r shiny: How to print a message in the app after the user forgets to upload a file?

Question

I am building a rudimentary shiny app. First, I created a data frame 'x' and saved it in my working directory:

x <- data.frame(a = 1:4, b = 2:5)
write.csv(x, 'x.csv', row.names = F)

In my shiny I'd like to:

1. Upload file 'x.csv'

2. Click my action button 'Click Here' and run a few commands upon clicking it.

3. Get a message printed in the Shiny app itself: "Load a file!" if I click on my button "Click here" after forgetting to upload the file first.

My code works, but I can't figure out how to make my message appear.

My code:

library(shiny)

ui <- fluidPage(
  br(),
  # User should upload file x here:
  fileInput("file_x", label = h5("Upload file 'x'!")),
  br(),
  # Users clicks the button:
  actionButton("do_it", "Click Here"),
  br(),
  # Print last value of the button 'do_it':
  verbatimTextOutput("print_action")
)


server <- function(input, output, session) {

  observeEvent(input$do_it, {

    # Just a check of my button's actions:
    output$print_action <- renderPrint({input$do_it})

    # Validating the input - next 5 lines are not working:
    # validate(
    #   need(
    #     try(is.null(input$file_x), "Load a file!")
    #     )
    # )

    # Reading in the file:
    fileData <- reactive({
      infile <- input$file_x
      if (is.null(infile)) {
        return(NULL)
      }
      read.csv(infile$datapath)
    })
    x <- fileData()

    # Writing out the same file - but under a different name:
    filename <- paste0("x", input$do_it, ".csv")
    write.csv(x, file = filename, row.names = FALSE)

  })
}

shinyApp(ui, server)

Answer 1

I think rather than displaying text, maybe modalDialog is better suited for what you are trying to achieve. I have implemented both solutions below, so you can compare.

Note that I also modified the reading of the csv slightly. It is bad practice to set a reactive from inside an observer. In those cases, it is better to use a reactiveVal, and update that from an observer.

Hope this helps!

library(shiny)

ui <- fluidPage(
  br(),
  # User should upload file x here:
  fileInput("file_x", label = h5("Upload file 'x'!")),
  br(),
  # Users clicks the button:
  actionButton("do_it", "Click Here"),
  br(),
  br(),
  # Print last value of the button 'do_it':
  verbatimTextOutput("print_action")
)

server <- function(input, output, session) {

  observeEvent(input$do_it, {

    if(is.null(input$file_x))
    {
      # show pop-up ...
      showModal(modalDialog(
        title = "Oh no!",
        paste0("You have not uploaded a file, silly person!"),
        easyClose = TRUE,
        footer = NULL
      ))
      # ... or update the text
      my_text('Please upload a file.')
    }
    else
    {
      # Reading in the file:
      infile <- input$file_x
      if (is.null(infile)) {
        return(NULL)
      }
      x <- read.csv(infile$datapath)
      fileData(x) # set the reactiveVal called fileData to the file inputs.

      # Writing out the same file - but under a different name:
      filename <- paste0("x", input$do_it, ".csv")
      write.csv(x, file = filename, row.names = FALSE)
      my_text('Succes!')
    }
  })

  fileData <- reactiveVal()
  my_text <- reactiveVal('')
  output$print_action <- renderText({my_text()})
}

shinyApp(ui, server)