Restrict widgets to specific posts

Hi and thank you.

THe code snippet you posted can handle that. You’d have to create multiple conditions like this:

function custom_rw_display_widget ( $display, $instance ) {
	if ( $instance['title'] == 'Paris offers' ) { // your Paris offer widget title
		if ( is_single( array( 1, 2, 3 ) ) ) { // ann array of post ids related to Paris
			$display = true; // or false if you want to hide it
		}
	} elseif ( $instance['title'] == 'Berlin offers' ) {
		if ( is_single( array( 4, 5, 6 ) ) ) { // an array of post ids related to Berlin
			$display = true; // or false if you want to hide it
		}
	}
	// and so on
	return $display;
}
add_filter( 'rw_display_widget', 'custom_rw_display_widget', 10, 2 );

However this may be time consuming, depending on how many widgets and offers you have and how actively it’s being updated. But you say you posts are categorized with Citytrip category. Is that a category taxonomy or a custom taxonomy? In any case if your posts are assigned to Citytrip category that contains subcategories for e.g. with city names (Paris, Berlin, Cracow, etc.) you can do this better, faster, more automatic.

function custom_rw_display_widget ( $display, $instance ) {
	// break if we're not on single post
	if ( ! is_single() )
		return $display;

	// get current post id
	$post_id = get_the_ID();
	
	if ( $instance['title'] == 'Paris offers' ) {
		// let's check if current post is assigned to a Paris subcategory in Citytrip category
		// 1 is an ID of Paris
		if ( has_term( 1, 'category', $post_id ) ) {
			$display = true;
		}
	} elseif ( $instance['title'] == 'Berlin offers' ) {
		// 2 is an ID of Berlin subcategory in Citytrip category 
		if ( has_term( 2, 'category', $post_id ) ) {
			$display = true;
		}
	}
	// and so on
	return $display;
}
add_filter( 'rw_display_widget', 'custom_rw_display_widget', 10, 2 );

It’s possible for pages only. Other single posts have to be handled via the code above. Maybe we’ll provide dome interface for this, but it’s not available yet.