How to enforce Drab to reload full loop?

I use Drab and I want just to change the value of assign inside of a loop.

I have this loop code:

<%= for page <- @holder.pages do %>
  <%= if == @page_id do %>
      <td><%= page.title %></td>
  <% end %>
<% end %>

I change @page_id like this:

Drab.Live.poke(socket, page_id: id)

This function called when clicking on a button.
Everything works just fine when I place @page_id on some static element like h1.
But it doesn’t work inside of the loop.
Error: undefined function page/0

Can someone explain me why does it happen? I think Drab doesn’t fully reload loop - only if expression.


I tried to place this loop logic to commander function like this:

holder = peek(socket, :holder)
page = Enum.find(lesson.pages, fn(p) -> == id end) id end)
poke(socket, page_title: page.title)

But I got nil instead of page.
I have no idea what’s wrong with this code. I tried a lot of tricks and workarounds.


Oh @grych, you around? I think the embedded order of data holding is a bit off. ^.^

@jeffrey You may want to report this on the Drab issue tracker. :slight_smile:


Hi @OvermindDL1, thanks for waking me up from my winter hibernation :wink:
Hi @jeffrey, I will take a look and will get back to you. It might be a limitation of how Drab is processing the code blocks (inside do…end), I am not sure if it is properly documented (here).


Unfortunately, it is not a bug. This is a limitation of Drab.Live’s local variables scope && the way how Drab is replacing parts of the page with the evaluated expression with new assign values.

When you’re poking the new version of assign @page_id, Drab tries to update it in every expression on a page, which contains that value. In this case, it is if == @page_id, do:.... But in the particular moment it does not know the value of page variable - that’s why it shows undefined function page() while evaluating this expression.

It is done by purpose. I could search expressions for assigns in their do..end blocks, and re-render the the whole for comprehension while changing @page_id. And actually it was made like this before, but I realized that in the real world it provides to full reload of almost the whole page and more issues, like #42.

In this particular situation, when you poke @page_id, you really want to update the whole for compehension. One of the solution would be to put @page_id assign in the comprehension filter:

<%= for page <- @holder.pages, == @page_id do %>
    <td><%= page.title %></td>
<% end %>

This moves @page_id to the for expression, and now, after you poke it, it re-evaluates the whole one.

I know it is not very intuitive, but so far I couldn’t find the better solution.

Anyway, please create an issue on github, this situation needs more explanation/docs and/or at least the better error description. Probably it should not even compile.



Ok. Thanks for explaining!

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I’ve got the idea on how to make such things easier to maintain for a developer. What about if you can leave your code as it is, and ensure the parent comprehension is refreshed by just updating both assigns?

poke socket, page: peek(:page), page_id: new_page_id

(or even introduce new API)

poke socket, page_id: new_page_id, refresh_assigns: [:page]

In the current version poking both assigns will generate the same error, because Drab will try to update @page_id anyway, even if it is completely pointless as it is inside the for comprehension, which is updated anyway.

Drab should be aware of such cases and update only the parent statement. It would be also a performance improvement.

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Btw I tried to update both assigns when I was looking for solution. So I think it’s intuitive.
But I think new API for updating additional assigns even more intuitive.

Cool. I will do that, one more small step for Drab to be better!

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It is solved now, the change will be included in 0.7.1.

Solution for your case:

poke socket, holder: peek(socket, holder), page_id: new_page_id

I did not introduce a new API for poke, as I don’t want to add a special options like refresh_assigns. You may have an assign named @refresh_assigns in you page :slight_smile:

If you have an idea for a good API for this case, please do not hesitate.

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