<?php

class AFA_Post_Notice {

	public function __construct($editor) {
		$editor->initialize();
	}

	public function initialize() {
		add_action( 'admin_enqueue_scripts', array($this, 'enqueue_styles') );
		add_action( 'admin_enqueue_scripts', array($this, 'enqueue_scripts') );

		add_action('admin_menu', array($this, 'register_admin_page') );
		add_action('admin_init', array($this, 'register_plugin_settings_options') );
	}

	public function register_admin_page() {
		add_options_page(
			'Post Notice Settings', // Title
			'AFA Post Notice',  // Name
			'manage_options',  // Capabilities
			'afa-post-notice', // Slug
			array($this, 'plugin_options_page') // Callback
		);
	}

	public function plugin_options_page() {
		require_once (WP_PLUGIN_DIR . '/afa-post-notice/views/afa-post-notice-options.php');
	}

	public function register_plugin_settings_options() {
		register_setting( 'afa-post-notice-option', 'post_notice_screens' );
		register_setting( 'afa-post-notice-option', 'where_to_show' );
	}

	public function enqueue_styles() {
		wp_enqueue_style(
			'afa-post-notice-admin',
			plugins_url( 'afa-post-notice/assets/css/admin.css' ),
			array(),
			'0.1.0'
		);
	}

	public function enqueue_scripts() {
		wp_enqueue_script(
			'afa-post-notice-admin',
			plugins_url( 'afa-post-notice/assets/js/admin.js' ),
			array('jquery'),
			'0.1.0'
		);
	}
}



******


<div>
    <h2>Post Notice Settings</h2>
    <form action="options.php" method="post">
        <?php settings_fields('afa-post-notice-option'); ?>
        <?php do_settings_sections('afa-post-notice-option'); ?>
        <table class="form-table">
            <tr valign="top">
                <th scope="row">Select Screen</th>
                <td>
                    <?php
						$args = array(
						   'public'   => true
						);
					?>
                    <?php $post_types = get_post_types($args); ?>
                    <select name="post_notice_screens[]" id="post_notice_screens" multiple>
                        <?php foreach($post_types as $key => $postType) : ?>
                        <option
                        	value="<?php echo $key; ?>"
                        	<?php echo ( get_option( 'post_notice_screens') && in_array($key, get_option( 'post_notice_screens')) ) ? 'selected="selected"' : null; ?>
                        ><?php echo $key; ?></option>
                        <?php endforeach; ?>
                    </select>
            </tr>
            <tr valign="top">
                <th scope="row">Where To Show</th>
                <td>
                	<?php $beforeToShow = esc_attr( get_option('where_to_show') ); ?>
                	<input type="radio" name="where_to_show" value="before" <?php echo ($beforeToShow == 'before') ? 'checked="checkded"' : null; ?>> Before <br>
                	<input type="radio" name="where_to_show" value="after" <?php echo ($beforeToShow == 'after') ? 'checked="checkded"' : null; ?>> After
                </td>
            </tr>
        </table>
        <?php submit_button(); ?>
    </form>
</div>

******

public function display_notice($content) {
	$post_notice = get_post_meta( get_the_ID(), 'afa-post-notice-display', true );

	if ( $post_notice != '' ) {

		$whereToShow = esc_attr( get_option('where_to_show') );
		$notice_html = '<div class="afa-post-notice-display">' . $post_notice . '</div>';

		if($whereToShow == 'before' || !$whereToShow) {
			$content = $notice_html . $content;
		} else {
			$content = $content . $notice_html;
		}

	}

	return $content;
}

*****


function add_meta_box() {

	$allowedScreens = get_option('post_notice_screens');
	$screen = get_current_screen();

	if( ! in_array($screen->id, $allowedScreens) ) {
		return;
	}

	add_meta_box(
		'afa-post-notice',
		'Post Notice',
		array( $this, 'post_notice_display' ),
		$screen->id,
		'normal',
		'high'
	);

}