SVGs & EEx Tags: Inlining from External File?

There are already a couple of helper packages on Github to manage inlining external SVG files into templates which is great from a readability point of view. However I’m guessing because of the way templates are evaluated, EEx tags treated as strings.

Taking the thoughtbot package as the base for a simple example, do something along the lines of:

def inline_svg(file_name) do
  path = static_path(file_name)
  case do
    {:ok, file} -> {:safe, file}
    {:error, _} -> raise "No SVG found at #{path}"

defp static_path(file_name) do
  path = "assets/static/svg"
  [path, "#{file_name}.svg"] |> Path.join() |> Path.expand

That will render the SVG but include the EEx as strings rather than the output of <%= some_func() %> included in the SVG file as expected because of the {:ok, file} -> {:safe, file}. But removing that and going with {:ok, file} -> (file), or wrapping the svg_inline() function with raw() still doesn’t give me the desired outcome.

So I’m wondering where in Phoenix’s order should I be attempting to run this so that tags included in the inlined SVG are rendered as expected?


As far as “pure” EEx is concerned there is absolute no difference, as raw EEx do not care about “tags”, as it do not understands them. If you are talking about Phoenix HTML, then code you have presented should work as expected. Are you sure that returned string is wrapped in tuple?

There’s a discussion on run-time loading and execution of Eex templates here: Dynamic EEx templates for web app

It sounds like the tags aren’t being evaluated because the templates aren’t getting compiled into functions.

Yeah this is purely in the context of Phoenix HTML.

I created a quick repo to demonstrate the issue. Using the function above, it’s definitely returning the tuple:

> DynImg.inline_svg("eex_test")
 "<svg width=\"100%\" height=\"60px\" viewBox=\"0 0 113 60\" version=\"1.1\" xmlns=\"\" xmlns:xlink=\"\">\n\t<g stroke=\"none\" stroke-width=\"1\" fill=\"none\" fill-rule=\"evenodd\" font-family=\"ArialMT, Arial\" font-size=\"24\" font-weight=\"normal\">\n<g transform=\"translate(-244.000000, -76.000000)\" fill=\"#000\">\n<text>\n                <tspan x=\"240.822\" y=\"122\"><%= \"Inlined by function\" %></tspan>\n</text>\n</g>\n    </g>\n</svg>"}

and for plain old SVG images it works exactly as expected. It’s only when I try to inline files with <%= %>, essentially treating the SVGs as templates that I have a problem.

Following @mindok’s link I seem to have to explicitly call EEx, in this case Phoenix.HTML.Engine and pass it through again but after spending the morning on it I’m not sure if I’m even any closer to an answer.

Your problem is that you’re using a function here. The function will be called at runtime, but at runtime the svg’s won’t be available (when using releases) or at least the path might no longer match. You’d want to use a macro, so you read the svg’s content at compile time and only have the result inlined with all the other eex template contents.

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Thanks @LostKobrakai.

Chatting on IRC I was pointed towards macros as well. Thinking about it, it does make sense it just never crossed my mind to consider the difference between macros & functions.

Writing macros isn’t something I’m familiar with so it’ll probably take me a while to rtfm.

Your macro:

defmacro inline_svg(file_name) do
  path = static_path(file_name)
  case do
    {:ok, file} -> quote do: {:safe, unquote(file)}
    {:error, _} -> raise "No SVG found at #{path}"

Thanks @hauleth but same issue after importing your macro into the view (