No gray screen when app/server disconnects when using waiter

[MalditoBarbudo]

When using waiter and an error occurs triggering the shiny:disconnected event, no gray screen appears. The app is not responsive anymore, but the lack of a visual hint of disconnection can be frustrating until you realize the app is not responding, especially in complex apps.

This can be reproduced clicking the Generate error button in the following example:

library(shiny)
library(waiter)

ui <- fluidPage(
  use_waiter(),
  actionButton("draw", "draw plot"),
  actionButton("gen_error", 'Generate error'),
  plotOutput("plot")
)

server <- function(input, output){
  
  # sets: 
  # input$plot_waiter_shown
  # input$plot_waiter_hidden
  w <- Waiter$new(id = "plot")
  
  dataset <- reactive({
    input$draw
    
    w$show()
    
    on.exit({
      w$hide()
    })
    
    Sys.sleep(3)  
    
    runif(100)
  })
  
  output$plot <- renderPlot(plot(dataset()))
  
  observeEvent(
    input$gen_error,
    {
      stop('no grey, why?')
    }
  )
  
}

shinyApp(ui, server)

I've tracked the error to this code in assets/waiter/custom.js in the latest code in master:

$(document).on('shiny:disconnected', function(event) {
  console.log("seems like a disconnect, hiding waiter"); // this line was added to know when this was triggered
  hide_waiter();
  Shiny.setInputValue("waiter_hidden", true, {priority: 'event'});
});

Following this, in the hide_waiter js function it seems to fail in this step:

  var selector = 'body';

  if(id !== null)
    selector = '#' + id;

  var dom = document.querySelector(selector);

  var overlay = dom.getElementsByClassName("waiter-overlay");
  \\ ... the rest of the function.

The last line fails as the dom is already null, as shown in the chromium console:
> waiter.js:124 Uncaught TypeError: Cannot read property 'getElementsByClassName' of null

I can think on two solutions for this:

1. Remove the event listener in custom.js. If the app is disconnected anyways, there is no need of removing the waiter.

2. Add a try-catch to the hide_waiter function, to catch this case and avoid the js error.

@JohnCoene If you can tell me what option you prefer (or have another one on mind), I can make a pull request with the fix ;)

[JohnCoene]

Ah! Well spotted thank you. I just removed it for now, might add it back later on :)

Thanks again!

[MalditoBarbudo]

Nice!
Thanks for the fast fix!