How to do a query only on a specific admin page?

I’m making a page for settings API using Codestar Framework, and loading fields using their filterable configure – question is not related to Codestar. In one of such dropdown field, I need to load all the posts from a custom post type that are added within 30 days. To make the things nice and clean I made a custom function:

<?php
/**
 * Get posts of last 30 days only.
 * 
 * @return array Array of posts.
 * --------------------------------------------------------------------------
 */
function pre_get_active_posts_of_30_days() {

    //admin-only function
    if( !is_admin() )
        return;

    global $project_prefix; //set a project prefix (i.e. pre_)
    $latest_posts = new WP_Query(
                            array(
                                'post_type'         => 'cpt',
                                'post_status'       => 'publish',
                                'posts_per_page'    => -1,
                                'date_query'        => array(
                                                        array(
                                                            'after'     => '30 days ago',
                                                            'inclusive' => true,
                                                        ),
                                                    )
                            )
                        );

    $posts_this_month = array();

    if( $latest_posts->have_posts() ) : while( $latest_posts->have_posts() ) : $latest_posts->the_post();

        $validity_type  = get_post_meta( get_the_ID(), "{$project_prefix}validity_type", true );
        if( $validity_type && 'validity date' === $validity_type ) {
            $validity   = get_post_meta( get_the_ID(), "{$project_prefix}validity", true );
            $tag        = days_until( $validity ) .' days left'; //custom function
        } else if( $validity_type && 'validity stock' === $validity_type ) {
            $tag        = 'Stock';
        } else {
            $tag        = '...';
        }

        $posts_this_month[get_the_ID()] = get_the_title() .' ['. $tag .']';

    endwhile; endif; wp_reset_postdata();

    return $posts_this_month;
}

Question is not even with the function.

I want to do the query only on that particular top_level_custom-settings-api page. The function is loading on every page load of admin. I tried using get_current_screen() but this function’s giving me not found fatal error.

Edit

No @bonger, I remembered that. I tried your code in this way:

add_action('current_screen', 'current_screen_callback');
function current_screen_callback($screen) {
    if( is_object($screen) && $screen->id == 'top_level_custom-settings-api' ) {
        add_action( 'admin_init', 'pre_get_active_posts_of_30_days' );
    }
}

The code does work well, but it doesn’t control my function loading only on that particular page. I checked the query on other pages, the query is there too. And I tried changing the conditional to something wrong, like $screen->id == 'top_level_-api', it still works that way. 🙁

I’m afraid, I know I have a serious lack in behind-the-scene action and filter things. Would love to have a good read for that too.

It’s worthwhile pointing out that using admin_init within the current_screen filter is too late because admin_init has already fired.

Instead do:

add_action('current_screen', 'current_screen_callback');

function current_screen_callback($screen) {
    if( is_object($screen) && $screen->id === 'top_level_custom-settings-api' ) {
        add_filter('top_level_screen', '__return_true');
    }
}

Elsewhere in your top_level_page_callback callback responsible for executing the query:

function top_level_page_callback() {

    $active_posts = null;

    if ( ($is_top_level = apply_filters('top_level_screen', false)) ) {

        $active_posts = pre_get_active_posts_of_30_days();

    }

    //etc...

}

That’s one way to do it…

Or you could use, add_action('load-top_level_custom-settings-api', 'callback');

Other than the current_screen hook, where else were you trying to call pre_get_active_posts_of_30_days() from? Because you had to have been calling it in a global scope of some sort for it to be running on all pages and not just the target page.