StephanLehmke

Using "generated" class names in Tailwind under Phoenix 1.7+

Is there an inherent problem with dynamically generating tailwind class names in a component?

I wanted to make the button color configurable. This is mostly from the auto-generated core_components.ex. I only added the “color” attribute, so I could say something like <.button color="red">.

  attr :type, :string, default: nil
  attr :class, :string, default: nil
  attr :color, :string, default: "zinc"
  attr :rest, :global, include: ~w(disabled form name value)

  slot :inner_block, required: true

  def button(assigns) do
    ~H"""
    <button
      type={@type}
      class={[
        "phx-submit-loading:opacity-75 rounded-lg bg-#{@color}-900 hover:bg-#{@color}-800 py-2 px-3",
        "text-sm font-semibold leading-6 text-white active:text-white/80",
        @class
      ]}
      {@rest}
    >
      <%= render_slot(@inner_block) %>
    </button>
    """
  end

The HTML source of the page looks OK to me:

    <button class="phx-submit-loading:opacity-75 rounded-lg bg-red-900 hover:bg-red-800 py-2 px-3 text-sm font-semibold leading-6 text-white active:text-white/80">
  Speichern und beenden
</button>
    <button class="phx-submit-loading:opacity-75 rounded-lg bg-zinc-900 hover:bg-zinc-800 py-2 px-3 text-sm font-semibold leading-6 text-white active:text-white/80" name="Weiter">
  Speichern
</button>

But the colors don’t show up unless they are also explicitly used somewhere else.

There is essentially the same problem with my attempt to define a component for a grid column with configurable span:

  @doc """
  Renders a grid column.
  """
  attr :span, :string, default: "1"
  slot :inner_block, required: true

  def gridcol(assigns) do
    ~H"""
    <div class={"col-span-#{@span}"}>
    <%= render_slot(@inner_block) %>
    </div>
    """
  end

The col-span seems to be ignored although the HTML source looks Ok.

9 comments

/phoenix #component #tailwind

13 1440 9

2026-06-26 14:24:23 UTC

Marked As Solved

sodapopcan

Ya, Tailwind won’t work with dynamic classes that way unless you list out all of the possibilities somewhere. From the docs:

Tailwind CSS works by scanning all of your HTML files, JavaScript components, and any other templates for class names, generating the corresponding styles and then writing them to a static CSS file.

So it does not see the literal strings bg-#{@color}-900 and col-span-#{@span} a valid class names so it ignores them.

If you want to provide custom colors I would use vanilla CSS. Another hacky solution would be to just list all possible classes in a comment, though that isn’t a great solution.

Post #2

Also Liked

arcanemachine

You can declare them in the safelist section of your Tailwind config file (default: assets/tailwind.config.js). For example:

module.exports = {
  content: ["./js/**/*.js", "../lib/*_web.ex", "../lib/*_web/**/*.*ex"],
  theme: {
    // ...
  },
  safelist: [
    "bg-primary",
    "bg-secondary",
    "bg-accent",
    "bg-neutral",
    "bg-info",
    "bg-success",
    "bg-warning",
    "bg-error",
  ],
  plugins: [
    // ...
  ],
};

EDIT: After thinking about it and RTFD, it turns out you can use regex as well:

  safelist: [
    "bg-(primary|secondary|accent|neutral|info|success|warning|error)","
  ]

Post #4

StephanLehmke

Ok, that seems doable.
I’ll try it out, thanks!

Post #5

sodapopcan

That’s cool, I wasn’t aware of safelist, though it’s still just a “cleaner” solution over writing them all out in a comment—a bit unscalable if you have a lot of colors and have to cover bg, text and so on. I’m seeing the docs even say it’s a last resort. Though if you are only doing the background color and text color, it could be ok.

@StephanLehmke At first I was assuming you were talking about allowing users to provide custom colors but if you just want different colour buttons, it’s far more preferable to define them either, as you mentioned, as different components, or use an attribute. It’s probably even better to defined them based on their purpose rather than colour, but that is up to you. You shouldn’t need much duplication.

def button(%{type: :primary} = assigns) do
  assigns = assign(assigns, :class,  "text-blue-50 bg-blue-900")

  ~H"""
  <.button class={@class} />
  """
end

def button(%{type: :muted} = assigns) do
  assigns = assign(assigns, :class,  "text-zinc-700")

  ~H"""
  <.button class={@class} />
  """
end

def button(assigns) do
  assigns = assign_new(assigns, :class, fn -> "text-zinc-900 text-zinc-100" end)

  ~H"""
  <button class={@class}>
    <%= render_slot(@inner_block) %>
  </button>
  """
end

# Usage:

<.button type={:primary} />

(I didn’t try out that code but it’s the general idea)

Post #6