template_redirect for single posts w/ custom fields

Is there any way I can use is_single() inside of my plugin’s functions.php file? Currently, this is what my code looks like:

if(is_single()) :
    function my_template() {
        include(PLUGINDIR . '/supersqueeze/all.php');
        exit;
    }
    add_action('template_redirect', 'my_template');
endif;

But for some reason, it is not working at all. If I remove if(is_single()), it works but is for all pages.

And then once I get that working, I’ll need to filter it once more to see if the post has a certain custom field value, lets say the name will be Design and the value will be Custom.

Thanks in advance for anyone who can help me out.

The problem with your code is that you’re checking is_single() when your plugin is first loaded, before the global query has run, so is_single() still returns false. You should move the is_single() check within your my_template function:

function my_template() {
    if(is_single() && have_posts() && 'Custom' == get_post_meta(get_the_ID(), 'Design')) {
    include(PLUGINDIR . '/supersqueeze/all.php');
    exit;
    }
}
add_action('template_redirect', 'my_template');