Understanding the Limitations of reactivePoll in Shiny Dashboards: A Solution

Understanding reactivePoll in Shiny

Introduction

In Shiny, reactivePoll is a function that creates a reactive poll that checks for changes at regular intervals. It’s commonly used to update dashboards or UI elements with new data. However, in this blog post, we’ll explore an issue where the value function of reactivePoll isn’t triggered as expected.

The Problem

The problem is described in a Stack Overflow question where a user tries to use reactivePoll in a Shiny dashboard. When the app first runs, it works fine, but after the initial refresh, the value function isn’t triggered again. This leaves the UI with stale data.

Here’s an excerpt from the original code:

library(shiny)
library(shinythemes)
library(shinyWidgets)
library(shinydashboard)
library(shinycssloaders)
library(RPostgreSQL)
library(pool)
library(config)
library(plotly)
library(data.table)

Sys.setenv(R_CONFIG_ACTIVE = "xyz")
config <- config::get()

pool <- dbPool(
  drv = dbDriver("PostgreSQL"),
  host = config$host,
  dbname = config$dbname,
  port = config$port,
  user = config$user,
  password = config$password
)

onStop(function() {
  poolClose(pool)
})

get_data <- function(pool) {
  abc <- dbGetQuery(pool, "SELECT * FROM tablename") # Query to pull data
  return(abc)
}

abc <- get_data(pool = pool)

ui <- dashboardPage(
  dashboardHeader(title = 'Dashboard'),
  dashboardSidebar(
    sidebarMenu(
      menuItem("pqr", tabName = "pqrs")
    )
  ),
  dashboardBody(
    tabItems(
      tabItem(
        tabName = 'pqrs',
        hemaTab("pqr", abc = abc)
      )
    )
  )
)

server <- function(input, output, session) {
  pollData <- reactivePoll(60000, session,
                           checkFunc = function() {
                             print("Entered Check")
                             Sys.time()
                             print(Sys.time())
                           },
                           valueFunc = function() {
                             print("Entered value")
                             get_data(pool)
                           }
  )
  
  order(input, output, session, data = pollData())
}

shinyApp(ui = ui, server = server)

In this example, the reactivePoll is used to update a UI element every minute. However, after the first refresh, the value function isn’t triggered again.

Understanding reactivePoll

The Stack Overflow answer suggests that reactivePoll needs to be called inside a reactive environment in order to work properly.

library(shiny)
ui <- fluidPage(
  mainPanel(
    verbatimTextOutput('text')
  )
)

server <- function(input, output, session) {
  pollData <- reactivePoll(600, session,
                           checkFunc = function() {
                             print("Entered Check")
                             Sys.time()
                             print(Sys.time())
                           },
                           valueFunc = function() {
                             print("entered value")
                             return('x')
                           }
  )
  
  ord <- function(data) {
    print(data)
  }

  observe(ord(pollData())) # Only triggers once
  # observe(ord(pollData())) # Triggers every time
}

The answer suggests that reactivePoll works like other reactive expressions: it checks to see if it’s invalidated. If it isn’t, it returns the saved value; if it is, then it runs again and returns the updated value.

However, in the provided example, we observe a different behavior when using order() with pollData(). This suggests that shinydashboard functions might not be enough like a reactive environment to access the value of reactivePoll.

Solution

To solve this issue, we can wrap the order() function around pollData() in an observe() function. This ensures that reactivePoll is called every time the UI changes.

library(shiny)
ui <- fluidPage(
  mainPanel(
    verbatimTextOutput('text')
  )
)

server <- function(input, output, session) {
  pollData <- reactivePoll(600, session,
                           checkFunc = function() {
                             print("Entered Check")
                             Sys.time()
                             print(Sys.time())
                           },
                           valueFunc = function() {
                             print("entered value")
                             return('x')
                           }
  )
  
  ord <- observe(pollData()) # Triggers every time

  ord
}

By wrapping order() in an observe() function, we ensure that the value of reactivePoll is updated every time the UI changes.

Conclusion

In conclusion, reactivePoll works like other reactive expressions: it checks to see if it’s invalidated. If it isn’t, it returns the saved value; if it is, then it runs again and returns the updated value. However, in Shiny dashboards created with shinydashboard, we might not be able to access the value of reactivePoll directly.

To solve this issue, we can wrap the order() function around pollData() in an observe() function. This ensures that reactivePoll is called every time the UI changes.

Remember, Shiny’s reactive expressions are powerful tools for creating dynamic and interactive dashboards. By understanding how they work, you’ll be able to create more complex and engaging applications with ease.

Last modified on 2024-01-13