R shiny bi-directional reactive widgets

Question

I am struggling to figure out how to get 2 R Shiny widgets to update each other. For example, a slider widget that can update a text box widget, and visa versa, where the en

Answer 1

If you enclose each input slider under a reactive observe, you can achieve your goal. This not only works for two inputs, but for several inputs as well. Let me explain by an example:

observe({
# Create a reactive relationship with slider1
input$slider1
# Update the second slider - slider2
updateTextInput(session, "slider2", NULL, input$slider1)
)}

Similarly for second input, you'd have to repeat the code, by:

observe({
# Create a reactive relationship with slider2
input$slider2
# Update the second slider - slider1
updateTextInput(session, "slider1", NULL, input$slider2)
)}

Answer 2

The trick is to create a dynamic UI. In this way, you can update a slider-drawing expression on changes in the other UI elements and rebuild a slider widget using a different default value:

server.R

shinyServer(
    function(input, output, clientData, session) {

    output$slider1 <- renderUI({
       slider2.value <- input$inSlider
       default.slider1 <- if (is.null(slider2.value)) 15 else slider2.value
       sliderInput("control_num",
                   "This controls values:",
                    min = 1, max = 20, value = default.slider1)
    })

    output$slider2 <- renderUI({
       slider1.value <- input$control_num
       default.slider2 <- if (is.null(slider1.value)) 15 else slider1.value
       sliderInput("inSlider", "Slider input:",
                    min = 1, max = 20, value = default.slider2)
    })

  })

ui.R

    shinyUI(fluidPage(
      titlePanel("One Way Reactive Slider"),
      fluidRow(
        column(3,
               wellPanel(
                 h4("Slider Inputs"),
                 uiOutput('slider1'),
                 uiOutput('slider2')
        ))

      )
    ))