This article is for Shiny and R Markdown developers who wish to write custom HTML components that “just work” with . Readers should already have some basic understanding of the Sass language as well as the sass package.

A basic themeable component

Before going through a full-blown dynamically themeable custom component, let’s start from a relatively straight-forward example of implementing a custom person() component. Say we have the following R function to generate some HTML with classes that we’ll write custom Sass/CSS styles for:

person <- function(name, title, company) {
  div(
    class = "person",
    h3(class = "name", name),
    div(class = "title", title),
    div(class = "company", company)
  )
}

And here’s some custom Sass to style those classes. Since these Sass rules listen to Bootstrap Sass variables like $gray-600, person() styles works great with different bs_theme() input:

.person {
  display: inline-block;
  padding: $spacer;
  border: $border-width solid $border-color;
  @include border-radius;
  @include box-shadow;
  outline: 0;
  width: 300px;
  .title {
    font-weight: bold;
  }
  .title, .company {
    color: $gray-600;
  }
  margin: $grid-gutter-width;
  margin-right: 0;
  // On mobile, span entire width
  @include media-breakpoint-down(sm) {
    display: block;
    width: auto;
    margin-right: $grid-gutter-width;
  }
}
.person:last-of-type {
  margin-right: $grid-gutter-width;
}

If we were to save these Sass rules to a file named person.scss, then we can then bs_add_rules() to the bs_theme() and use our themeable person() component like so:

ui <- fluidPage(
  theme = bs_theme(bg = "#002B36", fg = "#EEE8D5") %>%
    bs_add_rules(sass::sass_file("person.scss")),
  person("Andrew Carnegie", "Owner", "Carnegie Steel Company"),
  person("John D. Rockefeller", "Chairman", "Standard Oil")
)
shinyApp(ui, function(input, output) {})

Dynamically themeable component

To make the custom person() component dynamically themeable (i.e., make it work with session$setCurrentTheme()), we need an R function that generates an htmltools::htmlDependency() from a given theme. While not required, suppose this function, person_dependency, lives in an R package called {mypkg} which includes the person.scss (and pre-compiled person.css) file under the inst/ directory. Then we could do the following:

name <- "person"
version <- "1.0.0"
person_dependency <- function(theme) {
  if (is_bs_theme(theme)) {
    scss <- system.file(package = "mypkg", "person.scss")
    bs_dependency(
      input = sass::sass_file(scss),
      theme = theme,
      name = name,
      version = version
    )
  } else {
    htmlDependency(
      name = name,
      version = version,
      stylesheet = "person.css",
      package = "mypkg",
      all_files = FALSE
    )
  }
}

#' @export
person <- function(name, title, company) {
  div(
    class = "person",
    h3(class = "name", name),
    div(class = "title", title),
    div(class = "company", company),
    bs_dependency_defer(person_dependency)
  )
}

Note that when theme is a bs_theme() object, then person.scss is compiled with Bootstrap Sass variables and mixins included via bs_dependency() (which returns the compiled CSS as an htmlDependency()). Otherwise, if theme is not a bs_theme() object, then person() is being used in a context where bslib is not relevant, so a pre-compiled CSS file is returned instead. Pre-complied CSS isn’t necessarily a requirement, but it’s a good idea for increasing performance and reducing software dependencies for end users.

HTML widgets

For htmlwidgets that can be themed via CSS, we recommend supplying a bs_dependency_defer() to the dependencies argument of createWidget() (similar to the person() component from the last section), which will make the widget dynamically themeable. For widgets that can not be themed via CSS, the best option may be to query the active theme inside a preRenderHook() via bs_current_theme(), and then translate any relevant information to the widget’s instance data, for example:

my_widget <- function(...) {
  createWidget(
    name = "mywidget", ...,
    preRenderHook = my_widget_hook
  )
}
my_widget_hook <- function(instance) {
  theme <- bslib::bs_current_theme()
  if (!bslib::is_bs_theme(theme)) {
    return(instance)
  }
  instance$x$theme <- modifyList(
    instance$x$theme, as.list(
      bslib::bs_get_variables(theme, c("bg", "fg"))
    )
  )
  instance
}