WordPress Plugin: Ping screenshots capture generation using an hidden iframe only once for published posts

ping-screenshot-capture.php

<?php
/**
 * Plugin Name: Ping Screenshots Capture
 * Description: Ping screenshots capture generation using an hidden iframe only once for published posts
 * Version:     1.0.0
 * Author:      birgire
 */

namespace PingScreenshotCapture;

add_action( 'admin_init', function()
{
	$main = new Main;
		
	// Adjust to your needs
	$main
		->set_src( 'http://example.tld' )
		->init();
} );


/**
 * Main Class
 */
class Main 
{
 
	/**
	 * @val string
	 */
	private $src;
 
	/**
	 * Set source
	 */
	 public function set_src( $src )
	 {
		 $this->src = $src;
		 return $this;
	 }
	 
 	/**
	 * Init
	 */
	public function init() 
	{
        	add_action( 'add_meta_boxes', array( $this, 'add_metabox'  )        );
	}
 
	/**
	 * Add the meta box.
	 */
	public function add_metabox() 
	{
        	add_meta_box(
	            'ping-screenshot-capture-meta-box',
        	    __( 'Ping Screenshot Capture', 'psc' ),
        	    array( $this, 'render' ),
			'post',
			'advanced',
			'default'
		); 
	}
 
	/**
	 * Render the meta box.
	 */
	public function render( $post ) 
	{
		$pinged = get_post_meta( $post->ID, 'pinged_screenshot_capture', true );
		
		if(     empty( $pinged ) 
			&& 'publish' === get_post_status( $post ) 
		)
		{
			// Ping
			$out = sprintf( 
				'<p>%s</p><iframe style="visibility: hidden; height: 0; width: 0; border: none; " src="%s"></iframe>', 
				esc_html__( "Let's Ping", 'psc' ),
				esc_url( $this->src )
			);
			
			// Record that we've just pinged
			update_post_meta( $post->ID, 'pinged_screenshot_capture', 'done' );
		}
		elseif( ! empty( $pinged ) ) 
		{
			$out = esc_html__( 'Already Pinged', 'psc' );			
		}
		else
		{
			$out = esc_html__( 'Will be pinged during publish', 'psc' );
		}
	
	        // Display		
		printf( '<div>%s</div>', $out );
		
	}
 
} // end class