Smart Frame Redirecting with the Server

When an Ajax request happens via a <turbo-frame> and that frame has our data-turbo-form-redirect attribute, we're now communicating that to Symfony by sending a new header on the request called Turbo-Frame-Redirect. We're now going to use that to change any redirect responses to, sort of, "fake redirects" so that the fetch() function in JavaScript doesn't automatically follow them.

Creating the Event Subscriber

We're going to add this magic with an event subscriber. In the src/ directory, let's create a new EventSubscriber/ directory... and inside, a new PHP class called, how about, TurboFrameRedirectSubscriber. Make this implement EventSubscriberInterface... and then go to the "Code -> Generate" menu - or "Command + N" a Mac - and select "Implement Methods" to generate the one method we need: getSubscribedEvents(). Inside, return one event - ResponseEvent::class - set to onKernelResponse.

ResponseEvent is one of the last events that happens during the request-response process. It happens after our controller has been called... so the Response object has already been created.

Above this, add the public function onKernelResponse() method with a ResponseEvent $event argument.

Cool. So the logic inside of here will be fairly simple: if the request has the Turbo-Frame-Redirect header and the response is a redirect, then we're going to change the response to something else.

Replacing the Response

To keep things organized, add a new private method called shouldWrapRedirect(). This will need the Request object - so we can read the header - and the Response object that the controller created. This will return a bool.

Before we work on that method, back in onKernelResponse(), call this: if not $this->shouldWrapRedirect()... passing $event->getRequest() and $event->getResponse(). If we should not wrap the redirect, return and do nothing.

In a minute we'll add the logic down here to change the response.

But let's finish shouldWrapRedirect(). Start by checking to see if the $response is not a redirection. If it's not, return false. The only responses we need to change are redirects: we don't want to change normal frame loads or frame form submits that are returning with validation errors.

The only other check we need is for the header. Copy the header name from turbo-helper.js. Then return $request->headers->get('Turbo-Frame-Redirect'). So if the header exists and is set to something "truthy" like 1, this method will return true. Else, it will return false. Actually, I'm missing a tiny detail, but I'll fix it in a minute.

Finally, back in onKernelResponse(), at this point, we know that this request was made inside of a frame that has our data-turbo-form-redirect attribute and we know that the controller returned a redirect.

And so, create a new response object: new Response(), passing null for the content - we don't need to return anything - a 200 status code - so not a redirect - and then an array of headers. Invent a new header called Turbo-Location set to the URL that we want to redirect to. We can get that from the original response: $event->getResponse()->headers->get('Location').

Finally, to use this response instead of the original, say $event->setResponse($response).

Ok! That's all we need to do in Symfony: we're now replacing the redirect response in this situation with something different.

Reading the Response Header and Navigating

The last little piece of work is back in JavaScript. We already have a beforeFetchResponse() method, which is currently looking to see if a request was successful and redirected... and checking for the turboFormRedirect data attribute.

We can simplify this a lot. All we need to do now is check to see if the response has this Turbo-Location header. If it does, then we know that we should read that header and navigate.

Remove most of the code on top and add const redirectLocation = set to fetchResponse.response.headers.get('Turbo-Location').

Then, if we do not have a redirectLocation, we know this is not a situation where we need to do anything fancy. So, just return.

Then, the rest is perfect, except instead of fetchResponse.location. use redirectLocation.

That's it. We don't even need our getCurrentFrame() method anymore. It took more work inside of Symfony, but the JavaScript side of things is nice!

Oh, but before we try this, back in our subscriber, before the return statement, add a (bool) type-cast. This will guarantee the method returns a boolean.

Ok, now let's try it: Go back to the cart page and refresh. Remember: the whole goal is to be able to submit this form and have it not make duplicate requests to the redirected page. If we accomplish that, we'll be rewarded by seeing the success flash message. And... yes! There it is!

Look up here on the Ajax requests. We submitted the cart form here... and then there was only one request for the product show page, not two. Mission accomplished!

Thanks to our new fancy system, we can also - easily - solve an annoying problem. What happens if the user tries to open something in a <turbo-frame> - like a modal - but they got logged out in the background... maybe after taking a really long coffee break. Instead of just having this load broken, let's write about 10 lines of code to gracefully handle this everywhere.