How to add form alter hook in drupal 8 - drupal-8

Question

I am very much new to drupal 8, I am trying to alter a form under custom content type "Question". The form has an id "node-question-form".
I have setup a module and trying to add hook_FORM_ID_alter() but its never getting called. Even the simplest hook is not working. eg:
function constellator_form_alter(&$form, &$form_state, $form_id){
echo "alter the form"; exit;
}
where 'constellator' is the module name.
I have been stuck with since morning and nothing is working for me, Any help will be greatly appriciated.
Cheers

Answer 1

hook_form_alter() and hook_FORM_ID_alter() are called while a form is being created and before displaying it on the screen.
These functions are written in the .module file.
Always clear the cache after writing any hook function. This makes Drupal understand that such a function has been declared.
Use drush cr if using drush version 8 else click on Manage->Drupal 8 logo->Flush all Caches to clear the caches.
Now you can check if the function is being called or not.
The best way to check that is to install the Devel module, enable it. Along with Devel, Kint is installed. Enable Kint too from the Admin UI.
After doing that,you can check whether the hook is being called or not in the following way:
function constellator_form_alter(&$form, &$form_state, $form_id){
kint($form);
}
This will print all the form variables for all forms in the page.
If you want to target a particular form in the page, for eg. you form, node-question-form, type:
function node_question_form_form_alter(&$form, &$form_state, $form_id){
kint($form);
}
Using echo, the way you did, you can confirm whether the function is being called or not, without any hassle, by viewing the Source Code for the page and then searching for the text that you have echoed, using some search option of browser, like, Ctrl+f in case of Google Chrome.

Answer 2

If you want to change sorting options and/or direction (ASC / DESC), you can use this example (tested with Drupal 9).
Here I force the sorting direction according to the "sort by option" set by the user in an exposed filter. (if the user want to sort by relevance, we set the order to ASC, if the user want to sort by date, we set the order to DESC to have the latest content first).
/**
* Force sorting direction for search by date
*
*/
function MYTHEME_form_alter(&$form, &$form_state, $form_id)
{
if (!$form_id == 'views_exposed_form"' || !$form['#id'] == 'views-exposed-form-search-custom-page-1') {
return;
}
$user_input = $form_state->getUserInput();
if (empty($user_input['sort_by'])) {
return;
}
if ($user_input['sort_by'] == 'relevance') {
$user_input['sort_order'] = 'ASC';
} elseif ($user_input['sort_by'] == 'created') {
$user_input['sort_order'] = 'DESC';
}
$form_state->setUserInput($user_input);
}
Note that "views-exposed-form-search-custom-page-1" is the id of my form,
"relevance" and "created" are the sort field identifier set in drupal admin.

Answer 3

function hook_form_alter(&$form, \Drupal\Core\Form\FormStateInterface &$form_state, $form_id) {
echo 'inside form alter';
}