R Shiny Code gets executed multiple times when sending it to Javascript

Question

I have this app:

    #
# This is a Shiny web application. You can run the application by clicking
# the 'Run App' button above.
#
# Find out more about building applications with Shiny here:
#
#    http://shiny.rstudio.com/
#

library(shiny)

# Define UI for application that draws a histogram
ui <- fluidPage(includeScript("www/script.js"),

   # Application title
   titlePanel("Old Faithful Geyser Data"),

   # Sidebar with a slider input for number of bins 
   sidebarLayout(
      sidebarPanel(
         sliderInput("bins",
                     "Number of bins:",
                     min = 1,
                     max = 50,
                     value = 30)
      ),

      # Show a plot of the generated distribution
      mainPanel(
         plotOutput("distPlot")
      )
   )
)

# Define server logic required to draw a histogram
server <- function(session, input, output) {

   output$distPlot <- renderPlot({
      # generate bins based on input$bins from ui.R
      x    <- faithful[, 2] 
      bins <- seq(min(x), max(x), length.out = input$bins + 1)

      observe({
          if(input$bins > 25) {
              Message1 = input$bins
              session$sendCustomMessage("bla", Message1)
          }
      })

      # draw the histogram with the specified number of bins
      hist(x, breaks = bins, col = 'darkgray', border = 'white')
   })
}

# Run the application 
shinyApp(ui = ui, server = server)

My Oberserver checks if the value is larger than 25. I send the value to Javascript.

    $( document ).ready(function() {
    Shiny.addCustomMessageHandler("bla", dosomething);

    function dosomething(Message1) {
        alert(Message1)
    }
});

The code works perfectly, BUT every time i change the slider, the code seems to get executed one more time than before. After changing it 2 times, I get 3 alerts for example. Why is that happening and what can I do against it?

DeanAttali · Accepted Answer · 2018-10-16 20:16:34Z

1

The reason this is so broken is that your observe() is inside the renderPlot() function. Generally speaking, observers should not be inside render functions, it's almost always a recipe for very strange undefined behaviours to happen!

Simply moving the observer outside of the render function fixes your problem. This also fixes another problem you didn't mention, that the alert box was actually showing the previous number rather than the current one.

For completeness, this is the correct server code:

server <- function(session, input, output) {

  output$distPlot <- renderPlot({
    # generate bins based on input$bins from ui.R
    x    <- faithful[, 2] 
    bins <- seq(min(x), max(x), length.out = input$bins + 1)

    # draw the histogram with the specified number of bins
    hist(x, breaks = bins, col = 'darkgray', border = 'white')
  })


  observe({
    if(input$bins > 25) {
      Message1 = input$bins
      session$sendCustomMessage("bla", Message1)
    }
  })
}

answered Oct 16, 2018 at 20:16

DeanAttali

26.5k10 gold badges104 silver badges122 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

DeanAttali Over a year ago

Don't forget to accept the answer (click the arrow beside my answer) if this should be marked as solved

Collectives™ on Stack Overflow

R Shiny Code gets executed multiple times when sending it to Javascript

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related