app.R

library(shiny)
library(shiny.semantic)

ui <- semanticPage(
  title = "Dropdown example",
  dropdown_input("simple_dropdown", LETTERS[1:5], value = "A", type = "search selection multiple"),
  p("Selected letter:"),
  textOutput("selected_letter"),
  actionButton("simple_button", "Update input to D"),
  actionButton("simple_button2", "Update input to use all letters")
)


server <- shinyServer(function(input, output, session) {
  output$selected_letter <- renderText(paste(input[["simple_dropdown"]], collapse = ", "))

  observeEvent(input$simple_button, {
    update_dropdown_input(session, "simple_dropdown", value = "D")
  })

  observeEvent(input$simple_button2, {
    update_dropdown_input(session, "simple_dropdown", choices = LETTERS, value = input$simple_dropdown)
  })
})

shinyApp(ui = ui, server = server)