Add basic `appshot.shiny.appobj` functionality

[schloerke]

Adds an appshot method for shiny app objects.

• Saves all elements in the same environment as the server function of the app.
• Saves all currently attached packages.
• In a separate process,
  • Loads the environment, app, and packages.
  • Run the app
• Take a screenshot

[schloerke]

Could not use observe() from Shiny because the system2() command to call phantom within webshot became blocked.

library(shiny)

ui <- fluidPage(
  plotOutput("plot")
)

server <- function(input, output, session) {
  output$plot <- renderPlot({
    plot(cars)
  })
}

obj <- shinyApp(ui, server)

run_and_snapshot <- function(shinyAppObj) {
  initialized <- FALSE
  observe({
    if (!initialized) {
      invalidateLater(5000)
      initialized <<- TRUE
      return()
    }
    
    # Do whatever
    message("Take snapshot")
    shiny::stopApp()
  })
  
  print(obj)
  message("Got here")
}

[jcheng5]

@schloerke Can you elaborate on that? Calling system2 didn't ever return if you called it from where message is in the code snippet?

[schloerke]

Updated to use shiny::observe and keep checking until the callr r session closes, rather than looking for the file.

[wch]

It would also be good to have a timeout check, just in case the R process with webshot hangs for some reason.

[wch]

Maybe add on.exit(r_session$kill()) to make sure that it goes away? Although now that I think of it, that won't ensure that phantomjs process will exit -- in fact, it's probably the reverse, since if the child R process receives the hard kill signal, it won't be able to send any signals to the phantomjs process. Anyway, something should happen to ensure the child R process and grandchild phantomjs process exit.

Now that I think of it again, maybe killing the R process will cause the phantomjs process to exit. But the behavior might also be different on Windows.

[wch]

This delay should be a parameter. Same for appshot.character.

[schloerke]

I think that is fair for us to keep it internal. It is a delay for webshot to start processing. The delay in taking the picture could be increased by the 'delay' parameter. By keeping it static at 0.5, it is a minimum of 0.5 seconds in processing before the webshot is taken.

[wch]

Switch to callr::r_bg. We might not even need the processx package as a (direct) dependency.

[wch]

Use times instead of counts.

[wch]

Does this need to be invisible()?

[schloerke]

@wch Added

• shiny rmdshot example
• appshot shiny timeout adds delay value
• removed render quotes for do.call
• make wrapper to callr::r_bg to upgrade envvars from NULL to callr default env value