How to implement AJAX within a class storing shortcode atts as class variables

Question

I'm trying to write an OOP plugin, pass my shortcode arguments to the class, store them as class variables and then use them in an AJAX call.

Here is an example:

class WPAJAXClassTest {
    private $message;
    public function __construct() {
        add_action('wp_enqueue_scripts', array($this,'enqueue_scripts'));
        add_action( 'wp_ajax_testAJAX', array($this,'testAJAX'));
        add_action( 'wp_ajax_nopriv_testAJAX', array($this,'testAJAX'));
    }
    function wpAJAXClassTest($args) {
        $msg=shortcode_atts(array(
            'message' => 'Hello World!'
            ), $args
        );
        $this->setMessage($msg['message']);
        $html="<button id='testbutton'>test</button><p id='testreturn'>".$msg['message']." - ".$this->message."</p>";
        return $html;
    }
    public function setMessage($m) {
        $this->message=$m;
    }
    function enqueue_scripts() {
        $plugin_url = plugins_url()."/wpajaxclasstest";
        wp_enqueue_script('wpajaxclasstest_script', $plugin_url.'/js/plugin.js', array("jquery"));
        wp_localize_script( 'wpajaxclasstest_script', 'myASPlugin', array('plugin_dir' => $plugin_url));
        wp_localize_script( 'wpajaxclasstest_script', 'myAJAX', array('ajaxurl' => admin_url( 'admin-ajax.php' )));
    }
    function testAJAX() {
        header("Content-Type: application/json");
        $returnArray=array();
        $returnArray['message']=$this->message;
        echo json_encode($returnArray);
        exit();
    }
}

I have a simple javascript which calls the AJAX function

$.ajax({
    url: myAJAX.ajaxurl,
    data: json,
    method: "POST",
    error: function(data) {
        alert("Error! "+JSON.stringify(data));
    },
    success: function(data) {
        alert(JSON.stringify(data));
    }
});

My issue is that when I use my shortcode [wpAJAXClassTest] or [wpAJAXClassTest message='Goodbye Cruel World!'] I always get a json return of {"message": "null"} and not the class variable $message I was expecting.

I don't want to use a static getinstance as I want to be able to have multiple instances of the same plugin on the same page.

How do I get my AJAX function to access class variable?

To Clarify:

I want to have 2 buttons on a page driven by shortcode tags

[testAJAX message="hello"]

and

 [testAJAX message="world"]

when I click one button the AJAX function should return {"message": "hello"} and the other should return {"message": "world"}

Answer 1

to pass a variable from JavaScript to the AJAX call, you have to put the message in the data :

data : {"message" : "my message"}

and you retrieve it in the PHP code :

function testAJAX() {

    $returnArray = array();
    $returnArray['message'] = $_POST["message"];

    wp_send_json($returnArray);
}

you can read about the use of wp_send_json here :
https://codex.wordpress.org/Function_Reference/wp_send_json