shiny-post-handler.R

library(shiny)
library(jsonlite)

# we'll store received data in a local json file
write_json(list(), "data.json")

post_handler <- function(req, response) {
  # we'll catch everything that's POST for this demo but you'll want to make
  # sure you don't step on shiny's built-in POST handlers
  # (I'm pretty sure this handler is called after shiny's handlers but :shrug:)
  if (identical(req$REQUEST_METHOD, "POST")) {
    # you probably want to limit the upload file size w/ the first arg of $read()
    data <- req$rook.input$read()
    data <- jsonlite::fromJSON(rawToChar(data))
    message(str(data))
    if (is.null(data$token) || is.null(data$name)) {
      # expect a token and a name for this demo but you'd want to do auth/routing here... 
      # if the request is bad return the base response of 404 not found
      return(response)
    } else {
      # update our local data store (or write to db etc)
      y <- list()
      y[[data$token]] <- data$name
      x <- utils::modifyList(read_json("data.json"), y, keep.null = TRUE)
      write_json(x, "data.json")
      # return all okay response!
      return(httpResponse(200, "text/plain", "OK\n"))
    }
  }
  # return regular shiny response
  response
}

options(shiny.http.response.filter = post_handler)

ui <- fluidPage(
  h2("Hello", uiOutput("name", inline = TRUE, container = span))
)

server <- function(input, output, session) {
  # reveal token which we'll use as if it were a user id
  message(session$token)

  # poll our data store (local json) for updates, using token as ID
  name <- reactiveFileReader(1000, session, "data.json", function(x) {
    data <- read_json(x)
    if (session$token %in% names(data)) {
      data[[session$token]]
    }
  })

  output$name <- renderUI({
    if (is.null(name())) return("...")
    paste0(name(), "!")
  })
}

shinyApp(ui, server)

To test it set token and name in bash and send json via curl:

token=____
name=____
curl -X POST -H "Content-Type: application/json" -d "{\"name\": \"$name\", \"token\": \"$token\"}" http://127.0.0.1:3185

(don't forget to update the url to point to your shiny app)