fruitExplorer.R

library("shiny")
library("shinyWidgets")

fruits <- c("Banana", "Blueberry", "Cherry", "Coconut", "Grapefruit", "Kiwi", "Lemon", "Lime", "Mango", "Orange", "Papaya")

pickerMaker <- function(selected = c(), selectedText = "{0}/{1} fruits") {
  pickerInput(
    inputId = "picker",
    label = "Choices :",
    choices = fruits,
    selected = selected,
    options = list(
      `actions-box` = TRUE,
      `selected-text-format` = "count > 2",
      `count-selected-text` = selectedText
    ),
    multiple = TRUE
  )
}

ui <- fluidPage(
  titlePanel("Fruit Explorer"),
  sidebarLayout(sidebarPanel(uiOutput("picker")),
  mainPanel(textOutput("res")))
)

server <- function(input, output) {
  
  nTotal <- length(fruits)
  nPrevSelected <- reactiveVal(0)
  nSelected <- reactive(length(input$picker))
  observe(nPrevSelected(length(input$picker)))
  
  pickerState <- reactive({
    if (nPrevSelected() < nTotal && nSelected() == nTotal) {
      "toTotal"
    } else if (nPrevSelected() == nTotal && nSelected() < nTotal) {
      "fromTotal"
    } else if (nSelected() > 0) {
      "noop"
    } else {
      "init"
    }
  })
  
  output$picker <- renderUI({
    switch(pickerState(),
      toTotal = pickerMaker(input$picker, "Total"),
      fromTotal = pickerMaker(input$picker),
      noop = req(FALSE, cancelOutput = TRUE),
      init = pickerMaker())
  })
  
  output$res <- renderPrint({
    input$picker
  })
}

shinyApp(ui = ui, server = server)