TinyMCE custom button retrieve value from custom field

Question

I'm creating a plugin that adds a custom button to the default TinyMCE editor in WordPress admin.

The following code builds this simple plugin

mkay-tinymce-extra-button.php

function mkay_enqueue_plugin_script( $plugin_array ){
    $plugin_array['subscriptionPlugin'] = plugin_dir_url( __FILE__ ) . 'mkay-tinymce-extra-buttons.js';
    return $plugin_array;
}
add_filter( 'mce_external_plugins', 'mkay_enqueue_plugin_script' );

function mkay_register_buttons_editor( $buttons ){
    array_push( $buttons, 'subscription' );
    return $buttons;
}
add_filter( 'mce_buttons', 'mkay_register_buttons_editor' );

mkay-tinymce-extra-buttons.js

(function(){
tinymce.create('tinymce.plugins.subscriptionPlugin', {
    init: function(editor, url){
        editor.addButton('subscription', {
            title: 'Link per l\'iscrizione',
            cmd: 'subscriptionCmd',
            icon: false,
            text: 'Link iscrizione'
        });

        editor.addCommand('subscriptionCmd', function(){
            var selectedContent = editor.selection.getContent();
            var box = '<a href="/iscrizione/?data_corso=[custom field value]" class="subscription-link">'+selectedContent+'</div>';
            editor.execCommand('mceInsertContent', 0, box);
        });
    }
});

tinymce.PluginManager.add('subscriptionPlugin', tinymce.plugins.subscriptionPlugin);
})();

Till here the plugin seems to work well.

Now I need to take the value of a custom field from the post and put it in place of [custom field value] in js file. Then if possible show the custom button only if post is draft or published and the custom field is not empty.

Does someone have any idea? Any help's appreciated.

Answer 1

Rather than finding the custom field value when the user hits the button, change the [custom field value] to a shortcode name (perhaps "[mkay_subscription_button_value]"?) and then create a shortcode that can replace the value of that key with the post's custom meta. Something like this:

function 246286_display_custom_field( $atts ) {
    $data = get_post_meta( get_the_ID(), 'mkay_custom_field', true );
    return $data;
}
add_shortcode( 'mkay_subscription_button_value', '246286_display_custom_field' );

The value of doing this is: if the post's custom meta changes after the button has been pressed, the output on the page still remains valid.