Custom callbacks doesn't work

Thanks for this! I've noticed myself that callables cannot be used here, and only functions, which is really annoying.

+++ b/core/lib/Drupal/Core/Datetime/Element/Datetime.php
@@ -274,9 +274,7 @@ public static function processDatetime(&$element, FormStateInterface $form_state
+          call_user_func_array($callback, [&$element, $form_state, $date]);

@@ -305,10 +303,8 @@ public static function processDatetime(&$element, FormStateInterface $form_state
+          call_user_func_array($callback, [&$element, $form_state, $date]);

I think we still need to check if this is valid, so replacing function_exists with is_callable, and then using call_user_func_array should work.

it needs tests and surely must use is_callable instead of function_exists

Tested below patch with following datetime field

    $test = [$this, 'testdatecallbackfromclass'];
    $form['datetime'] = [
      '#type' => 'datetime',
      '#title' => t('date'),
      '#description' => t('Description about the date.'),
      '#date_date_callbacks' => [$test, 'testdatecallbackfrommodule'],
    ];

@andypost : Can you give some idea about test to write, should i write tests in core/modules/datetime/tests/src/Functional/DateTimeFieldTest.php file ? and what can be added to compare.

@Vj I suppose there should be tests for datetime form element (very probably it does not exists for core/lib/Drupal/Core/Datetime/Element/Datetime.php)

Example is core/tests/Drupal/KernelTests/Core/Element/PathElementFormTest.php but I can't find similar one for this element

@andypost

Tried creating test for the first time :P
Please review and let me know if it works or any changes required.

@Vj it looks great! Please attach patch with test only to show that without fix it will fail

testonly patch should fail

This looks great. Thanks all!

Fixed these code style issues locally, and committed/pushed to 8.7.x, thanks!

FILE: ...sts/Drupal/KernelTests/Core/Datetime/DatetimeElementFormTest.php
----------------------------------------------------------------------
FOUND 3 ERRORS AND 2 WARNINGS AFFECTING 5 LINES
----------------------------------------------------------------------
  7 | WARNING | [x] Unused use statement
  9 | ERROR   | [x] There must be one blank line after the last USE
    |         |     statement; 3 found;
 12 | ERROR   | [ ] More than 2 empty lines are not allowed
 23 | ERROR   | [x] Additional blank lines found at end of doc
    |         |     comment
 68 | WARNING | [x] A comma should follow the last multiline array
    |         |     item. Found: ]
----------------------------------------------------------------------
PHPCBF CAN FIX THE 4 MARKED SNIFF VIOLATIONS AUTOMATICALLY
----------------------------------------------------------------------

Can we fix this in 8.6.x, too?
Patch applies cleanly.

As expected, bot is happy on 8.6.x.

Cherry-picking this to 8.6.x would unblock #2672950: Notice: Undefined index: #default_value in Drupal\Core\Datetime\Element\Datetime::valueCallback() (line 103 which in turn unblocks / simplifies #2419131: [PP-1] #states attribute does not work on #type datetime.

This seems like an utterly non-disruptive bugfix with test coverage.

RTBC.

Thanks!
-Derek

Sorry, in case that wasn't clear, I had re-triggered patch #9 to test against 8.6.x and it's all green...

Thanks,
-Derek

This indeed looks very non-disruptive.

I agree we can backport this.