The magic behind Shiny is driven by reactivity. As you learned in this lesson, there are three types of reactive components in a Shiny app.

Reactive source: User input that comes through a browser interface, typically.
Reactive conductor: Reactive component between a source and an endpoint, typically used to encapsulate slow computations.
Reactive endpoint: Something that appears in the user's browser window, such as a plot or a table of values.

ui <- fluidPage(
  titlePanel('BMI Calculator'),
  theme = shinythemes::shinytheme('cosmo'),
  sidebarLayout(
    sidebarPanel(
      numericInput('height', 'Enter your height in meters', 1.5, 1, 2),
      numericInput('weight', 'Enter your weight in Kilograms', 60, 45, 120)
    ),
    mainPanel(
      textOutput("bmi"),
      textOutput("bmi_range")
    )
  )
)

server <- function(input, output, session) {
  rval_bmi <- reactive({
    input$weight/(input$height^2)
  })
  output$bmi <- renderText({
    bmi <- rval_bmi()
    paste("Your BMI is", round(bmi, 1))
  })
  output$bmi_range <- renderText({
    bmi <- rval_bmi()
    health_status <- cut(bmi, 
      breaks = c(0, 18.5, 24.9, 29.9, 40),
      labels = c('underweight', 'healthy', 'overweight', 'obese')
    )
    paste("You are", health_status)
  })
}

shinyApp(ui, server)

In this exercise, you will be presented with a set of reactive components. Classify each as a reactive source, conductor, or endpoint.

Reactivity concepts: observe & reactive

Building Web Applications with Shiny in R

実践的なインタラクティブ演習