access.php

<?php
/*
Plugin Name: Access
Plugin URI: http://github.com/ryanve/access
Description: Control content access via a taxonomy that accepts user IDs, roles, or capabilities.
Version: 0.5.1
Author: Ryan Van Etten
Author URI: http://ryanve.com
License: MIT
*/

add_action('init', function() {
    $tax = 'access';
    $name = __('Access');
    $cases = array('denied', 'limited', 'granted');
    $avoid = array('nav_menu_item', 'revision');
    $types = array_diff(get_post_types(), $avoid);
    register_taxonomy($tax, $types, apply_filters("@$tax:ui", array(
        'hierarchical' => 1
      , 'public' => current_user_can('delete_others_posts')
      , 'labels' => array(
            'all_items' => __('All')
          , 'popular_items' => __('Popular')
          , 'edit_item' => __('Edit')
          , 'view_item' => __('View')
          , 'update_item' => __('Update')
          , 'search_items' => __('Search')
          , 'add_new_item' => __('Add')
          , 'new_item_name' => __('Name')
          , 'add_or_remove_items' => __('Add or remove')
          , 'choose_from_most_used' => __('Most used')
          , 'not_found' => __('Not found')
          , 'separate_items_with_commas' => 'Use WP roles, capabilities, or user IDs.'
          , 'name' => $name
          , 'singular_name' => $name
        ))
    ));

    # Accommodate late post types.
    add_action('init', function() use ($tax, &$types, &$avoid) {
        if (taxonomy_exists($tax))
            foreach (array_diff(get_post_types(), $types, $avoid) as $type)
                register_taxonomy_for_object_type($tax, $type);
        unset($types, $avoid);
    }, 100);
    
    # Define the "test access" callback via filter to enable override.
    add_filter("@$tax:test", function($fn, $user = null) use ($tax) {
        # Callback result grants (truthy) or denies (falsey) access. 
        # Grant when not applicable, when no terms are applied, or when *any* term passes.
        $applicable = taxonomy_exists($tax) && !is_404() and $user = $user ?: wp_get_current_user();
        return $applicable ? function($post = null) use ($tax, $user) {
            $grant = 1;
            $terms = get_the_terms($post, $tax);
            if ($terms && is_array($terms) and $grant--)
                foreach (wp_list_pluck($terms, 'slug') as $slug)
                    if ($grant = is_numeric($slug) ? $user->id === $slug : $user->has_cap($slug))
                        break;
            return !!$grant;
        } : '__return_true';
    }, 0, 2);
    
    add_action('loop_start', function(&$query) use ($tax, $cases) {
        $test = apply_filters("@$tax:test", null);
        $denies = array();
        foreach ($query->posts as $i => $post)
            is_int($i) && call_user_func($test, $post) or $denies[] = array_splice($query->posts, $i, 1)[0];
        $case = $cases[$denies ? $query->posts ? 1 : 0 : 2];
        $hold = 'message';
        $hook = "@$tax:$hold";
        $msg = apply_filters($hook, '', $query->posts, $denies);
        foreach (array_diff($cases, array($case)) as $not)
            $msg = apply_filters("$hook:!$not", $msg, $query->posts, $denies);
        $msg = apply_filters("$hook:$case", $msg, $query->posts, $denies);
        if ($msg) echo "<div class='loop-$tax $tax-$hold $hold-$case'>$msg</div>\n\n";
    });
    
    # Define the "contextual CSS" callback via filter to enable override.
    add_filter("@$tax:contextualize", function($fn) use ($tax, $cases) {
        $all = array_map(function($c) use ($tax) { return "$tax-$c"; }, $cases);
        return function($classes) use ($tax, $cases, $all) {
            $classes = $classes ? array_diff((array) $classes, $all) : array();
            $grant = call_user_func(apply_filters("@$tax:test", null)) ? 2 : 0;
            $case = $cases[$grant];
            $classes[] = $all[$grant];
            do_action("@$tax:$case@" . current_filter());
            return $classes;
        };
    }, 0);
    
    is_admin() or add_action('wp', function() use ($tax) {
        if ($contextualize = apply_filters("@$tax:contextualize", null))
            foreach (array('post_class', 'body_class') as $hook)
                add_filter($hook, $contextualize);
    });
}, 1);