Shiny dynamic UI - observers & lapply

app.R

# An example based on http://shiny.rstudio.com/articles/dynamic-ui.html

library(shiny)

ui = basicPage(
  fluidRow(
    actionButton(inputId = "add_buttons", label = "Add 5 Buttons")
  ),
  uiOutput("more_buttons") # this is where the dynamically added buttons will go.
)

server = function(input, output)
{
  # We need to track the actionButtons and their respective observeEvents
  rvs = reactiveValues(buttons = list(), observers = list()) 
  
  observeEvent(input$add_buttons,{
    l = length(rvs$buttons) + 1
    
    for(i in l:(l+4)) {
      rvs$buttons[[i]] = actionButton(inputId = paste0("button",i), label = i)
    }
    
    rvs$observers = lapply(
      l:(l+4), 
      function(i) {
        observeEvent(input[[paste0("button",i)]], 
                     print(sprintf("You clicked button number %d",i))
        )
      }
    )
  }
  )
  
  output$more_buttons = renderUI({
    do.call(fluidRow, rvs$buttons) # Add the dynamic buttons into a single fluidRow
  })
}

shinyApp(ui, server)