// config/app.php
'providers' => [
    ...
    vishaldudhatra\FCM\FcmNotificationServiceProvider::class,
    ...
];

return [
   
    ...
    ...
    /*
     * Add the Firebase API key
     */
    'fcm' => [
        'key' => env('FCM_SECRET_KEY')
     ]
];

public function via($notifiable)
{
    return ['fcm'];
}

use vishaldudhatra\FCM\FcmMessage;

...

public function toFcm($notifiable) 
{
    $message = new FcmMessage();
    $message->content([
        'title'        => 'Foo', 
        'body'         => 'Bar', 
        'sound'        => '', // Optional 
        'icon'         => '', // Optional
        'click_action' => '' // Optional
    ])->data([
        'param1' => 'baz' // Optional
    ])->priority(FcmMessage::PRIORITY_HIGH); // Optional - Default is 'normal'.
    
    return $message;
}

/**
 * Route notifications for the FCM channel.
 *
 * @param  \Illuminate\Notifications\Notification  $notification
 * @return string
 */
public function routeNotificationForFcm($notification)
{
    return $this->device_token;
}

use vishaldudhatra\FCM\FcmMessage;

...

public function toFcm($notifiable) 
{
    $message = new FcmMessage();
    $message->to('the-topic', $recipientIsTopic = true)
    ->content([...])
    ->data([...]);
    
    return $message;
}

use vishaldudhatra\FCM\FcmMessage;

...

public function toFcm($notifiable) 
{
    $message = new FcmMessage();
    $message->contentAvailable(true)
        ->priority('high')
        ->condition("'user_".$notifiable->id."' in topics")
        ->data([...]);
    
    return $message;
}

use vishaldudhatra\FCM\FcmMessage;

...

public function toFcm($notifiable) 
{
    $message = new FcmMessage();
    $message->setHeaders([
        'project_id'    =>  "48542497347"   // FCM sender_id
    ])->content([
        'title'        => 'Foo', 
        'body'         => 'Bar', 
        'sound'        => '', // Optional 
        'icon'         => '', // Optional
        'click_action' => '' // Optional
    ])->data([
        'param1' => 'baz' // Optional
    ])->priority(FcmMessage::PRIORITY_HIGH); // Optional - Default is 'normal'.
    
    return $message;
}