shiny multiple select input select choice many times - r

Question

In this minimal example I want to have an option to select choices more than once i.e. to produce input value as e.g. A,B,B,B,A,A,C
Option hideSelected = FALSE makes selected options still visible, but not selectable again.
According to https://github.com/rstudio/shiny/issues/518 there is such an option in selectize but I can't find such an option even here: https://github.com/selectize/selectize.js/blob/master/docs/usage.md
server <- function(input, output, session) {
output$multipleSelect <- renderUI({
selectizeInput("selectMany",
label = "I want to select each multiple times",
choices = LETTERS[1:3],
multiple = TRUE,
options = list(hideSelected = FALSE))
})
}
ui <- function() {
fluidPage(
uiOutput("multipleSelect")
)
}
shinyApp(ui, server)

Answer 1

Since Shiny hasn't implemented this yet and if you'd like to stick to selectInput, a workaround would be using a selectInput but clears the selection everytime the user makes a choice. Then you can put another DT output to show the currently selected elements and let the user delete elements from there. I'm using verbertimTextOutput just for demo purpose.
library(shiny)
ui <- fluidPage(
selectInput(
"selectMany",
label = "Many",
choices = LETTERS[1:3],
multiple = TRUE
),
verbatimTextOutput("debug")
)
server <- function(input, output, session) {
elements <- reactiveVal(c())
observeEvent(input$selectMany, {
req(input$selectMany)
elements(c(elements(), input$selectMany[[1]]))
})
observeEvent(elements(), {
req(elements())
updateSelectInput(session, "selectMany",
selected = character(0),
choices = LETTERS[1:3]
)
})
output$debug <- renderPrint({
print(elements())
})
}
shinyApp(ui, server)

Answer 2

I've come up with some good idea of adding invisible space to choices list. I've also trick selectize by adding " " option at the beggining, which solves problem of lack of reactivity when removing last element.
Here is sth that pretty much does the job - excellent when adding items.
There are still two unresolvable problems:
dropdown list is closed each time (won't be fixable as input needs to be updated)
when removing item there is blink of too many options on dropdown list
code:
library(shiny)
library(dplyr)
server <- function(input, output, session) {
# set the default choices and set previous selection to initial selectInput vector
globalList <- reactiveValues(ManyChoices = LETTERS[1:3], SelectedPrev = c())
output$multipleSelect <- renderUI({
selectizeInput("selectMany",
label = "I want to select each multiple times",
choices = c(" ", globalList$ManyChoices),
selected = " ",
multiple = TRUE,
options = list(closeAfterSelect = TRUE, openOnFocus = TRUE))
})
observeEvent(input$selectMany, {
# if sth was added
if(length(input$selectMany) > length(globalList$SelectedPrev)) {
#find out what was modified
vDiff <- setdiff(input$selectMany, globalList$SelectedPrev) %>% setdiff(., " ")
# used when removing " " and selecting sth to double the selection
if(length(vDiff) == 0) vDiff <- input$selectMany[length(input$selectMany)]
req(input$selectMany != " ") # if only " " is selected then there is no need to update
# get the position of selected element
vIndex <- which(globalList$ManyChoices == vDiff)
vLength <- length(globalList$ManyChoices)
# create new choices in the correct order
globalList$ManyChoices <- c(globalList$ManyChoices[1:vIndex],
paste0(vDiff, " "),
if(vIndex < vLength) {globalList$ManyChoices[(vIndex + 1):vLength]})
} else {
# remove the version with additional space when value was removed
vDiff <- setdiff(globalList$SelectedPrev, input$selectMany)
globalList$ManyChoices <- setdiff(globalList$ManyChoices, paste0(vDiff, " "))
}
# update previous selection
globalList$SelectedPrev <- input$selectMany
# update input with same selection but modified choices
updateSelectizeInput(session = session,
inputId = "selectMany",
selected = c(" ", input$selectMany),
choices = c(" ", globalList$ManyChoices))
})
}
ui <- function() {
fluidPage(
uiOutput("multipleSelect")
)
}
shinyApp(ui, server)