shiny/_includes/tutorial/reactivity.md

To run the example type:

> library(shiny)
> runExample("03_reactivity")

ui.R

library(shiny)

# Define UI for dataset viewer application
shinyUI(pageWithSidebar(

  # Application title
  headerPanel("Reactivity"),

  # Sidebar with controls to provide a caption, select a dataset, and 
  # specify the number of observations to view. Note that changes made
  # to the caption in the textInput control are updated in the output
  # area immediately as you type
  sidebarPanel(
    textInput("caption", "Caption:", "Data Summary"),

    selectInput("dataset", "Choose a dataset:", 
                choices = c("rock", "pressure", "cars")),

    numericInput("obs", "Number of observations to view:", 10)
  ),


  # Show the caption, a summary of the dataset and an HTML table with
  # the requested number of observations
  mainPanel(
    h3(textOutput("caption")), 

    verbatimTextOutput("summary"), 

    tableOutput("view")
  )
))

server.R

library(shiny)
library(datasets)

# Define server logic required to summarize and view the selected dataset
shinyServer(function(input, output) {

  # By declaring databaseInput as a reactive function we ensure that:
  #
  #  1) It is only called when the inputs it depends on changes
  #  2) The computation and result are shared by all the callers (it 
  #     only executes a single time)
  #  3) When the inputs change and the function is re-executed, the
  #     new result is compared to the previous result; if the two are
  #     identical, then the callers are not notified
  #
  datasetInput <- reactive(function() {
    switch(input$dataset,
           "rock" = rock,
           "pressure" = pressure,
           "cars" = cars)
  })

  # The output$caption is computed based on a reactive function that
  # returns input$caption. When the user changes the "caption" field:
  #
  #  1) This function is automatically called to recompute the output 
  #  2) The new caption is pushed back to the browser for re-display
  # 
  # Note that because the data-oriented reactive functions below don't 
  # depend on input$caption, those functions are NOT called when 
  # input$caption changes.
  output$caption <- reactiveText(function() {
    input$caption
  })

  # The output$summary depends on the datasetInput reactive function, 
  # so will be re-executed whenever datasetInput is re-executed 
  # (i.e. whenever the input$dataset changes)
  output$summary <- reactivePrint(function() {
    dataset <- datasetInput()
    summary(dataset)
  })

  # The output$view depends on both the databaseInput reactive function
  # and input$obs, so will be re-executed whenever input$dataset or 
  # input$obs is changed. 
  output$view <- reactiveTable(function() {
    obs <- as.integer(input$obs)
    head(datasetInput(), n = obs)
  })
})