Non-static method Statamic\StaticSite\Generator::after() should not be called statically

[tao]

I tried adding the post generator function to my AppServiceProvider as the documentation suggests but I ran into an issue:

ErrorException  : Non-static method Statamic\StaticSite\Generator::after() should not be called statically

  at /.../app/Providers/AppServiceProvider.php:21
    17|     {
    18|         // Statamic::script('app', 'cp');
    19|         // Statamic::style('app', 'cp');
    20| 
  > 21|         Generator::after(function () {
    22|             // eg. copy directory to some server
    23|         });
    24|     }

  Exception trace:

  1   Illuminate\Foundation\Bootstrap\HandleExceptions::handleError("Non-static method Statamic\StaticSite\Generator::after() should not be called statically", "/.../app/Providers/AppServiceProvider.php", [])
      /.../app/Providers/AppServiceProvider.php:21

  2   App\Providers\AppServiceProvider::boot()
      /.../vendor/laravel/framework/src/Illuminate/Container/BoundMethod.php:32

[tao]

It seems to work fine if you use it as a real-time facade though:

<?php

namespace App\Providers;

use Illuminate\Support\ServiceProvider;
use Statamic\Statamic;
use Statamic\StaticSite\Generator;

class AppServiceProvider extends ServiceProvider
{
    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot(Generator $ssg)
    {
        // Statamic::script('app', 'cp');
        // Statamic::style('app', 'cp');

        $ssg->after(function() {
            // eg. copy directory to some server
        });
    }

    /**
     * Register any application services.
     *
     * @return void
     */
    public function register()
    {
        //
    }
}