Demo on a simple counter

server.R

# server.R

library("shiny")
library("shinyAce")

shinyServer(
  function(input, output) {
    
    # A reactiveValues object holds the value of the counter
    values <- reactiveValues(i = 0)
    
    # Print the value of the counter stored in values$i
    output$count <- renderUI({
      h5(paste0("counter = ", values$i))
    })
    
    # Display buttons in the mainPanel
    output$buttons <- renderUI({
      if (values$i == 0) { 
        list(actionButton("increment", "Next"))
      } else {
        if (values$i == 10) {
          list(actionButton("decrement", "Back"))
        } else { 
          list(actionButton("decrement", "Back"), actionButton("increment", "Next"))
        } 
      } 
    })
  
    # The observers re-run the code whenever the button is clicked
    # Use isolate to avoid getting stuck in an infinite loop
    observe({ 
      if(is.null(input$increment) || input$increment == 0){return()} 
      values$i <- isolate(values$i) + 1 
    }) 
    
    observe({ 
      if(is.null(input$decrement) || input$decrement == 0){return()} 
      values$i <- isolate(values$i) - 1 
    }) 
       
  }
)

ui.R

# https://groups.google.com/forum/#!topic/shiny-discuss/IWBghVCcMnI

# ui.R

library("shiny")
library("shinyAce")

sourceCode <- list(  # or save this in global.R
  aceEditor("ui"
    , value = paste(readLines("ui.R"), collapse = "\n")
    , mode = "r"
    , theme = "ambience"
    , height = "400px"
    , readOnly = TRUE
  ), br(),
  aceEditor("server"
    , value = paste(readLines("server.R"), collapse = "\n")
    , mode = "r"
    , theme = "ambience"
    , height = "400px"
    , readOnly = TRUE
  )
)

shinyUI(
  basicPage(
    h4("Shiny App Demo: Reactive Counter")
    ,
    tags$hr()
    ,
    uiOutput("buttons")
    ,
    uiOutput("count")
    ,
    tags$hr()
    ,
    helpText("Description: A counter starting at 0 and with maximum value 10")
    ,
    tags$br()
    ,
    checkboxInput("showSourceCode", label = "Show shiny source code?", value = FALSE)
    ,  # 'value = FALSE' not working in Chrome
    conditionalPanel(
      condition = "input.showSourceCode == true", sourceCode
    )
  )
)