I’m having trouble understanding how works the #submit property of a submit element in Drupal 8, when Ajax is used.

I found this code, as an example : https://gist.github.com/leymannx/72d41cf0baa4dee62d6ddc89bc7c7a5a

There is a submit button in the form, described by the code below :

$  form['container']['actions']['add_item'] = [   '#type'   => 'submit',   '#value'  => $  this->t('Add another name'),   '#submit' => ['::MYMODULE_MYFORM_add_item'],   '#ajax'   => [     // Could also use [ $  this, 'colorCallback'].     'callback' => '::MYMODULE_MYFORM_ajax_callback',     'wrapper'  => 'my-container', // CHECK THIS ID   ], ]; 

There are 2 callback functions used here : the value of the #submit property and the value of callback in the #ajax property.

What happens exactly when the user clicks on the button ? As I understand it, MYMODULE_MYFORM_add_item function is first called, then MYMODULE_MYFORM_ajax_callback function.

But, in a custom module, I’m currently doing something like this :

function mymodule_form_user_register_form_alter(&$  form, FormStateInterface $  form_state, $  form_id) {      $  form['account']['fill_username'] = array(         '#type' => 'submit',         '#value' => t('Fill username with Bob'),         '#weight' => -1,         '#submit' => ['mymodule_fill_username_submit'],         '#ajax' => array(             'callback' => 'mymodule_fill_username_callback',             'wrapper' => 'edit-account'         )     ); }  function mymodule_fill_username_submit( $  form, FormStateInterface &$  form_state ){     \Drupal::logger('mymodule')->notice('mymodule_fill_username_submit');     $  form_state->setValue('name', 'Bob');     $  form_state->setRebuild( true ); }  function mymodule_fill_username_callback( $  form, FormStateInterface &$  form_state ){     \Drupal::logger('mymodule')->notice('mymodule_fill_username_callback');     return $  form['account']; } 

The aim is to inject a value in the “Username” field, just to try and understand how this works.

As I click on the button, mymodule_fill_username_callback is called (I see the log) but mymodule_fill_username_submit never. And, of course, the field is not filled at all…

I don’t understand what happens here. Nor how it’s supposed to work.

Can anybody help ?