Slim invokable class as group callable example (PR #2229)

example.php

<?php

use Example\MyGroup;
use Slim\App;

$app = new App;

// I'd use DI for this
$dependency = new SomeDependency;
$group = new MyGroup($dependency);

// Ideally the callable resolver could be used to resolve group callables too, and I'd just give the class name
$app->group("/example", $group);

$app->run();

MyGroup.php

<?php

namespace Example;

use Slim\App;
use SomeDependency;

class MyGroup
{
    public function __construct(SomeDependency $dependency)
    {
        $this->dependency = $dependency;
    }
    
    public function __invoke(App $app)
    {
        // register your routes here
    }
}