check_category");

Answer 1

home.php is an archive of posts, and when you view home.php and function in plugin run, (on 'wp' hook) there is no current post.

So the function post_is_in_descendant_category call in_category($descendants, null) that can never return true if there is no current post.

For this reason, you should run the plugin function on 'the_post' action hook, if you want it runs for every post in the archive, something like:

add_filter("the_post", "check_category");

function check_category($post) {
  // here your code
  global $rtlDir;
  if ( $category_to_check = get_term_by( 'slug', 'category-3', 'category' ) ){
    if( post_is_in_descendant_category($category_to_check->term_id, $post) ){
        $rtlDir = true;
    } else {
        $rtlDir = false;
    }
  }
}

This should work, but don't seems a good solution: this run an additional db query for every post...

If you explain why you run this code, and what you want to obtain probably is possible to find a more performant solution.