Reporting of output styles (colors and fonts)

[cpsievert]

Adds the ability to obtain computed CSS styles of output containers via getCurrentOutputInfo() within any renderPlot(), renderImage(), or any output binding with a .shiny-report-theme class. Currently the computed styles include bg (background-color), fg (color), accent (link-color), font (font-family), and renderedFamily (the actual font-family chosen by the browser....this information is only available if FontFaceSet.check() is available). For example:

library(shiny)

shinyApp(
  fluidPage(
    tags$style(HTML("#info {color: #FFFFFF; background-color: #000000}")),
    tags$style(HTML("#info a {color: #00FF00}")),
    h3("Below is computed styles for the info container"),
    tagAppendAttributes(
      textOutput("info"), 
      class = "shiny-report-theme"
    )
  ),
  function(input, output) {
    output$info <- renderText({
      info <- getCurrentOutputInfo()
      jsonlite::toJSON(
        list(
          bg = info$bg(), 
          fg = info$fg(), 
          accent = info$accent(), 
          font = info$font()
        ), 
        auto_unbox = TRUE
      )
    })
  }
)

Note that after this change, htmlwidgets::shinyWidgetOutput() will gain a reportTheme argument to make it easier for htmlwidgets developers to include the .shiny-report-theme class (and, thus, access styles with getCurrentOutputInfo()).

In addition, a new shiny.useragg option was added to use ragg::agg_png() over Cairo::CairoPNG and non-quartz png() (if ragg is installed). For now, this option will default to FALSE, but will likely default to TRUE in a future release.

Testing notes

Install remotes::install_github("rstudio/shiny"), then run the example app above and confirm that you see the same information as the screenshot. That's ok if you see differences in the font information, but everything else (i.e., bg, fg, accent) should be the same.

There's also a fully automated testing app here https://github.com/rstudio/shinycoreci-apps/blob/master/apps/181-report-theme/app.R