R shiny: display "loading..." message while function is running

Very simply, you can use built-in shiny functions showModal() at the start of the function and removeModal() at the end. If you remove the footer, said modal cannot be clicked out of.

Example:

observeEvent(input$button, {
     showModal(modalDialog("Doing a function", footer=NULL))
     #Do the stuff here....
     #...
     #...
     #Finish the function
     removeModal()
})

Though this question is old I think it is still relevant. I have another solution to offer that displays the activity indicator on the button that starts a lengthy process next to the button label.

We need an action button with a label in a span and some way of identifying that label.

actionButton("btnUpdate", span("Update", id="UpdateAnimate", class=""))

We also need some CSS animation that can be added to the button label, e.g. like this:

            tags$head(tags$style(type="text/css", '
            .loading {
                display: inline-block;
                overflow: hidden;
                height: 1.3em;
                margin-top: -0.3em;
                line-height: 1.5em;
                vertical-align: text-bottom;
                box-sizing: border-box;
            }
            .loading.dots::after {
                text-rendering: geometricPrecision;
                content: "⠋\\A⠙\\A⠹\\A⠸\\A⠼\\A⠴\\A⠦\\A⠧\\A⠇\\A⠏";
                animation: spin10 1s steps(10) infinite;
                animation-duration: 1s;
                animation-timing-function: steps(10);
                animation-delay: 0s;
                animation-iteration-count: infinite;
                animation-direction: normal;
                animation-fill-mode: none;
                animation-play-state: running;
                animation-name: spin10;
            }
            .loading::after {
                display: inline-table;
                white-space: pre;
                text-align: left;
            }
            @keyframes spin10 { to { transform: translateY(-15.0em); } }
            '))

Now we can use shinyjsto manipulate the span class which dynamically adds the animation behind the button label. We add the animation once a user presses the button:

    observeEvent(input$btnUpdate, { # User requests update
        # ... 

        shinyjs::addClass(id = "UpdateAnimate", class = "loading dots")
        shinyjs::disable("btnUpdate")
        
        # ...
    })

When the operation has finished we can remove the class from the span and end the animation:

    output$distPlot <- renderPlot({
        # ...
        
        Sys.sleep(1) # just for show, you probably want to remove it in a real app
        # Button settings        
        shinyjs::enable("btnUpdate")
        shinyjs::removeClass(id = "UpdateAnimate", class = "loading dots")

        # ...
    })

The full code of the sample app is available as gist on GitHub.

I solved the problem by adding the following code to sidebarPanel():

HTML('<script type="text/javascript">
        $(document).ready(function() {
          $("#DownloadButton").click(function() {
            $("#Download").text("Loading...");
          });
        });
      </script>
')

You can use ShinyJS: https://github.com/daattali/shinyjs

When the actionButton is pressed, you can easily toggle a text component showing "loading...", and when the calculation is finished, you can then toggle this component to hidden.

I found a solution, that works fine for me. I am using the Bootstrap modal. It is shown when the execution of the function starts and is hidden again, when it ends.

modalBusy <- function(id, title, ...){

 msgHandler =  singleton(tags$head(tags$script('Shiny.addCustomMessageHandler("jsCode",
                                            function(message) {
                                              console.log(message)
                                              eval(message.code);
                                            });'
                                            )
                                )
                      )

 label_id = paste(id, "label", sep='-')
 modal_tag <- div(id=id, 
               class="modal hide fade", 
               "aria-hidden"=FALSE, 
               "aria-labelledby"=label_id, 
               "role"="dialog", 
               "tabindex"="-1",
               "data-keyboard"=FALSE,
               "data-backdrop"="static")
 header_tag <- div(class="modal-header",
                h3(id=label_id, title))
 body_tag <- div(class="modal-body",
              Row(...))   
 footer_tag <- div(class="modal-footer")
 modal_tag <- tagAppendChildren(modal_tag, header_tag, body_tag, footer_tag)
 tagList(msgHandler, modal_tag) 
}

To show and to hide it use the functions

showModal <- function(id,session) {
  session$sendCustomMessage(type="jsCode",
                            list(code= paste("$('#",id,"').modal('show')"
                                             ,sep="")))
}

hideModal <- function(id,session) {
  session$sendCustomMessage(type="jsCode",
                            list(code= paste("$('#",id,"').modal('hide')"
                                             ,sep="")))
}

Call the showModal function before your function Call and the hideModal function afterwards!

Hope this helps.

Seb