Chaining events in Laravel
Tags: php, laravel, eventdriven
Published: 2023-05-17 06:59:59

The Laravel event management system provides a flexible and powerful mechanism for implementing event-driven architecture in your application. It promotes decoupling, modularity, reusability, and testability while allowing for easy extensibility and adaptability.

In Laravel, an event is a class that represents a specific occurrence or action that takes place within your application.

> Events serve as a way to notify other parts of your application when something significant happens.

Events allow for a decoupled and flexible architecture by separating the logic that triggers an event from the logic that handles the event. This means that different components of your application can be loosely coupled and unaware of each other, making it easier to add or remove functionality without directly modifying existing code.

When an event is fired in Laravel, it triggers a sequence of actions. These actions can include executing event listeners, sending notifications, updating data, logging, or any other logic you define.

To create an event run the following artisan command:

```
php artisan make:event CableStatusChanged
```
This command generates an event class file in the app/Events directory. You can then define properties and methods within this class to encapsulate the data and behavior associated with the event.

The below code snippet is an example of the CableStatusChange event. It receives its parameters through the constructor and stores them as public properties.

```
<?php

namespace App\Events;

use App\Models\Cable;
use Illuminate\Broadcasting\InteractsWithSockets;
use Illuminate\Broadcasting\PrivateChannel;
use Illuminate\Foundation\Events\Dispatchable;
use Illuminate\Queue\SerializesModels;

class CableStatusChanged
{
    use Dispatchable, InteractsWithSockets, SerializesModels;

    /**
     * Create a new event instance.
     *
     * @return void
     */
    public function __construct(public Cable $cable, public int $cablePairStatusId) {

    }

}
```
Once you have created an event you need to create an event listener.

Event listeners are responsible for responding to events and performing actions based on those events.

When an event is fired in Laravel, it will be dispatched to its corresponding listeners. The listeners can then execute code or perform tasks in response to the event. This allows for the decoupling of different components within your application, making it easier to add or remove functionality without directly modifying the code that triggers the event.

To create an event listener in Laravel, you can use the artisan command make:listener like this:

```
php artisan make:listener ChangeCableStatus --event=CableStatusChanged
```
After running the command, Laravel will generate a new listener class file in the app/Listeners directory. You can find and modify this file to define the logic for handling the event. Within the listener class, you'll typically implement the handle() method, which contains the code that should be executed when the event is fired.

Once you’ve created an event listener, you need to register it in your application. This can be done in the EventServiceProvider class, located in the app/Providers directory. Inside the listen property of the service provider, you can specify the events and their corresponding listeners:

```
class EventServiceProvider extends ServiceProvider
{
    /**
     * The event to listener mappings for the application.
     *
     * @var array<class-string, array<int, class-string>>
     */
    protected $listen = [
        CableStatusChanged::class => [
            ChangeCableStatus::class,
            RegisterCableStatusChange::class,
        ]
    ];

}
```
In the above example, you can see how to chain listeners for the same event.

> Chaining listeners allows you to define multiple listeners that will be executed sequentially when the event is fired. Each listener in the chain can perform its actions or handle different aspects of the event.

It is important to remember that the order of listeners in the chain matters. If ChangeCableStatus modifies the event data or cancels the event, subsequent listeners may receive different data or not be executed at all. Therefore, it’s important to consider the order of listeners when chaining them.

> By chaining listeners, you can create a modular and flexible event-driven system in Laravel, where different listeners can handle specific aspects of an event or perform different tasks in response to the same event.

```
class ChangeCableStatus {
    /**
     * Handle the event.
     *
     * @param  \App\Events\CableStatusChanged  $event
     * @return void
     */
    public function handle(CableStatusChanged $event) {
        $event->cable->connection_points()
            ->get()
            ->each
            ->update(['cable_pair_status_id' => $event->cablePairStatusId]);
    }

}
```
The event listener in the example above receives the event and updates the application database accordingly.

The reason for using each() method is to enable record-level logging for mass updates in the application.

In Laravel, you can fire an event using the event() helper function or the Event facade. Here's how you can fire an event using the Eventfacade:

```
$cablePairStatusIdIsDirty &&
    CableStatusChanged::dispatch($this->cable, $this->cablePairStatusId);
```
In both cases, you’re creating a new instance of the event class (CableStatusChanged) and passing any necessary data as its constructor arguments.

As a summary let me highlight the advantages of Laravel’s event management system:

1. **Decoupled Architecture**: Events allow for a decoupled architecture by separating the code that triggers an event from the code that handles the event. This promotes loose coupling between different components of your application, making it easier to modify or extend functionality without directly modifying existing code.

2. **Modularity and Reusability**: Events provide a modular and reusable approach to handling specific occurrences or actions within your application. You can define events and their corresponding listeners, allowing you to add or remove listeners as needed. This promotes code organization and reusability across different parts of your application.

3. **Flexibility**: With event-driven architecture, your application becomes more flexible and adaptable. By firing events and attaching listeners, you can easily introduce new features or change existing behavior by adding or modifying event listeners. This promotes scalability and makes it easier to maintain and evolve your application over time.

4. **Improved Testability**: Events make it easier to write unit tests for specific functionality within your application. You can test event listeners independently, ensuring they respond appropriately to events and perform the expected actions. This improves the overall testability and maintainability of your codebase.

5. **Event Sourcing and Logging**: Laravel events can be used for event sourcing, where events are stored and used to rebuild the application state. This enables auditing, historical tracking, and undo/redo functionality. Events can also be logged to provide a comprehensive record of important occurrences in your application, facilitating debugging and troubleshooting.

6. **Asynchronous Processing**: Laravel’s event system supports asynchronous event handling, allowing listeners to be processed asynchronously. This can be particularly useful when dealing with time-consuming or resource-intensive tasks, as it offloads the processing to a background queue, improving performance and responsiveness.