(Experimental) Xaringan with configurable styles
Xaringan and other R Markdown-based presentation formats are great for quickly and reproducibly building presentations, but if you want to customise colours of fonts, you need to dip into the sometimes-scary world of Cascading Style Sheets (CSS).
The idea with {bigtop} is to create a Xaringan theme that leverages CSS custom properties to make limited customisations available right from the frontmatter of the RMarkdown document, like:
---
title: "My sweet slide deck"
author: "James Goldie"
styles:
fonts:
header:
- "Montserrat"
- "Helvetica Neue"
body:
- "Helvetica Neue"
- "Arial"
- "sans"
colors:
- red
- green
- yellow
---Here's how CSS custom properties can work:
style.css
h1 {
color: red;
background-color: var(--main-bg-color);
}test.html
<html>
<head>
<title>Test test test</title>
<link rel="stylesheet" type="text/css" href="style.css">
<!-- insert css variables here! -->
<style>
:root {
--main-bg-color: pink;
}
</style>
</head>
<body>
<h1>Test test test</h1>
<p>This'll help me work it out!</p>
</body>
</html>Here, the CSS file describes the style of the <h1> element in the HTML—but its background color is set to a CSS custom property. That property is then defined in a <style> tag inside <head>.
The benefit of this approach is that the CSS file doesn't need to be touched. It's possible that a properly specified HTML template could allow CSS info to be injected without even using the pre- or post-render hooks in RMarkdown (although I suspect this approach will be a little limited, in that no processing of the frontmatter values will be possible).
(Actually, I'm not sure that the HTML template needs to be modified either: CSS custom properties even work if the property definition comes before the included CSS file. This means I could use the ... although I'm not sure that's designed to touch the frontmatter. Nope, it's for specifying separate files like stylesheets.)includes argument to html_document
A more complex process could use the hooks to process the frontmatter before it's inserted into the template. Because xaringan::moon_reader hardcodes the HTML template (the one in the package), I'm going to have to reimplement it. That sucks a little.
I'm trying to work out how rmarkdown::render() and xaringan::moon_reader() work—I want to see if there's a place I can break in and send YAML to a CSS file (in the way that Rmarkdown content might be sent through an HTML template).
I think the action starts around line 420 of rmarkdown::render(), where the YAML from the input file is parsed. Then:
- line 497: what's an "intermediates generator"?
- line 530: "call any pre-knit handler" (the only arg this takes is this original input contents... so basically the whole file rmd).
- line 535: post-knot handler takes
yaml_front_matter(ie. parsed YAML),knit_input,runtimeandencoding. - Lines 555–763: if
requires_knit(otherwise, justcall_post_knit_handler()): - A bunch of Pandoc stuff that I probably don't care about (Xaringan uses inline Markdown, not Pandoc)
- line 950: "if there is a post-processor then call it" (it takes
yaml_front_matter,utd8_input,output_format,cleanand!quiet) - That's it!
Okay, functions like xaringan::moon_reader() actually return the result of a call to rmarkdown::output_format() (which I think is why you see references in render() to things like output_format$pre_knit etc.). This is where format-specific things like pre_knit, post_processor, etc. are defined.
https://github.com/yihui/xaringan/blob/85c0e7f268ba94306bb33f2cfb70ad846d03b9ad/R/render.R#L165
... (which comes all the way from rmarkdown::render() at the top) is passed to the pre_knit function and also to rmarkdown::html_document2, which is the base_format. According to html_document, extra_dependencies and ... are "Additional function arguments to pass to the base R Markdown HTML output formatter html_document_base". But that function ignores ....
I don't think I can pass anything all the way in from rmarkdown::render(). But maybe I can fork moon_reader() or make a version of it that includes some extra code in pre_processor.
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