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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练习说明

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

交互式实操练习

通过完成这段示例代码来试试这个练习。

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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