If you want to alter the form of a checkout pane, you must implement a form alter for the entire checkout form and check if your pane exists. This is a bit messy. Normally I would just swap a checkout pane plugin's class and be done with it. But this might be a good developer experience additional.
Inject the module_handler service into CheckoutFlowWithPanesBase class and invoke an alter called checkout_pane_form and checkout_pane_PANE_ID_form. The checkout flow is passed as context.
In \Drupal\commerce_checkout\Plugin\Commerce\CheckoutFlow\CheckoutFlowWithPanesBase::buildForm:
$form[$pane_id] = $pane->buildPaneForm($form[$pane_id], $form_state, $form);
\Drupal::moduleHandler()->alter([
'commerce_checkout_pane_form',
"commerce_checkout_pane_{$pane_id}_form",
], $form[$pane_id], $this);
Add commerce_checkout.api.php to document the hook.
hook_commerce_checkout_pane_form_alter(array &$pane_form, CheckoutFlowWithPanesInterface $checkout);
hook_commerce_checkout_pane_PANE_ID_form_alter(array &$pane_form, CheckoutFlowWithPanesInterface $checkout);
Needs work
2.0
Checkout
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Fixed 1 and 2 points from #7.
> 3. Also, swapping the checkout pane plugin is still an option.
Yes, it is. But sometimes it's overkill when all you need is to simply change some labels.