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That would not work. Just because a callback is registered to a hook doesn't mean that the hook will be triggered when expected. The only thing that would sort-of-kind-of work is using the 'shutdown' hook, which you mentioned before:

add_action( 'shutdown', function() {
  if ( !did_action( 'template_tag_like_hook' ) )
    echo 'Problem.';
} );

Of course, this would be printed at the very bottom, after the </html> tag, on the front-end (since that's where template tags are normally used), which is not of much use.

You could try to store the message in wp_options and then display it in the admin area, but that would open a whole new can of worms: invalidation, caching plugins etc.

Using hooks would works well for 'normal' and 'weak' dependencies. The only drawback is that you would still need to use function_exists() or is_plugin_active() if you want to stop if the dependency is not met. Using the 'all' filter for that would be too expensive IMO.

Using hooks would works well for 'normal' and 'weak' dependencies. The only drawback is that you would still need to use function_exists() or is_plugin_active() if you want to stop if the dependency is not met. Using the 'all' filter for that would be too expensive IMO.

@scibu This was targeted at "your" topic. (I already dropped talking about mine). :)

So basically, if you need a dependency - and you have a nice author - then he could offer a hook instead/as replacement for a template tag. Because the plugin would only hook into it if the hook would be present, or simply do nothing. And on the other side you wouldn't have an error, when the plugins not present.

Here's the tough part (or more of a Q): To write an admin notice to inform the user about the dependancy "You need to install »DisneyWonderLinks«", you could check the array_keys( $GLOBALS['wp_filter']['template_tag_like_hook'] ). I'm not sure if this would work, but afaik the array should be accessible on both (public/admin) sides.

Using hooks would works well for 'normal' and 'weak' dependencies. The only drawback is that you would still need to use function_exists() or is_plugin_active() if you want to stop if the dependency is not met. Using the 'all' filter for that would be too expensive IMO.