Introduction to Shiny

Introducing a simplified reactive app

The real magic of Shiny happens when you have an app with inputs and outputs

ui <- fluidPage(
  textInput(inputId = "name", 
            label = "What's your name?"),
  textOutput("greeting")
)

server <- function(input, output, session) {
  output$greeting <- renderText({
    paste0("Hello ", input$name, "!")
  })
}

Looking for “name” and “greeting”

The mental model

Unlike with R where updates like this occur when we run the code, here Shiny is being told how it could create the string if it needs to. The possibilities are:

Introducing a new input

Four inputs were mentioned previously:

• sliderInput()
  
• selectInput()
  
• textInput()
  
• numericInput()

Building a new app

Starting again with a new script, typing shinyapp and then Shift + Tab add the code to ui, save and Run App

 titlePanel("Basic Demo"), # Title
 checkboxGroupInput(
    inputId = "name1",
    label = "What are your favourite languages?",
    choices = c("SQL", "Python", "R")
  ),

Adding an action button

Next we’ll add an action button so, after making a selection, the user presses a button for the next thing to happen:

  actionButton(inputId = "name2",
               label = "List the selections",
               icon = icon("question"))

Panels

Before we move to the interactive part of shiny we’ve currently got the layout where everything is appearing on the page as if it were a document.

Panel functions

To give the app more structure that are more familiar we need to introduce three functions:

ui <- fluidPage(titlePanel("Basic Demo"), 
                sidebarLayout(
                sidebarPanel(
                    checkboxGroupInput(
                      inputId = "name1",
                      label = "What are your favourite languages?",
                      choices = c("SQL", "Python", "R")
                    ),
                    actionButton(
                      inputId = "name2",
                      label = "Selected",
                      icon = icon("question")
                    )
                ),
                mainPanel(textOutput(outputId = "name3"))
                )
)

Tips

output$ and input$ in the server

server code

server <- function(input, output, session) {
    observeEvent(input$name2, {
    output$name3 <- renderText(input$name1)
    })
}

Copy and paste this, save and run

Exercise - renaming placeholders

Replace the names name1, name2, name3 with more descriptive names, paying attention to where the repetitions occur

observeEvent()

bindEvent()

bindEvent() can be used to execute only when a button is pressed. We’ll replace the code

  observeEvent(input$name2, {
    output$name3 <- renderText(input$name1)
  })

with

output$name3 <- renderText({
  input$name1
}) |>
  bindEvent(input$name2)

The formal names - imperative and declarative

Mastering Shiny explains this really well:

Laziness

Although it’s a strength of Shiny that declarative programming is lazy it does mean that code can have bugs but no errors.

If you recall when we changed names1 to other names there was no error if these did not appear in both the ui and server.

This applies to functions so if don’t pair them correctly:

output$name3 <- renderPlot({ # should be renderText()
  input$name1
}) |>
  bindEvent(input$name2)

Execution order

The familiar analysis scripts are run in the order of lines.

Quarto reports for example, need to particularly have everything in the right order to run.

This isn’t the case for Shiny!

Exercise (from Mastering Shiny)

Given this UI:

ui <- fluidPage(
  textInput("name", "What's your name?"),
  textOutput("greeting")
)

Fix the simple errors found in each of the three server functions below. First try spotting the problem just by reading the code; then run the code to make sure you’ve fixed it.

server1 <- function(input, output, server) {
  input$greeting <- renderText(paste0("Hello ", name))
}

server2 <- function(input, output, server) {
  greeting <- paste0("Hello ", input$name)
  output$greeting <- renderText(greeting)
}

server3 <- function(input, output, server) {
  output$greting <- paste0("Hello", input$name)
}

Solution

server1 <- function(input, output, session) {
  output$greeting <- renderText(paste0("Hello ", input$name))
}

server2 <- function(input, output, server) {
  greeting <- reactive(paste0("Hello ", input$name))
  output$greeting <- renderText(greeting())
}

server3 <- function(input, output, session) {
  output$greeting <- renderText({
    paste0("Hello ", input$name)
    })
}

End session

Acknowledgements

Mastering Shiny by Hadley Wickham

Building Web Apps with R Shiny from PsyTeachR