How to make a dynamic form in Drupal with Ajax, select autocomplete

So, let's see how to make a dynamic form using an example:

  // function for creating a form
function get_select ($ form, & $ form_state) {
// the number of options in select_two, at the first construction of the form is always equal to 1
$ numbers = empty ($ form_state ['values'] ['select_one'])? 1: $ form_state ['values'] ['select_one'];

// filling the array of values ​​for select_two
$ select_two_values ​​= array ();
for ($ i = 1; $ i <= $ numbers; $ i ++) {
$ select_two_values ​​['option _'. $ i] = 'Value'. $ i;
}

$ form ['select_one'] = array (
'#title' => 'First list',
'#description' => 'Select how many values ​​will be in the second list',
'#type' => 'select',
'#options' => array ('1' => '1 value', '2' => '2 values', '3' => '3 values'),
// make the form dynamic
'#ajax' => array (
// function to be called when the select_one value changes
'callback' => 'get_select_one_values',
// id of the element on the page where the results should be written
'wrapper' => 'wrapper',
'method' => 'replace',
'effect' => 'fade',
),
);

$ form ['select_two'] = array (
'#title' => 'Second list',
'#description' => 'Select a value from the second list',
'#type' => 'select',
'#options' => $ select_two_values,
// prefix and suffix wraps select_two with id = wrapper - this is where the results will be written
'#prefix' => '& lt; div id = "wrapper">',
'#suffix' => '& lt / div>',
);

return $ form;
}

// function that will be called when the select_one value changes
function get_select_one_values ​​($ form, $ form_state) {
// the select_two field will be rebuilt
return $ form ['select_two'];
}
 

Thus, the dynamic form on Drupal has been successfully created, now when the value from the first list changes, the values ​​in the second list will be automatically updated.