How Can I Implement Flash Messages?

Flash messages are used to display one-time messages to a user. A typical use case is for setting and later displaying a successful submission via a Post/Redirect/Get (PRG) workflow, where the flash message would be set during the POST request, but displayed during the GET request. (PRG is used to prevent double-submission of forms.) As such, flash messages usually are session-based; the message is set in one request, and accessed and cleared in another.

Expressive does not provide native session facilities out-of-the-box, which means you will need:

  • Session functionality.
  • Flash message functionality, for handling message expiry from the session after first access.

A number of flash message libraries already exist that can be integrated via middleware, and these typically either use PHP's ext/session functionality or have a dependency on a session library. Two such libraries are slim/flash and damess/expressive-session-middleware.

slim/flash

Slim's Flash messages service provider can be used in Expressive. It uses PHP's native session support.

First, you'll need to add it to your application:

$ composer require slim/flash

Once you have, you'll need to create a factory to return middleware that will add the flash message provider to the request:

namespace App;

use Slim\Flash\Messages;

class SlimFlashMiddlewareFactory
{
    public function __invoke($container)
    {
        return function ($request, $response, $next) {
            // Start the session whenever we use this!
            session_start();

            return $next(
                $request->withAttribute('flash', new Messages()),
                $response
            );
        };
    }
}

Now, let's register it with our middleware pipeline. In config/autoload/middleware-pipeline.global.php, make the following additions:

return [
    'dependencies' => [
        'factories' => [
            'App\SlimFlashMiddleware' => App\SlimFlashMiddlewareFactory::class,
            /* ... */
        ],
        /* ... */
    ],
    'middleware_pipeline' => [
        'always' => [
            'middleware' => [
                'App\SlimFlashMiddleware',
                /* ... */
            ],
            'priority' => 10000,
        ],
        /* ... */
    ],
];

Where to register the flash middleware

Sessions can sometimes be expensive. As such, you may not want the flash middleware enabled for every request. If this is the case, add the flash middleware as part of a route-specific pipeline instead.

From here, you can add and read messages by accessing the request's flash attribute. As an example, middleware generating messages might read as follows:

use Zend\Diactoros\Response\RedirectResponse;

public function __invoke($request, $response, $next)
{
    $flash = $request->getAttribute('flash');
    $flash->addMessage('message', 'Hello World!');

    return new RedirectResponse('/other-middleware');
}

And middleware consuming the message might read:

public function __invoke($request, $response, $next)
{
    $flash = $request->getAttribute('flash');
    $messages = $flash->getMessages();
    // ...
}

From there, it's a matter of providing the flash messages to your template.

damess/expressive-session-middleware and Aura.Session

damess/expressive-session-middleware provides middleware for initializing an Aura.Session instance; Aura.Session provides flash messaging capabilities as part of its featureset.

Install it via Composer:

$ composer require damess/expressive-session-middleware

In config/autoload/dependencies.global.php, add an entry for Aura.Session:

return [
    'dependencies' => [
        'factories' => [
            Aura\Session\Session::class => DaMess\Factory\AuraSessionFactory::class,
            /* ... */
        ],
        /* ... */
    ],
];

In config/autoload/middleware-pipeline.global.php, add a factory entry for the damess/expressive-session-middleware, and add it to the middleware pipeline:

return [
    'dependencies' => [
        'factories' => [
            DaMess\Http\SessionMiddleware::class => DaMess\Factory\SessionMiddlewareFactory::class,
            /* ... */
        ],
        /* ... */
    ],
    'middleware_pipeline' => [
        'always' => [
            'middleware' => [
                DaMess\Http\SessionMiddleware::class,
                /* ... */
            ],
            'priority' => 10000,
        ],
        /* ... */
    ],
];

Where to register the session middleware

Sessions can sometimes be expensive. As such, you may not want the session middleware enabled for every request. If this is the case, add the session middleware as part of a route-specific pipeline instead.

Once enabled, the SessionMiddleware will inject the Aura.Session instance into the request as the session attribute; you can thus retrieve it within middleware using the following:

$session = $request->getAttribute('session');

To create and consume flash messages, use Aura.Session's flash values. As an example, the middleware that is processing a POST request might set a flash message:

use Zend\Diactoros\Response\RedirectResponse;

public function __invoke($request, $response, $next)
{
    $session = $request->getAttribute('session');
    $session->getSegment(__NAMESPACE__)
            ->setFlash('message', 'Hello World!');

    return new RedirectResponse('/other-middleware');
}

Another middleware, to which the original middleware redirects, might look like this:

public function __invoke($request, $response, $next)
{
    $session = $request->getAttribute('session');
    $message = $session->getSegment(__NAMESPACE__)
                       ->getFlash('message');
    // ...
}

From there, it's a matter of providing the flash messages to your template.

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