r - Create an input variable that is dependent on another input variable in flexdashboard shiny widget

I am trying to create a user input in flexdashboard that is dependent on another user input. Example dataset: alphabet_data <- read.table(
text = "Alphabet Number

ABC 1
DEF 4
ABD 5
ABC 2
ABC 3
ABD 6
ABD 7
ABD 8",
header = TRUE,
stringsAsFactors = FALSE)

User selects "alphabet" in selectizeInput, say ABD, based on that I want the user to get the selectizeInput options for "number" to only to be shown 5,6,7,8.

1. I tried observeEvent on "alphabet", to create the new dependent input fresh


2. I created the dependent input "number" with NULL choices, and used observe event to updateselectizeInput.


3. I created a new table based on alphabet choice, and then used that table within reactive to create "number" input.


4. There's code below to reproduce the issue.

---

title: "Untitled"
output:
flexdashboard::flex_dashboard:
orientation: columns
vertical_layout: fill

runtime: shiny

library(flexdashboard)
library(tidyverse)

alphabet_data <- read.table(
        text = "Alphabet       Number
        ABC       1
        DEF       4
        ABD       5
        ABC       2

        ABC       3
        ABD       6
        ABD       7
        ABD       8",
        header = TRUE,
        stringsAsFactors = FALSE)

Column {.sidebar data-width=650}

Chart A

selectizeInput(
    inputId  = "alphabet",
    label    = "Alphabet",
    choices  = unique(alphabet_data$Alphabet),
    multiple = TRUE,
    options  = list(maxItems = 2)
)


selectizeInput(
        inputId  = "number",
        label    = "Number",
        choices  = NULL,
        multiple = TRUE,
        options  = list(maxItems = 2)
)

selected_alphabet <- eventReactive(

    eventExpr = input$alphabet,

    valueExpr = {
    alphabet_data %>% 
            filter(Alphabet %in% input$alphabet)
})

reactive({
    observeEvent(
        eventExpr   = input$alphabet,

        handlerExpr = {
            updateSelectizeInput(
                inputId = "number",
                choices = selected_alphabet()$number
            )
        }
    )
})

Column {data-width=350}

Chart B

output$alphabet <- renderPrint(input$alphabet)
textOutput(outputId = "alphabet")

Chart C

renderPrint(selected_alphabet())

Chart D

output$number <- renderPrint(input$number)
textOutput(outputId = "number")

I expect when the user select ABD alphabet, the options for number to showcase as 5,6,7,8.

Answer

I'm having trouble running your sample script, so I wrote a similar one.

You have two options:

1. Use renderUI() or insertUI() to generate UI components in server.


2. use updateSelectInput() to upadte UI components.

I wrote a demo in shiny, though it's not using flexdashboard, it does the same thing:

library(shiny)

ui <- fluidPage(
  fluidRow(
      tags$h1("level 1"),
      column(

          width = 6,
          selectizeInput("selectizeInput1","Input 1",choices = letters,selected = "",multiple = TRUE)
      ),
      column(
          width = 6,
          textOutput("textOutput1")
      )
  ),
  fluidRow(
      tags$h1("level 2"),

      column(
          width = 6,
          selectizeInput("selectizeInput2","Input 2",choices = "",selected = "",multiple = TRUE)
      ),
      column(
          width = 6,
          textOutput("textOutput2")
      )
  ),
  fluidRow(

      tags$h1("level 3"),
      column(
          width = 6,
          selectizeInput("selectizeInput3","Input 3",choices = "",selected = "",multiple = TRUE)
      ),
      column(
          width = 6,
          textOutput("textOutput3")
      )
  )


)

server <- function(input, output, session) {
    # level 1
    output$textOutput1 <- renderText(input$selectizeInput1)

    # level 2
    observe({
        updateSelectInput(

            session = session,
            inputId = "selectizeInput2",
            choices = input$selectizeInput1,
            selected = input$selectizeInput1
        )
        output$textOutput2 <- renderText(input$selectizeInput2)

    })



    # level 3
    observe({
        updateSelectInput(
            session = session,
            inputId = "selectizeInput3",
            choices = input$selectizeInput2,
            selected = input$selectizeInput2
        )
        output$textOutput3 <- renderText(input$selectizeInput3)


    })
}

shinyApp(ui, server)

For better understanding, you can read this article or try out this app