Delay reactions with eventReactive()

Shiny's reactive programming framework is designed such that any changes to inputs automatically updates the outputs that depend on it. In some situations, we might want to explicitly control the trigger that causes the update.

The function eventReactive() is used to compute a reactive value that only updates in response to a specific event.

rval_x <- eventReactive(input$event, {
 # calculations
})

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Building Web Applications with Shiny in R

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Istruzioni dell'esercizio

Use eventReactive() to delay the execution of computing BMI until the user clicks on the button.

esercizio interattivo pratico

Prova questo esercizio completando questo codice di esempio.

ui <- fluidPage(
  titlePanel('BMI Calculator'),
  sidebarLayout(
    sidebarPanel(
      textInput('name', 'Enter your name'),
      numericInput('height', 'Enter height (in m)', 1.5, 1, 2, step = 0.1),
      numericInput('weight', 'Enter weight (in Kg)', 60, 45, 120),
      actionButton("show_bmi", "Show BMI")
    ),
    mainPanel(
      textOutput("bmi")
    )
  )
)

server <- function(input, output, session) {
  # MODIFY CODE BELOW: Use eventReactive to delay the execution of the
  # calculation until the user clicks on the show_bmi button (Show BMI)
  rval_bmi <- reactive({
    input$weight/(input$height^2)
  })
  output$bmi <- renderText({
    bmi <- rval_bmi()
    paste("Hi", input$name, ". Your BMI is", round(bmi, 1))
  })
}

shinyApp(ui = ui, server = server)

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