Drupal 8 - Reading node fields values within a custom module - php

Question

I am using Drupal 8.2.6 and I would like to create a block that would appear on a custom content type page.
It is kind of a booking block which sends an e-mail to the site admin that a visitor would like to book a product (the custom content type).
I assume I would need a form which only consists of a submit button and a block which renders the form.
But the real point would be sending the mail with the product's reference to the site admin.
As I found here, I could get the values I need using this snippet:
if ($node = \Drupal::routeMatch()->getParameter('node')) {
$field_my_custom_value = $node->field_my_custom_value->value;
}
But I am not sure in which scope of my code I should use it. This example was for rendering the values within a custom block, where as my case would be sending a mail with the values.
And could anyone remind me as well how to send a mail from a custom module in Drupal 8?
Thanks a lot

Answer 1

So, after solving it myself after a day's whole worth of documentation, here are the solutions, as I am going to revert back my question to earlier revisions, in case anyone needs it.
So, given the snippet in the question above, I declared the variables in the buildForm() function
public function buildForm(array $form, FormStateInterface $form_state) {
$field_value = '';
if ($node = \Drupal::routeMatch()->getParameter('node')) {
$field_value = $node->field_name->value;
}
$form['field_value'] = array(
'#type' => 'value',
'#value' => $field_value,
);
// And then you add the definition for other form items and submit button
}
And for sending the mail using the value, you retrieve the value using $form_state like this:
public function submitForm(array &$form, FormStateInterface $form_state) {
$module = 'your_module_name';
$key = 'any_key_you_would_like';
$to = 'receiver#email.address';
$langcode = 'en';
$params = array(
'body' => 'Node is booked',
'subject' => $form_state->getValue('field_value'),
);
$mailer = \Drupal::service('plugin.manager.mail');
$mailer->mail($module, $key, $to, $langcode, $params, NULL, TRUE);
}
Some values are quite tricky to get from the node, such as the node title which uses $node->getTitle() instead of $node->field_name->value so you would like to check your fields using Drupal 8's Devel + Kint module to know what attributes and methods to use.