Drupal and Web Request, most needed functionality

How to get the current request object

Procedural

In procedural code get the request from the static wrapper \Drupal:

$request = \Drupal::request();

The method Request(), returns a request object.
The Request object is a symfony technology and not Drupal's, so to understand its logic methods and internals, you can read more about it at: symfony.com/doc/current/components/http_foundation.html#request

so if you need the path created the request, use the following code:

$path = \Drupal::request()->getPathInfo();

Service

In a service get the current request from the injected argument @request_stack:

The module_name/module_name.services.yml file :

services:  
	custom.service:    
		class: Drupal\module_name\Service\CustomService    
		arguments:      
			- '@request_stack'

The module_name/src/Service/CustomService.php file :

use Symfony\Component\HttpFoundation\RequestStack;
 
/** 
	@var \Symfony\Component\HttpFoundation\RequestStack 
*/ 
protected $requestStack; 

class customService {  
	public function __construct(RequestStack $requestStack) {    
		$this->requestStack = $requestStack;  
	}  
	public function doSomething() {    
		// use $this->requestStack->getCurrentRequest()  
	} 
}

Controller

In a controller you can pull the request from the route argument stack by including a typed request parameter. 

An example from UserController:

use Symfony\Component\HttpFoundation\Request;  

public function resetPass(Request $request, $uid, $timestamp, $hash) {
	// use $request  
}

It's not necessary to define the route parameter in the route definition. 

The request is always available.

Form

In a form method get the request from getRequest():

  public function submitForm(array &$form, FormStateInterface $form_state) { 
  	$request = $this->getRequest();    
  	...  
  }

Don't use the injected requestStack property directly, because it is not available for all Form API callbacks.