Understanding reactive programming (baby steps).

app.R

library(ggplot2)
library(shiny)

# Global variables can go here.
values <- runif(200)


# Define the UI.
ui <- fluidPage(
  radioButtons("n", "Approximate number of bins",
               c(3, 5, 10), selected = 5),
  plotOutput("plot")
)


# Define the server code.
server <- function(input, output) {
  
  # The `input` object contains reactive values (implementation of reactive
  # sources). They are set by input from the web browser.
  
  # Use a reactive expression to cache the results of any procedure that happens
  # in response to user input.
  current_bw <- reactive({ 1 / as.numeric(input$n) })
  
  # The `output` object is an observer (implementation of reactive endpoint).
  # Observers do not return values; they have side effects.
  output$plot <- renderPlot({
    ggplot(as.data.frame(values), aes(values)) +
      geom_histogram(binwidth = current_bw())
  })
}

# Return a Shiny app object.
shinyApp(ui = ui, server = server)

This single-file Shiny app is up and running at: https://mkcor.shinyapps.io/reactivity/