MiddlewareAwareTrait.php

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<?php
namespace Slim;

use RuntimeException;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Message\ResponseInterface;
use UnexpectedValueException;

trait MiddlewareAwareTrait
{
    protected $tip;

    protected $middlewareLock = false;

    protected function addMiddleware(callable $callable)
    {
        if ($this->middlewareLock) {
            throw new RuntimeException('Middleware can’t be added once the stack is dequeuing');
        }

        if (is_null($this->tip)) {
            $this->seedMiddlewareStack();
        }
        $next = $this->tip;
        $this->tip = function (
            ServerRequestInterface $request,
            ResponseInterface $response
        ) use (
            $callable,
            $next
        ) {
            $result = call_user_func($callable, $request, $response, $next);
            if ($result instanceof ResponseInterface === false) {
                throw new UnexpectedValueException(
                    'Middleware must return instance of \Psr\Http\Message\ResponseInterface'
                );
            }

            return $result;
        };

        return $this;
    }

    protected function seedMiddlewareStack(callable $kernel = null)
    {
        if (!is_null($this->tip)) {
            throw new RuntimeException('MiddlewareStack can only be seeded once.');
        }
        if ($kernel === null) {
            $kernel = $this;
        }
        $this->tip = $kernel;
    }

    public function callMiddlewareStack(ServerRequestInterface $request, ResponseInterface $response)
    {
        if (is_null($this->tip)) {
            $this->seedMiddlewareStack();
        }
        $start = $this->tip;
        $this->middlewareLock = true;
        $response = $start($request, $response);
        $this->middlewareLock = false;
        return $response;
    }
}