Understanding the Shiny Server Delay When Loading CSS
Introduction
When building Shiny applications, developers often encounter performance issues related to loading stylesheets. In this article, we’ll delve into the world of Shiny Server and explore why loading CSS files seems to introduce a delay in certain scenarios.
We’ll start by examining the provided code and identify potential causes for the delay. Then, we’ll discuss some key concepts and techniques that can help resolve performance issues related to CSS loading.
The Problem Statement
The original question highlights an issue where the Shiny Server application takes time to load CSS stylesheets, particularly in the context of authorization pages with conditional rendering.
To better understand the problem, let’s examine the provided code snippet:
shinyUI(
fluidPage(
ui <- dashboardPage(
header <- dashboardHeader(uiOutput("header")),
sidebar <- dashboardSidebar(uiOutput("sidebarpanel")),
body <- dashboardBody(uiOutput("body"))
)
)
)
The server.R file contains the Shiny Server logic:
shinyServer(function(input, output, session) {
# ...
observe({
if (USER$Logged == FALSE) {
# ...
if (!is.null(input$Login)) {
if (input$Login > 0) {
Username <- isolate(input$userName)
Password <- isolate(input$passwd)
Id.username <- which(my_username == Username)
Id.password <- which(my_password == Password)
if (length(Id.username) > 0 & length(Id.password) > 0) {
if (Id.username == Id.password) {
USER$Logged <- TRUE
} else {
sendSweetAlert(session = session, title = "Incorrect Username or Password!",
text = "Please check.", type = "error",
btn_labels = "Ok")
}
}
}
}
}
})
output$body <- renderUI({
dashboardBody(tags$style(HTML('
/* logo */
.skin-blue .main-header .logo {
background-color: #FFFFFF;
}
/* logo when hovered */
.skin-blue .main-header .logo:hover {
background-color: #FFFFFF;
}
/* sidebar */
.skin-blue .main-sidebar {
background-color: #873260;
}
/* logo when hovered */
.skin-blue .main-header .navbar {
background-color: #873260;
}
'))),
if (USER$Logged == FALSE) {
box(title = "Login", textInput("userName", "Username"),
passwordInput("passwd", "Password"),
br(),
actionButton("Login", "Log in"))
} else {
dashboardSidebar(
sidebarMenu(
menuItem("Home", icon = icon("home")),
menuItem("Panel 1", icon = icon("th-large")),
menuItem("Panel 2", icon = icon("dashboard")),
menuItem("Panel 3", icon = icon("dashboard")),
menuItem("Panel 4", icon = icon("pencil")),
menuItem("Panel 5", icon = icon("pencil")),
menuItem("Panel 6", icon = icon("th"))
)
)
}
})
output$sidebarpanel <- renderUI({
if (USER$Logged == TRUE) {
dashboardSidebar(
sidebarMenu(
menuItem("Home", icon = icon("home")),
menuItem("Panel 1", icon = icon("th-large")),
menuItem("Panel 2", icon = icon("dashboard")),
menuItem("Panel 3", icon = icon("dashboard")),
menuItem("Panel 4", icon = icon("pencil")),
menuItem("Panel 5", icon = icon("pencil")),
menuItem("Panel 6", icon = icon("th"))
)
)
}
})
})
Potential Causes for the Delay
Several factors might contribute to the delay when loading CSS stylesheets:
- File Size: Large CSS files can take time to load, especially if they contain complex styles or multiple dependencies.
- Number of Stylesheets: When a single stylesheet is used by multiple elements in the application, it may cause a delay as the browser needs to download and process each element’s styles separately.
- Conditional Rendering: The use of conditional rendering (as seen in the original code) can lead to an additional overhead when loading CSS files.
Solving Performance Issues with CSS Loading
To resolve performance issues related to CSS loading, consider the following strategies:
- Optimize File Size: Minify and compress CSS files using tools like Gzip or Brotli.
- Use a Single Stylesheet: Instead of using multiple stylesheets for individual elements, use a single stylesheet that defines all necessary styles.
- Apply Styles Only When Needed: Implement conditional rendering only when the condition is met, reducing unnecessary overhead.
Example Solution
To demonstrate these strategies, let’s modify the original code:
shinyUI(
fluidPage(
ui <- dashboardPage(
header <- dashboardHeader(tags$style(HTML('
/* logo */
.skin-blue .main-header .logo {
background-color: #FFFFFF;
}
/* logo when hovered */
.skin-blue .main-header
.logo:hover {
background-color: #FFFFFF;
}
/* sidebar */
.skin-blue .main-sidebar {
background-color: #873260;
}
/* logo when hovered */
.skin-blue .main-header .navbar {
background-color: #873260;
}
'))),
body <- dashboardBody(uiOutput("body"))
)
)
)
By applying styles only when needed, we reduce the number of CSS files required.
Conclusion
The delay in loading CSS stylesheets can be attributed to various factors, including file size, number of stylesheets, and conditional rendering. By optimizing file size, using a single stylesheet, and applying styles only when needed, developers can improve performance issues related to CSS loading in their Shiny Server applications.
Last modified on 2025-03-14