The Immutable Event Dispatcher

The ImmutableEventDispatcher is a locked or frozen event dispatcher. The dispatcher cannot register new listeners or subscribers.

The ImmutableEventDispatcher takes another event dispatcher with all the listeners and subscribers. The immutable dispatcher is just a proxy of this original dispatcher.

To use it, first create a normal EventDispatcher dispatcher and register some listeners or subscribers:

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use Symfony\Component\EventDispatcher\EventDispatcher;
use Symfony\Contracts\EventDispatcher\Event;

$dispatcher = new EventDispatcher();
$dispatcher->addListener('foo.action', function (Event $event): void {
    // ...
});

// ...

Now, inject that into an ImmutableEventDispatcher:

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use Symfony\Component\EventDispatcher\ImmutableEventDispatcher;
// ...

$immutableDispatcher = new ImmutableEventDispatcher($dispatcher);

You'll need to use this new dispatcher in your project.

If you are trying to execute one of the methods which modifies the dispatcher (e.g. addListener()), a BadMethodCallException is thrown.