Shiny: update selectizeInput choices based on selected

Question

I am trying to update the choices of a selectizeInput based on the current selected choices. Here is my attempt (causes loop):

library(shiny)
run_ui <- function() {

  ui <- selectizeInput('words', 'Search words:', choices = NULL, selected = NULL, multiple = TRUE, options = NULL)

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

    # change 'Search words' ----
    observeEvent(input$words, {

      # handle no words (reset everything)
      if (is.null(input$words)) {
        cowords <- letters

      } else {
        # update cowords (choices for selectizeInput)
        cowords <- unique(c(input$words, sample(letters, 5)))
      }

      # update UI
      print('updating')
      updateSelectizeInput(session, 'words', choices = cowords, selected = input$words, server = TRUE)

    }, ignoreNULL = FALSE)
  }
  runGadget(shinyApp(ui, server), viewer = browserViewer())
}

run_ui()

How can I achieve this?

Answer 1

If you want to stick to server = TRUE, it's maybe not a trivial problem.

One possible work-around could be to debounce the input that you are observing, and then check and only update in case there is a change. This could look as follows - I added some print statements such that you can better follow what's happening.

library(shiny)

run_ui <- function() {

  ui <- selectizeInput('words', 'Search words:', choices = NULL, selected = NULL, multiple = TRUE, options = NULL)

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

    val <- "a"
    pasteCollPlus <- function(...) {
      paste(..., collapse = "+")
    }

    wordSelect <- debounce(reactive({input$words}), millis = 50)

    # change 'Search words' ----
    observeEvent(wordSelect(), {

      # handle no words (reset everything)
      if (is.null(input$words)) {
        cowords <- letters
      } else {
        # update cowords (choices for selectizeInput)
        cowords <- unique(c(input$words, sample(letters, 5)))
      }

      if (isTRUE(pasteCollPlus(val) == pasteCollPlus(input$words))) {
        print(paste("No update - val is", pasteCollPlus(val)))
      } else {
        # update UI
        print(paste("updating selection to", pasteCollPlus(input$words)))
        print(paste("val is", pasteCollPlus(val)))
        val <<- input$words
        updateSelectizeInput(session, 'words', choices = cowords, selected = input$words, server = TRUE)
      }

    }, ignoreNULL = FALSE)

  }
  runGadget(shinyApp(ui, server), viewer = browserViewer())
}

run_ui()

---

Edit

Another work-around would be to handle the bouncing pattern explicitly, in order to block it. This is maybe even less elegant, but could be more robust for more involved / complex cases (apps). An example for this follows:

library(shiny)
run_ui <- function() {

  ui <- selectizeInput('words', 'Search words:', choices = NULL, selected = NULL, multiple = TRUE, options = NULL)

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

    val <- "a"
    newVal <- NULL
    pasteCollPlus <- function(...) {
      paste(..., collapse = "+")
    }

    # change 'Search words' ----
    observeEvent(input$words, {

      # handle no words (reset everything)
      if (is.null(input$words)) {
        cowords <- letters
      } else {
        # update cowords (choices for selectizeInput)
        cowords <- unique(c(input$words, sample(letters, 5)))
      }

      if (isTRUE(pasteCollPlus(val) == pasteCollPlus(input$words))) {
        print(paste("No update - val is", pasteCollPlus(val)))
        val <<- newVal
      } else {
        # update UI
        print(paste("updating selection to", pasteCollPlus(input$words)))
        print(paste("val is", pasteCollPlus(val)))
        print(paste("newVal is", pasteCollPlus(newVal)))

        val <<- NULL
        newVal <<- input$words

        updateSelectizeInput(session, 'words', choices = cowords, selected = input$words, server = TRUE)
      }

    }, ignoreNULL = FALSE)

  }
  runGadget(shinyApp(ui, server), viewer = browserViewer())
}

run_ui()

Answer 2

Do you need to use server-side selectize? If not, then your code would work fine as-is by simply removing that part.

library(shiny)
run_ui <- function() {

  ui <- selectizeInput('words', 'Search words:', choices = NULL, selected = NULL, multiple = TRUE, options = NULL)

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

    # change 'Search words' ----
    observeEvent(input$words, {

      # handle no words (reset everything)
      if (is.null(input$words)) {
        cowords <- letters

      } else {
        # update cowords (choices for selectizeInput)
        cowords <- unique(c(input$words, sample(letters, 5)))
      }

      # update UI
      print('updating')
      updateSelectizeInput(session, 'words', choices = cowords, selected = input$words)

    }, ignoreNULL = FALSE)
  }
  runGadget(shinyApp(ui, server), viewer = browserViewer())
}

run_ui()

Answer 3

The following solution simply updates the entire object through renderUI and re-draws it, rather than passing back an update via updateSelectizeInput(). This does allow choices to be fully managed on the server-side. A downside is that it fires with each change event, which means that the multiple=TRUE is moot since the object redraws with each change. If multiples are critical, I think the updateSelectizeInput() approach or any other solution that updates onChange, would run into the same issue. To allow multiple choices, the event would need to move to onBlur or a mouseout event. Otherwise, the event trigger doesn't know if a user intends to select only one choice and fire; or wait for the user to make multiple choices before firing. However, blur or mouseout might make it behave strangely from the user's perspective. A button forcing the update action would resolve this. Keeping the update based on the first select, solution as follows:

library(shiny)
run_ui <- function() {

  ui <- uiOutput(outputId="select_words")      
  server <- function(input, output, session) {

    # change 'Search words' ----
    output$select_words <- renderUI({    
      cowords <- letters
      if (!is.null(input$words)) cowords <- unique(c(input$words, sample(letters, 5)))
      print(paste("Updating words: ",paste0(cowords,collapse=",")))

      return (tagList(selectizeInput('words', 'Search words:', choices = cowords, selected = input$words, multiple = TRUE, options = NULL)))
    })
  }
  runGadget(shinyApp(ui, server), viewer = browserViewer())
}

run_ui()