Use req() in checkFunction ReactivePoll about shiny HOT 5 OPEN

Hi @HugoGit39, thanks for the question! The main issue is that req() throws an error that's normally handled gracefully by observers and reactives but that checkFunc isn't designed to handle. You can read the reactive value in checkFunc, though, so this pattern works:

  USER_LOGGED_IN <- reactiveVal(FALSE)

  observeEvent(input$login, {
    message("User has logged in.")
    # Pretend that the user has now logged in
    USER_LOGGED_IN(TRUE)
  })

  # Function to poll for updated data every 30 seconds
  data_poll <- reactivePoll(
    intervalMillis = 5000,
    session = session,
    checkFunc = function() {
      if (!USER_LOGGED_IN()) {
        message("Not logged in yet.")
        return()
      }

      # Return a timestamp to trigger polling
      message("Data check: ", format(Sys.time(), "%H:%M:%S"))
      format(Sys.time(), "%H:%M")
    },
    valueFunc = function() {
      # Generate new data
      req(USER_LOGGED_IN())
      generate_data()
    }
  )

I've put this modification in a small shinylive app from your example.

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@gadenbuie Thx!. But the checkFunc is running already from the start. Isnt there a way to start the checkFunc (or the reactivePoll function) when the user logs in?

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@gadenbuie Thx!. But the checkFunc is running already from the start. Isnt there a way to start the checkFunc (or the reactivePoll function) when the user logs in?

No, the way that reactivePoll() works is that checkFunc should be a cheap and quick function that can be called repeatedly. Whenever its value changes, it gets a new value by calling valueFunc.

The idea in my updated example is to have checkFunc return immediately when the user isn't yet logged in. Because that's only checking an R object, it'll resolve immediately. So it's okay that it's running before the value of data_poll() is actually used.

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Right, but it its still a nice feature to have reactivePoll start running after a certain period right? Does checkFunc takes a lot of computing power on the server-side?

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Right, but it its still a nice feature to have reactivePoll start running after a certain period right? Does checkFunc takes a lot of computing power on the server-side?

No, the code you use for checkFunc should be as quick and simple as possible. The idea is that checkFunc should run extremely quickly and be okay for Shiny to run more often than necessary without slowing down your app.

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