User Feedback

Lecture 26

Dr. Eric Friedlander

College of Idaho
CSCI 2025 - Winter 2026

Validation

Cancelling Execution

• Problem: Missing inputs often cause ugly R errors.
• Solution 1: req() (Require).
  • req(input$id) stops execution silently if input$id is missing, NULL, FALSE, or empty.
  • Downstream outputs remain blank (or keep old value) instead of crashing.

output$plot <- renderPlot({
  req(input$x) # Stop here if input$x is missing
  plot(input$x)
})

Explicit Validation

• Solution 2: validate() and need().
  • Checks a condition and displays a specific error message if unsatisfied.

output$table <- renderTable({
  validate(
    need(input$nrows > 0, "Number of rows must be positive")
  )
  head(mtcars, input$nrows)
})

Notifications

Sending Messages

• showNotification(): Displays a temporary message in the corner.
• Usage:
  • Confirmation (“Saved successfully”).
  • Warnings.
  • Errors.

observeEvent(input$save, {
  saveData(data())
  showNotification("Data saved successfully!", type = "message")
})

Progress Bars

Long-Running Tasks

• Feedback is critical for slow operations.
• withProgress(): Wraps a code block.
• incProgress(): Increments the bar.

output$plot <- renderPlot({
  withProgress(message = 'Making plot', value = 0, {
    
    incProgress(0.1, detail = "Part 1")
    Sys.sleep(0.5) # Simulate work
    
    incProgress(0.5, detail = "Part 2")
    Sys.sleep(0.5)
    
    plot(rnorm(100))
  })
})

Spinners

• The shinycssloaders package is a simpler way to add spinners.
• Setup: Wrap an output with withSpinner() in the UI.
• No server-side changes are needed.

# In UI
library(shinycssloaders)
withSpinner(plotOutput("plot"))

# In server
output$plot <- renderPlot({
  Sys.sleep(2) # Simulate work
  plot(rnorm(100))
})

Confirmation

Modals

• Use modalDialog() for actions that require explicit confirmation (e.g., deleting data).
• Trigger with showModal().

observeEvent(input$delete, {
  showModal(modalDialog(
    title = "Delete Data?",
    "Are you sure you want to delete this?",
    footer = tagList(
      modalButton("Cancel"),
      actionButton("confirm_delete", "Delete", class = "btn-danger")
    )
  ))
})

Wrap-Up

Feedback Loops

• Silent Rejection: req() (Best for missing inputs).
• Explicit Error: validate(need()) (Best for wrong inputs).
• Status Update: showNotification() (Best for success/info).
• Waiting: withProgress() (Best for slow tasks).
• Confirmation: modalDialog() (Best for dangerous actions).

Do Next

1. Read Chapter 8: User feedback from Mastering Shiny.
2. There’s NO recitation Gem for this textbook but I recommend creating your own and adding the textbook chapter and these slides.
3. I reommend working through the Exercises in the textbook.
4. Move on to Lecture 27!