httpuv application that listens for R commands in background

listen.R

# This application will listen for local connections on port 7000 and run
# commands it receives via HTTP POST. It will run commands using the later
# packages -- that is, whenever the R call stack is empty (it is idle at the R
# prompt), or whenever later::run_now() is called. Shiny keeps calling run_now()
# when running an application, so this can run commands when a Shiny application
# is running.
#
# This can be useful for enabling options while a Shiny application is running.
# For example, you can use it to run `options(shiny.trace=TRUE)` without
# restarting the application.
#
# curl -X POST http://localhost:7000/ --data "options(shiny.trace=TRUE)"
# wget --quiet  -O - http://localhost:7000/ --post-data="options(shiny.trace=TRUE)"


# Note that it requires the background-thread branch of httpuv:
#   devtools::install_github('rstudio/httpuv@background-thread')

# Examples of sending R commands via POST with curl or wget:
# curl -X POST http://localhost:7000/ --data "a<-1; a+100"
# wget --quiet  -O - http://localhost:7000/ --post-data="a<-1; a+100"
httpuv::startServer("127.0.0.1", 7000,
  list(call = function(req) {

    txt <- req$rook.input$read_lines()
    expr <- parse(text = txt)
    result_txt <- capture.output({
      result <- withVisible(eval(expr, envir = .GlobalEnv))
      if (result$visible == TRUE) {
        print(result$value)
      }
    })
    result_txt <- paste(result_txt, collapse = "\n")
    
    # Print result to R console
    cat(result_txt, "\n", sep = "")

    # Return result to client
    list(
      status = 200L,
      headers = list(
        'Content-Type' = 'text/plain'
      ),
      body = result_txt
    )
  })
)


# To stop:
httpuv::stopAllServers()