Trigger reactions with observeEvent()

There are times when you want to perform an action in response to an event. For example, you might want to let the app user download a table as a CSV file, when they click on a "Download" button. Or, you might want to display a notification or modal dialog, in response to a click.

The observeEvent() function allows you to achieve this. It accepts two arguments:

The event you want to respond to.
The function that should be called whenever the event occurs.

In this exercise, you will use observeEvent() to display a modal dialog with help text, when the user clicks on a button labelled "Help". The help text has already been assigned to the variable bmi_help_text.

This is a part of the course

“Building Web Applications with Shiny in R”

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Exercise instructions

UI:
- Add an action button named 'show_help', labelled "Help". You might have to scroll down or collapse the console to view the UI.
Server:
- Uncomment the code # showModal ...
- Wrap showModal(...) in observeEvent() so that the help text is displayed when a user clicks on the help button

Hands-on interactive exercise

Have a go at this exercise by completing this sample code.

ui <- fluidPage(
  titlePanel('BMI Calculator'),
  sidebarLayout(
    sidebarPanel(
      textInput('name', 'Enter your name'),
      numericInput('height', 'Enter your height in meters', 1.5, 1, 2),
      numericInput('weight', 'Enter your weight in Kilograms', 60, 45, 120),
      actionButton("show_bmi", "Show BMI")
      # CODE BELOW: Add an action button named "show_help"
      
    ),
    mainPanel(
      textOutput("bmi")
    )
  )
)

server <- function(input, output, session) {
  # MODIFY CODE BELOW: Wrap in observeEvent() so the help text 
  # is displayed when a user clicks on the Help button.
  
     # Display a modal dialog with bmi_help_text
     # MODIFY CODE BELOW: Uncomment code
     # showModal(modalDialog(bmi_help_text))
  
  rv_bmi <- eventReactive(input$show_bmi, {
    input$weight/(input$height^2)
  })
  output$bmi <- renderText({
    bmi <- rv_bmi()
    paste("Hi", input$name, ". Your BMI is", round(bmi, 1))
  })
}

shinyApp(ui = ui, server = server)

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