Drupal 8 subscribe to an event from inside block - drupal-8

Question

I am currently trying to wrap my head around Drupal 8 module development best practices. All I'm trying to do is to have a simple form Demoform on a page where a user can input an email address. When the form gets submitted I'd like to dispatch an event demo_form.save. Also I need a block that then displays the user's email address within the block (let's say sidebar second). I have already implemented an EventSubscriber before as a test, so the event gets properly dispatched etc. and I also subscribed to the event (but how to get the information inside a block) Now my question: what's the best practice for this workflow:
File DemoForm.php
class DemoForm extends ConfigFormBase {
...
$event = $dispatcher->dispatch('demo_form.save', $e);
...
}
File DemoEventSubscriber.php
class DemoEventSubscriber implements EventSubscriberInterface {
static function getSubscribedEvents() {
$events['demo_form.save'][] = array('onConfigSave', 0);
return $events;
}
public function onConfigSave($event) {
...
}
}
This works and I can access the input from the form inside the DemoEventSubscriber class and do whatever I want with it.
But now I'd like to display the email address inside the block markup. How should this best be done ?
File DemoBlock.php
class DemoBlock extends BlockBase {
public function build() {
// here return markup with email address from form
}
}
How do I combine the eventsubscriber and the block markup ? Can Blockbase itself implement the EventSubscriberInterface and be independent from DemoEventSubscriber.php ? Or do I need to register a service that transmits the form data and then access the service within the block's build() function ? Or is there another way I am missing ?
Thanks for any input.

Answer 1

I am not sure what you need the event for, but to dispatch the event, use the code you have already displayed in your submitForm() function of the DemoForm class.
Because you are using ConfigFormBase, I assume that you want to store the submitted e-mail address in config, use code like from the config form documentation:
/**
* {#inheritdoc}
*/
public function submitForm(array &$form, FormStateInterface $form_state) {
// Retrieve the configuration
$this->config('mymodule.settings')
// Set the submitted configuration setting
->set('email', $form_state->getValue('email'))
->save();
// Assuming you have injected the dispatcher.
$event = $this->dispatcher->dispatch('demo_form.save', $e);
parent::submitForm($form, $form_state);
}
Within you block, you can then access the configuration for example with the static wrapper or inject the service Simple Configuration API
$config = \Drupal::config('mymodule.settings');
$message = $config->get('email');
Note that with this you can always set only one e-mail address. I don't know if that was your purpose. If you want to collect multiple e-mails then you should store them in the database and not in config.