server.r

shinyServer(function(input, output) {
  output$distPlot <- renderPlot({
    # Take a dependency on input$goButton
    input$goButton

    # Use isolate() to avoid dependency on input$obs
    dist <- isolate(rnorm(input$obs))
    hist(dist)
  })
})

ui.r

library(shinyIncubator)

shinyUI(pageWithSidebar(
  headerPanel("Click the button"),
  sidebarPanel(
    sliderInput("obs", "Number of observations:",
                min = 0, max = 1000, value = 500),
    actionButton("goButton", "Go!"),
    p(br(), a("View source code", href="https://gist.github.com/wch/4963887"))
  ),
  mainPanel(
    plotOutput("distPlot")
  )
))