Add reactive session$getCurrentTheme()

[wch]

This adds a method session$getCurrentTheme(), which is reactive and returns the current theme for the session.

We may want to tweak the interface, but this works.

remotes::install_github('rstudio/shiny@wch-reactive-theme')

Demo app:

library(shiny)
library(bootstraplib)

shinyApp(
  ui = fluidPage(
    theme = bs_theme(bootswatch = "darkly"),
    selectInput("theme", "Select theme", choices = c("darkly", "sketchy", "solar", "lux")),
    numericInput("n", "n", 1),
    sliderInput("slider", "Slider Input", 1, 10, 5),
    p("This is some text"),
    br(),
    verbatimTextOutput("theme_info")
  ),
  server = function(input, output, session) {
    observe({
      theme <- bs_theme(bootswatch = input$theme)
      session$setCurrentTheme(theme)
    })
    
    output$theme_info <- renderPrint({
      print(Sys.time())
      session$getCurrentTheme()
    })
  }
)

[cpsievert]

LGTM once the mock session method is added.