README.md

output variables are used to send information to client to display, and in most cases, they are all that's needed for an app. In some cases, though, you need more flexibility in what the server does in response to inputs.

When you set an output variable to a value, that value is sent to the client to be displayed. In contrast, observers don't (by default) send anything to the client. Their purpose is simply to execute R code. For example, observers can be used to write to files or databases. They can also be used send custom messages to the client -- these are messages that don't fit into the normal output model.

DESCRIPTION

Type: Shiny
Title: Observer demo
License: MIT
Author: Winston Chang <winston@rstudio.com>
AuthorUrl: http://www.rstudio.com/
Tags: observer
DisplayMode: Showcase

server.R

shinyServer(function(input, output, session) {

  # Create a random name for the log file
  logfilename <- paste0('logfile',
                        floor(runif(1, 1e+05, 1e+06 - 1)),
                        '.txt')


  # This observer adds an entry to the log file every time
  # input$n changes.
  obs <- observe({    
    cat(input$n, '\n', file = logfilename, append = TRUE)
  })


  # When the client ends the session, suspend the observer.
  # Otherwise, the observer could keep running after the client
  # ends the session.
  session$onSessionEnded(function() {
    obs$suspend()

    # Also clean up the log file for this example
    unlink(logfilename)
  })


  output$text <- renderText({
    paste0("The value of input$n is: ", input$n)
  })

})

ui.R

shinyUI(fluidPage(
  titlePanel("Observer demo"),
  fluidRow(
    column(4, wellPanel(
      sliderInput("n", "N:",
                  min = 10, max = 1000, value = 200, step = 10)
    )),
    column(8,
      verbatimTextOutput("text"),
      br(),
      br(),
      p("In this example, what's visible in the client isn't",
        "what's interesting. The server is writing to a log",
        "file each time the slider value changes.")
    )
  )
))