ex_shiny_app.R

library(shiny)
ui <- fluidPage(
  sliderInput("obs", "Number of observations", 0, 1000, 500),
  actionButton("goButton", "Go!", icon = icon("refresh")),
  plotOutput("distPlot")
)

server <- function(input, output) {
  output$distPlot <- renderPlot({
    # Take a dependency on input$goButton. This will run once initially,
    # because the value changes from NULL to 0.
    input$goButton
    
    # Use isolate() to avoid dependency on input$obs
    dist <- isolate(rnorm(input$obs))
    hist(dist)
  })
}

shinyApp(ui, server)