_wp_preview_post_thumbnail_filter() │ WP 4.6.0

Filters post thumbnail lookup to set the post thumbnail.

Internal function — this function is designed to be used by the kernel itself. It is not recommended to use this function in your code.

No Hooks.

Returns

null|Array. The default return value or the post thumbnail meta array.

Usage

_wp_preview_post_thumbnail_filter( $value, $post_id, $meta_key );
$value(null|array|string) (required)
The value to return - a single metadata value, or an array of values.
$post_id(int) (required)
Post ID.
$meta_key(string) (required)
Meta key.

Changelog

Since 4.6.0 Introduced.

_wp_preview_post_thumbnail_filter() code WP 7.0

wp-includes/revision.php 
function _wp_preview_post_thumbnail_filter( $value, $post_id, $meta_key ) {
	$post = get_post();

	if ( ! $post ) {
		return $value;
	}

	if ( empty( $_REQUEST['_thumbnail_id'] ) || empty( $_REQUEST['preview_id'] )
		|| $post->ID !== $post_id || $post_id !== (int) $_REQUEST['preview_id']
		|| '_thumbnail_id' !== $meta_key || 'revision' === $post->post_type
	) {
		return $value;
	}

	$thumbnail_id = (int) $_REQUEST['_thumbnail_id'];

	if ( $thumbnail_id <= 0 ) {
		return '';
	}

	return (string) $thumbnail_id;
}