How to ensure "the_content" filter runs only for the main displayed content?

Question

I have a plugin which adds the method to "the_content" filter.

add_filter('the_content', 'myFilteringFunction', 10000);

Within that function I want to add some links on the beginning and the end of the content. But I only need to do it for the "main" content of the displayed page so - not in the any of the widgets, not in the footer, header etc.

Moreover I only want it to be included for the custom post type which I defined in the same plugin. So I figured out that kind of check, thinking it would be enough.

if( is_single() && get_query_var('post_type') == 'myCustomPostType' && is_main_query() )

Unfortunately it's not working as intended - at least not in every case.

On the page the plugin WP Types is installed, it's not working (the links are added despite the condition). Why?

Answer 1

According to Wordpress documentation, you can accomplish this using this code:

add_filter( 'the_content', 'filter_the_content_in_the_main_loop' );

function filter_the_content_in_the_main_loop( $content ) {

    // Check if we're inside the main loop in a single post page.
    if ( is_single() && in_the_loop() && is_main_query() ) {
        return $content . "I'm filtering the content inside the main loop";
    }

    return $content;
}

Or use the same logic via a single line at the beginning of the filter callback function:

if( !is_single() || !in_the_loop() || !is_main_query() ) return $content;

Answer 2

Try adding a condition to your filtering function that checks your post type against get_post_type.

if ( 'book' == get_post_type() )

If you wish to apply this filter to pages as well, try is_singular() and include your custom post type(s) as an argument.

is_singular('book');

This will return true if any of the following conditions are true:
is_single()
is_page()
is_attachment()
'book' == get_post_type()