Outputting something based on the presence of post variable in admin screens

Question

Lots of times, I might check for the presence of a variable in the post data and then do something. I'm having a hard time understanding what is going on when the user creates or updates a post. I have no problem doing any kind of backend function, but when I want to output something, I guess the page is re-directed? How do you normally handle this common task? How do I create an admin notice based on the presence of a post variable, or any immediate action?

What is going on here:

<?php
/*
 Plugin Name: Echo something on presence of variable
 Plugin URI: http://wordpress-bdd.com/
 Description: Hooking into the redirect with post variables
 */

class WhatIsGoingOn{

    public function __construct(){
        //add meta box:
        add_action('load-post.php', array($this, 'doAddMetaBoxes'));
        add_action('load-post-new.php', array($this, 'doAddMetaBoxes'));
        $this->whyCantIechoThisNotice();
     }

    public function doAddMetaBoxes(){
        add_meta_box('metaWithInput', 'Hello', array($this, 'echoMetaBox'), 'post', 'side', 'high');
    }

    public function echoMetaBox(){
        echo("<input type = 'text' name = 'myInput' id = 'myInput' />");
    }

     public function whyCantIechoThisNotice(){

        //check for presence of variable and do something:
        if (isset($_POST['myInput']) && (!($_POST['myInput'] == ""))){
            //If this line is uncomented, the die fires
            //die('I am here!');
            //How do I get the notice to echo?
                echo ("
                        <div class='notice notice-success is-dismissible'>
                                <p>This is a notice</p>
                        </div>
                ");
        }
    }

}

$WhatIsGoingOn = new WhatIsGoingOn;

Answer 1

exit; or store in $_SESSION. then on each page load check is_admin() and if your session var is set, then if so print the message, and remove the message from $_SESSION.