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18.

Turbo Streams: Update any Element

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Today, we're diving headfirst into the finale of the Turbo trilogy: Turbo Streams. Streams allow us to solve problems that we... just don't have a solution for yet.

Take, for instance, our homepage: we have this really nice data tables system... with one teeny tiny problem. When we type into this box, that number of results doesn't change. It's stuck at whatever it was on page load. The Turbo Frame is around this table, so the result count is outside of that.

This is where Turbo Streams comes in. When you're dealing with a Turbo Frame and you need to update something outside of it, you need a stream. Streams have a fancy name, but it's a simple idea. A Turbo Stream is actually a custom HTML element. I could take this, put it onto my page, and it would instantly execute. It would find the element on the page whose id is messages and append this content. And there are actions for everything: prepend, replace, update, etc. We can use a Turbo Stream to make any change we want to any element on the page... from anywhere. The power!

Adding a <turbo-stream> Right on the Page

To prove this, copy the Turbo Stream that's an update. Back on our site, inspect element on the "Space Inviters" name. Temporarily, give this an id called company_name so we can target it.

Now, anywhere else on the page - so how about down here in the footer - edit as HTML and paste that Turbo Stream. In this case, we want the target to be company_name and inside the template element, say "Space Invaders!". Now, check this out. As soon as I click out of this, the <turbo-stream> custom element will become active and will execute its action. Watch. Boom! It found that element and updated it!

Take a peek back at the footer: that <turbo-stream> is gone! It executes, then self-destructs and removes itself from the page. It's the most noble of custom elements.

And even if it were on the page for a moment, remember: all <turbo-streams> have a template element inside. We talked about that element on Day 11: anything inside a <template>... isn't really on the page at all: it's completely hidden and inactive. So even if this were on the page for a moment, it would be invisible.

Streams just work.

Updating the Result Count with a Stream

So let's use them to solve our problem! Open templates/main/homepage.html.twig and search for "results". Here's the element we need to update. To target this, give it an id: how about voyage-result-count:

145 lines | templates/main/homepage.html.twig
// ... lines 1 - 27
{% block body %}
<div class="flex">
// ... lines 30 - 36
<section class="flex-1 ml-10">
<form
// ... lines 39 - 54
<div id="voyage-result-count" class="whitespace-nowrap m-2 mr-4">{{ voyages|length }} results</div>
</form>
// ... lines 57 - 141
</section>
</div>
{% endblock %}

Copy that. When we search on the page, it's actually this <turbo-frame> that's navigating. So anywhere inside this - I'll go to the bottom - we can add a <turbo-stream>. Say: <turbo-stream action="replace", target="" and paste. Then add the <template> element - don't forget that - and I'll hard-code a message to start:

145 lines | templates/main/homepage.html.twig
// ... lines 1 - 27
{% block body %}
<div class="flex">
// ... lines 30 - 36
<section class="flex-1 ml-10">
// ... lines 38 - 56
<turbo-frame id="voyage-list" data-turbo-action="advance">
// ... lines 58 - 135
<turbo-stream action="replace" target="voyage-result-count">
<template>
Is this thing on?
</template>
</turbo-stream>
</turbo-frame>
</section>
</div>
{% endblock %}

Ok, watch what happens when I refresh. Boom! Because the <turbo-stream> element exists on page load, it immediately executes and replaces the element with the custom content.

Replacing the Real Content with a Block

So now... let's put in the real content. Essentially, we want to copy this entire div... and paste it down here. Except... without actually duplicating this.

To do this, we'll use a trick with Twig blocks. Surround the result count with a new block called result_count... then endblock below:

147 lines | templates/main/homepage.html.twig
// ... lines 1 - 27
{% block body %}
<div class="flex">
// ... lines 30 - 36
<section class="flex-1 ml-10">
<form
// ... lines 39 - 43
>
// ... lines 45 - 54
{% block result_count %}
<div id="voyage-result-count" class="whitespace-nowrap m-2 mr-4">{{ voyages|length }} results</div>
{% endblock %}
</form>
// ... lines 59 - 143
</section>
</div>
{% endblock %}

In Twig, you're free to add blocks wherever you want. When you do, they don't do anything immediately. When this renders, Twig will see this block.... ignore it... and render the div like normal.

But now, we can go down inside our <turbo-stream> and say {{ block('result_count') }}:

147 lines | templates/main/homepage.html.twig
// ... lines 1 - 27
{% block body %}
<div class="flex">
// ... lines 30 - 36
<section class="flex-1 ml-10">
// ... lines 38 - 58
<turbo-frame id="voyage-list" data-turbo-action="advance">
// ... lines 60 - 137
<turbo-stream action="replace" target="voyage-result-count">
<template>
{{ block('result_count') }}
</template>
</turbo-stream>
</turbo-frame>
</section>
</div>
{% endblock %}

I think we're ready! Start by going to the homepage so we see the full 30 results. And then as we type... ah, beautiful! The count updates as the results reload. Dang, that was easy!

For those of you that are nerds for details, first, we love you, and second, yes, on page load, we're rendering the result count twice: here... and, even though we can't see it, we're also rendering it down here inside the Turbo Stream. So it's being rendered twice inside the HTML. And that's not a problem at all, unless, for some reason, calculating the result count takes a lot of work. If you had that situation, you could set the count to a Twig variable, then render in both spots.

All right, tomorrow we'll start into the biggest, boldest part of this entire series: building a reusable modal system that just absolutely rocks. I'm so excited!