Why use a closure to assign instead of directly assigning a value to a key?

Question

I watched this video , and at 7:10 he added a db dependency and uses closure to assign a value.

My question is, why not just use direct assignment instead, I mean that I am not doing this:

$container['db'] = $capsule;

equivalent to this:

$container['db'] = function ($container) use ($capsule) {
    return $capsule;
}

If not, what's the difference and which way is better?

Answer 1

TL; DR , because defining dependencies as closures allows the dependency injection container to create them on demand, so you don’t have to worry about how to define them and manually manage their dependencies.

Pimple , , .

, Pimple , , :

$container['sample-param'] = 'foo';
echo $container['sample-param'];
//output: foo

, sample-param - , , . :

$container['myService'] = function($c) {
    $service = new \Some\Complicated\Service();
    //this service depends on cache service
    $service->setCache($c['cache']);
    return $service;
}

$container['cache'] = function($c) {
    $cache = new \Cache\Driver();
    //our cache driver needs a db connection
    $cache->setDbConnection($c['db']);
    return $cache;
}

$container['db'] = function($c) {
    //our database connection requires some parameters
    $dbConnection = new \Database\Connection($c['db-config']);
    return $dbConnection;
}

$container['db-config'] = [
    'username' => 'foo',
    'password' => 'bar',
    'host' => 'localhost'
];

//Now we want to user our service:
$container['myService']->doSomething();

, .

myService cache, myService. Pimple , , Pimple . myService, Pimple , myService , , . , Pimple ($ c - Pimple) dependecies ( cache). Pimple , , ..... , , , db-config, . .

, , ? , myService , . , , , , . myService , cache, .

Answer 2

Slim framework , Pimple .

http://pimple.sensiolabs.org/

- , - . : , . .

, :

// define some services
$container['session_storage'] = function ($container) {
    return new SessionStorage('SESSION_ID');
};

$container['session'] = function ($container) {
    return new Session($container['session_storage']);
};

, , .

Pimple \ArrayAccess (. ), , , . offsetSet offsetGet, , :

$container['a'] = function($c){ return new Blah();}

$thing = $container['a']; 

tl; dr Pimple, PHP.

Answer 3

This is completely different. The first $container['db'] = $capsule;
means that the array $containerhas a key db, and this key is equal $capsule, the second means that you have an array $container, and the key dbhas a close value. You can see the difference when resetting two results:

Closing example:

$capsule = 1;
$container['db'] = function ($container) use ($capsule) {
    return $capsule;
};
var_dump($container); 

Result:

Array(1) {
  ["db"]=>
  object(Closure)#1 (2) {
    ["static"]=>
    array(1) {
      ["capsule"]=>
      int(1)
    }
    ["parameter"]=>
    array(1) {
      ["$container"]=>
      string(10) "<required>"
    }
  }
}

Example without closing:

$capsule = 1;
$container['db'] = $capsule;    
var_dump($container);

Result:

result array(1) {
  ["db"]=>
  int(1)
}

You can read about closure in the php manual for a better understanding of their role. PHP closing guide