Laravel 4 : How are Facades resolved?

Question

I've kind of looking at what's going on Laravel 4 facades under the hood.

Let's take this Facade as an example:

File::get(someArgs);

If i'm not mistaken, the step by step (oversimplified) invocation would be:

//static method invocation which are all extended from Facade class
File::__callStatic(get, someArgs)
//returns an instance of FileSystem
File::resolveFacedeInstance('files') 
FileSystem->get(someArgs)

What I'am confused about is in the commented line below of the method File::resolveFacadeInstance() below:

protected static function resolveFacadeInstance($name)
{
    if (is_object($name)) return $name;

    if (isset(static::$resolvedInstance[$name]))
    {
        return static::$resolvedInstance[$name];
    }



    /**
    * The line that i'm confused about
    */

    return static::$resolvedInstance[$name] = static::$app[$name];
}

My questions are:

• How is File::$app even initialized or assigned a value inside the Facade class

• If File::get() is the invoked Facade

  static::$app[$name] would resolve to i think Application['files'] or Application->files which in turn calls Application->__get('files') since there's no files property inside Application class.

  How would FileSystem Class be return if this is only the content of this method?

  public function __get($key)
  {
      return $this[$key];
  }
  

Answer 1

I'll try to describe in short :

So, you already know that resolveFacadeInstance method is called via __callStatic method of Facade class and component's Facade (i.e. File extends Facade) extends this Facade class.

During the boot-up process of the framework, from public/index.php following line starts the execution of bootstrap/start.php file

$app = require_once __DIR__.'/../bootstrap/start.php';

So, in this (bootstrap/start.php) file you can see some code like

// the first line, initiate the application
$app = new Illuminate\Foundation\Application;
// ...
// ...
// notice this line
require $framework.'/Illuminate/Foundation/start.php';
// ...
// last line
return $app;

In this code snippet, require $framework.'/Illuminate/Foundation/start.php'; line starts the execution of Foundation/start.php file and in this file you may see something like this

// ...

Facade::clearResolvedInstances();
// Notice this line
Facade::setFacadeApplication($app);

This (given above) line sets application instanse to $app property in the Facade class

// support/Facades/Facade.php
public static function setFacadeApplication($app)
{
    static::$app = $app;
}

Then in the Foundation/start.php file at the bottom, you can see something like this

/*
|--------------------------------------------------------------------------
| Register The Core Service Providers
|--------------------------------------------------------------------------
|
| The Illuminate core service providers register all of the core pieces
| of the Illuminate framework including session, caching, encryption
| and more. It's simply a convenient wrapper for the registration.
|
*/

$providers = $config['providers'];

$app->getProviderRepository()->load($app, $providers);

$app->boot();

In this code snippet given above, all the core components registered by the framework and as you know that, every component has a service provider class (i.e. FilesystemServiceProvider) and in every service provider class there is a method register which is (for FilesystemServiceProvider)

/**
 * Register the service provider.
 *
 * @return void
 */
public function register()
{
    $this->app['files'] = $this->app->share(function() { return new Filesystem; });
}

Well, in this case $this->app['files'] setting (return new Filesystem) an anonymous function, which returns the filesystem when gets executed

$this->app['files'] = $this->app->share(function() { return new Filesystem; });

to $app['files'] so, when you call the File::get(), it finally calls the anonymous function and in this case, the following line

return static::$resolvedInstance[$name] = static::$app[$name];

Calls the function for static::$app['file']; and this function returns the instance but before returning, it stores the instance in the $resolvedInstance variable, so, next time it can return the instance from the variable without calling the anonymous function again. So, it looks like that, static::$resolvedInstance[$name] = static::$app[$name]; calls the anonymous function which returns the instance and this function was registered earlier, when the app was started through boot up process.

Important :

Application extends Container and Container extends ArrayAccess class and that's why, a property of the $app object could be (accessed) set/get using array notation.

I've tried to give you an idea but you have to look in to the code, step by step, you won't get it only reading/tracking the code once.