set_current_screen() 前に管理画面のポストタイプだったりを取得するクラス

after_setup_theme だったり init など、$GLOBALS['typenow']、$GLOBALS['taxnow'] が定義されていない段階で取得する。
###code

<?php

class WPQuickScreen {

	/**
	 * @var string
	 */
	private $pagenow;

	/**
	 * @var null|string
	 */
	private $typenow;

	/**
	 * @var null|string
	 */
	private $taxnow;

	private static $query_filter_diffinitions = array(
		'action'    => \FILTER_SANITIZE_ENCODED,
		'post_type' => \FILTER_SANITIZE_ENCODED,
		'taxonomy'  => \FILTER_SANITIZE_ENCODED,
		'post'      => \FILTER_VALIDATE_INT,
		// and more
	);

	public static function getInstance() {
		static $instance;
		return $instance ?: $instance = new self();
	}

	public function __get( $name ) {
		return property_exists( $this, $name ) ? $this->$name : null;
	}

	private function __construct() {
		if ( ! is_admin() ) {
			return;
		}
		global $pagenow;
		$this->pagenow = $pagenow;
		$this->parse_query();
	}

	private function parse_query() {
		$q = filter_input_array( INPUT_GET, self::$query_filter_diffinitions );
		switch ( $this->pagenow ) {
			case 'edit.php' :
			case 'post-new.php' :
				$this->typenow = $q['post_type'] ?: 'post';
				break;
			case 'post.php' :
				/**
				 * @see https://ja.forums.wordpress.org/topic/150122
				 */
				$this->typenow = $q['post'] ? get_post_type( $q['post'] ) : filter_input( \INPUT_POST, 'post_type' );
				break;
			case 'edit-tags.php' :
			case 'term.php' :
				$this->taxnow = $q['taxonomy'] ?: 'post_tag';
				break;
			case 'index.php' :
				// var_dump( 'Dashboard!!!!!' );
				break;
		}
	}

}

###usage

function.php

<?php
add_action( 'after_setup_theme', function() {
	if ( !is_admin() ) {
		return;
	}
	global $pagenow;
	var_dump( $typenow ); // null

	$qscreen = WPQuickScreen::getInstance();
	var_dump( $qscreen->typenow );
} );