A reactive expression is like a recipe that manipulates inputs from Shiny and then returns a value. Reactivity provides a way for your app to respond since inputs will change depending on how users interact with your user interface. Expressions wrapped by reactive() should be expressions that are subject to change.

Creating a reactive expression is just like creating a function:

calc_sum <- reactive({
  input$box1 + input$box2
})

# ...

calc_sum()

This application predicts the horsepower of a car given the fuel efficiency in miles per gallon for the car.

library(shiny)
shinyUI(fluidPage(
  titlePanel("Predict Horsepower from MPG"),
  sidebarLayout(
    sidebarPanel(
      sliderInput("sliderMPG", "What is the MPG of the car?", 10, 35, value = 20),
      checkboxInput("showModel1", "Show/Hide Model 1", value = TRUE),
      checkboxInput("showModel2", "Show/Hide Model 2", value = TRUE)
    ),
# ...

# ...
    mainPanel(
      plotOutput("plot1"),
      h3("Predicted Horsepower from Model 1:"),
      textOutput("pred1"),
      h3("Predicted Horsepower from Model 2:"),
      textOutput("pred2")
    )
  )
))

library(shiny)
shinyServer(function(input, output) {
  mtcars$mpgsp <- ifelse(mtcars$mpg - 20 > 0, mtcars$mpg - 20, 0)
  model1 <- lm(hp ~ mpg, data = mtcars)
  model2 <- lm(hp ~ mpgsp + mpg, data = mtcars)
  
  model1pred <- reactive({
    mpgInput <- input$sliderMPG
    predict(model1, newdata = data.frame(mpg = mpgInput))
  })
  
  model2pred <- reactive({
    mpgInput <- input$sliderMPG
    predict(model2, newdata = 
              data.frame(mpg = mpgInput,
                         mpgsp = ifelse(mpgInput - 20 > 0,
                                        mpgInput - 20, 0)))
  })

  output$plot1 <- renderPlot({
    mpgInput <- input$sliderMPG
    
    plot(mtcars$mpg, mtcars$hp, xlab = "Miles Per Gallon", 
         ylab = "Horsepower", bty = "n", pch = 16,
         xlim = c(10, 35), ylim = c(50, 350))
    if(input$showModel1){
      abline(model1, col = "red", lwd = 2)
    }
    if(input$showModel2){
      model2lines <- predict(model2, newdata = data.frame(
        mpg = 10:35, mpgsp = ifelse(10:35 - 20 > 0, 10:35 - 20, 0)
      ))
      lines(10:35, model2lines, col = "blue", lwd = 2)
    }

  legend(25, 250, c("Model 1 Prediction", "Model 2 Prediction"), pch = 16, 
           col = c("red", "blue"), bty = "n", cex = 1.2)
    points(mpgInput, model1pred(), col = "red", pch = 16, cex = 2)
    points(mpgInput, model2pred(), col = "blue", pch = 16, cex = 2)
  })
  
  output$pred1 <- renderText({
    model1pred()
  })
  
  output$pred2 <- renderText({
    model2pred()
  })
})

You might not want your app to immediately react to changes in user input because of something like a long-running calculation. In order to prevent reactive expressions from reacting you can use a submit button in your app. Let's take a look at last app we created, but with a submit button added to the app.

There's one new line added to the code from the last app:

shinyUI(fluidPage(
  titlePanel("Predict Horsepower from MPG"),
  sidebarLayout(
    sidebarPanel(
      sliderInput("sliderMPG", "What is the MPG of the car?", 10, 35, value = 20),
      checkboxInput("showModel1", "Show/Hide Model 1", value = TRUE),
      checkboxInput("showModel2", "Show/Hide Model 2", value = TRUE),
      submitButton("Submit") # New!
    ),

There are several other kinds of UI components that you can mix into your app including tabs, navbars, and sidebars. We'll show you an example of how to use tabs to give your app multiple views. The tabsetPanel() function specifies a group of tabs, and then the tabPanel() function specifies the contents of an individual tab.

library(shiny)
shinyUI(fluidPage(
  titlePanel("Tabs!"),
  sidebarLayout(
    sidebarPanel(
      textInput("box1", "Enter Tab 1 Text:", value = "Tab 1!"),
      textInput("box2", "Enter Tab 2 Text:", value = "Tab 2!"),
      textInput("box3", "Enter Tab 3 Text:", value = "Tab 3!")
    ),
    mainPanel(
      tabsetPanel(type = "tabs", 
                  tabPanel("Tab 1", br(), textOutput("out1")), 
                  tabPanel("Tab 2", br(), textOutput("out2")), 
                  tabPanel("Tab 2", br(), textOutput("out3"))
      )
    )
  )
))

library(shiny)
shinyServer(function(input, output) {
  output$out1 <- renderText(input$box1)
  output$out2 <- renderText(input$box2)
  output$out3 <- renderText(input$box3)
})

One of my favorite features of Shiny is the ability to create graphics that a user can interact with. One method you can use to select multiple data points on a graph is by specifying the brush argument in plotOutput() on the ui.R side, and then using the brushedPoints() function on the server.R side. The following example app fits a linear model for the selected points and then draws a line of best fit for the resulting model.

library(shiny)
shinyUI(fluidPage(
  titlePanel("Visualize Many Models"),
  sidebarLayout(
    sidebarPanel(
      h3("Slope"),
      textOutput("slopeOut"),
      h3("Intercept"),
      textOutput("intOut")
    ),
    mainPanel(
      plotOutput("plot1", brush = brushOpts(
        id = "brush1"
      ))
    )
  )
))

library(shiny)
shinyServer(function(input, output) {
  model <- reactive({
    brushed_data <- brushedPoints(trees, input$brush1,
                            xvar = "Girth", yvar = "Volume")
    if(nrow(brushed_data) < 2){
      return(NULL)
    }
    lm(Volume ~ Girth, data = brushed_data)
  })
  output$slopeOut <- renderText({
    if(is.null(model())){
      "No Model Found"
    } else {
      model()[[1]][2]
    }
  })
# ...

# ...
output$intOut <- renderText({
    if(is.null(model())){
      "No Model Found"
    } else {
      model()[[1]][1]
    }
  })
  output$plot1 <- renderPlot({
    plot(trees$Girth, trees$Volume, xlab = "Girth",
         ylab = "Volume", main = "Tree Measurements",
         cex = 1.5, pch = 16, bty = "n")
    if(!is.null(model())){
      abline(model(), col = "blue", lwd = 2)
    }
  })
})

Once you've created a Shiny app, there are several ways to share your app. Using shinyapps.io allows users to interact with your app through a web browser without needing to have R or Shiny installed. Shinyapps.io has a free tier, but if you want to use a Shiny app in your business you may be interested in paying for the service. If you're sharing your app with an R user you can post your app to GitHub and instruct the user to use the runGist() or runGitHub() function to launch your app.

• The Official Shiny Tutorial
• Gallery of Shiny Apps
• Show Me Shiny: Gallery of R Web Apps
• Integrating Shiny and Plotly
• Shiny on Stack Overflow