Chapter 4 Backend Server

The server is the backend that takes input values from the user to ultimately generate the output results that you want to display in the application. Let’s go through it line by line together.

Here are some of the server codes.

Note: These are not full server codes

# Backend of the application
server <- function(session, input, output) {
  # stored plot into reactiveValues
  values <- reactiveValues(plot = NULL)
  
  # Reactive data
  dt <- reactive({
    req(input$dt)
    if (input$dt == "mtcars") {
      dt <- mtcars
      # converts variables: cyl, qsec, vs, am, gear, carb from numeric to character
      dt[, c(2, 8:11)] <- lapply(dt[, c(2, 8:11)], as.character)
    } else{
      dt <- iris
    }
    return(dt)
  })
  
  
  # Select Variable for plot
  output$var <- renderUI({
    req(dt())
    selectInput(
      "var_plot",
      label = "Select Variable",
      choices = names(dt()),
      multiple = FALSE
    )
  })
  
  
  # Download the plot
  output$download <- downloadHandler(
    filename = function() {
      paste("plot_", input$var_plot, '.png', sep = '')
    },
    content = function(file) {
      ggsave(file, plot = values$plot)
    }
  )
}# end of server

From the codes, you can see the object called server through a function function(session, input, output). It contained output$var, output$dtable, output$plot_out, output$dl_butt, and output$download, also it contained some reactivities reactive(), reactiveValue(), observeEvent(), and updateTabsetPanel()