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Frame Redirecting and Clearing the Snapshot Cache

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Where else could we use our new turbo frame redirect system? Go to the cart. On the featured product sidebar, we could leverage a frame around the cart controls. Right now, this form submits to the whole page. And so, on success, the entire page is redirected to that product with a nice flash message. I love it! That's exactly the behavior I want.

But go back to the cart. This time, let's change the color... and be annoying: try to buy a negative quantity. Hit add. It still changed the entire page... which isn't as smooth as I'd like. It would be way cooler if the error showed up in the sidebar on the cart page.

Adding the turbo-frame

Time to add a frame! The template for this "add to cart" section lives at templates/product/_cart_add_controls.html.twig. This template is included on two pages: the product show page and also over on the sidebar. When we submit the form, as we just saw, it's handled by the product show page. This means that if we added a frame around this entire template, when we submit, the response will contain a matching turbo-frame... since the product show page renders this template.

In other words... adding a frame here should... just work. On top, add <turbo-frame> with id="add-to-cart-controls". Add the closing frame at the bottom.

<turbo-frame id="add-to-cart-controls">
{{ form_start(addToCartForm, {
attr: { class: 'cart-add-controls d-flex align-items-center justify-content-baseline' }
}) }}
{% if addToCartForm.color is defined %}
// ... lines 6 - 25
{% endif %}
// ... lines 27 - 34
{{ form_end(addToCartForm) }}
{{ form_errors(addToCartForm) }}
{% if addToCartForm.color is defined %}
{{ form_errors(addToCartForm.color) }}
{% endif %}
{{ form_errors(addToCartForm.quantity) }}

Just with that, refresh the page and go to the cart. Submit with a negative quantity. That is so much nicer. Now change to red change, set the quantity to 5 and hit add.

Um, did that work? The color changed back and the quantity reset... and I don't see any errors. But that wasn't very obvious. I also don't see the item in the cart until I refresh.

As usual, this behavior makes sense if you think about it. When we submit the form, it redirects to the product show page. And that renders a success message - which we don't see - and a "reset" add to cart form. Ya know, this would all work much better if we could go back to the original success behavior where we navigate the entire page after adding an item.

Activating data-turbo-form-redirect

Fortunately, that's exactly what our new frame system does! Let's add the attribute that we invented to this frame. I'll move it onto multiple lines to keep my sanity, then add data-turbo-form-redirect="true".

// ... lines 5 - 46

Testing time! Refresh the cart page. If we submit the form with errors, everything stays right here. But if we submit it successfully... yes! That redirected to the product show page!

We Preventing the Snapshot Cache From Clearing!

Though... dang! There are two weird things going on. First, we're missing our flash message! We'll talk about that later.

To see the second, watch the shopping cart header as we add more and more items. Yikes! It jumps backwards and forwards!

This is a result of the preview system. When we submit this form, for just a moment, it shows the cached preview of this page. For example, at this moment, the cached version - which comes from the last time we navigated away from this page - still holds the value 14. So when we hit add, it jumps back to 14 and ahead to 16. Now, a page with 15 sits in the cache.

This is not normally a problem. If you submit a POST request with Turbo and the response is successful, Turbo automatically clears its internal snapshot cache. It does that precisely to avoid this problem: a successful form submit typically means that something has changed on the server. So, to be safe, Turbo decides that it shouldn't use any of its old, cached pages.

Manually Clearing the Cache

So... if that's true, why are we seeing this problem? In turbo-helper, we're calling event.preventDefault() in the turbo:before-fetch-response listener. This tells Turbo to prevent rendering this response. But... it has a side effect: it also prevents it from clearing its snapshot cache!

But now that we know that, it's no problem: we can clear it manually by saying Turbo.clearCache().

125 lines | assets/turbo/turbo-helper.js
// ... lines 1 - 3
const TurboHelper = class {
// ... lines 5 - 103
beforeFetchResponse(event) {
// ... lines 105 - 113
// ... lines 118 - 121
// ... lines 123 - 125

Refresh and watch the cart header. Much better.

Bug with Not Clearing the Current Page

By the way, there is still one spot where this jumpy cart thingy happens. Go to the cart page and add an item. Watch the number when I click back to the shopping cart... it's 21 right now. See that? It temporarily jumped back to 20.

This happens due to, what I think is, a fairly straightforward bug in Turbo that I hope will be fixed in the future. Here's the scoop. As we just talked about, when you successfully submit a POST form, Turbo clears its snapshot cache. And we even manually did that a minute ago. But right after it clears the snapshot cache, as we're navigating away, it re-caches the page that we just submitted!

This means that, when we hit add, it clears the snapshot cache but then re-caches this page with a shopping cart number set to 21.

This is pretty rare situation. To trigger this, you need a form that submits to another page. And then the problem only happens if you navigate back to that form later. I'm going to ignore this.

Next: the bigger weird issue with our new system is that, when we add an item, it redirects... but there's no success flash message. This page actually does have a flash message... we saw it a few minutes ago: it should be showing right here. But something unexpected is happening behind the scenes that's hiding it.

Let's find out what next and improve our system to prevent it.