Stop reactions with isolate()

Ordinarily, the simple act of reading a reactive value is sufficient to set up a relationship, where a change to the reactive value will cause the calling expression to re-execute. The isolate() function allows an expression to read a reactive value without triggering re-execution when its value changes.

In this exercise, you will use the isolate() function to stop reactive flow.

This exercise is part of the course

Building Web Applications with Shiny in R

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Exercise instructions

Update the server code so that the text output updates only when user changes the height or weight, and NOT the name.

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 m)', 1.5, 1, 2, step = 0.1),
      numericInput('weight', 'Enter your weight (in Kg)', 60, 45, 120)
    ),
    mainPanel(
      textOutput("bmi")
    )
  )
)

server <- function(input, output, session) {
  rval_bmi <- reactive({
    input$weight/(input$height^2)
  })
  output$bmi <- renderText({
    bmi <- rval_bmi()
    # MODIFY CODE BELOW: 
    # Use isolate to stop output from updating when name changes.
    paste("Hi", input$name, ". Your BMI is", round(bmi, 1))
  })
}

shinyApp(ui = ui, server = server)

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