Shiny Lecture 2 (Shinier)

• In the last lecture, we covered basic creation of Shiny applications
• If you tried it and are like most, you had an easy time with ui.R but a harder time with server.R
• In this lecture, we cover some more of the details of shiny
• Since writing the last lecture, a more detailed tutorial has been created that is worth checking out (http://shiny.rstudio.com/tutorial/)

• Code that you put before shinyServer in the server.R function gets called once when you do runApp()
• Code inside the unnamed function of shinyServer(function(input, output){ but not in a reactive statement will run once for every new user (or page refresh)
• Code in reactive functions of shinyServer get run repeatedly as needed when new values are entered (reactive functions are those like render*)

ui.R

shinyUI(pageWithSidebar(
  headerPanel("Hello Shiny!"),
  sidebarPanel(
      textInput(inputId="text1", label = "Input Text1"),
      textInput(inputId="text2", label = "Input Text2")
  ),
  mainPanel(
      p('Output text1'),
      textOutput('text1'),
      p('Output text2'),
      textOutput('text2'),
      p('Output text3'),
      textOutput('text3'),
      p('Outside text'),
      textOutput('text4'),
      p('Inside text, but non-reactive'),
      textOutput('text5')
  )
))

server.R Set x <- 0 before running

library(shiny)
x <<- x + 1
y <<- 0

shinyServer(
  function(input, output) {
    y <<- y + 1
    output$text1 <- renderText({input$text1})
    output$text2 <- renderText({input$text2})
    output$text3 <- renderText({as.numeric(input$text1)+1})
    output$text4 <- renderText(y)
    output$text5 <- renderText(x)
  }
)

• type runApp()
• Notice hitting refresh incriments y but enterting values in the textbox does not
• Notice x is always 1
• Watch how it updated text1 and text2 as needed.
• Doesn't add 1 to text1 every time a new text2 is input.
• Important try runApp(display.mode='showcase')

• Sometimes to speed up your app, you want reactive operations (those operations that depend on widget input values) to be performed outside of a render*1 statement
• For example, you want to do some code that gets reused in several render* statements and don't want to recalculate it for each
• The reactive function is made for this purpose

server.R

shinyServer(
  function(input, output) {
    x <- reactive({as.numeric(input$text1)+100})      
    output$text1 <- renderText({x()                          })
    output$text2 <- renderText({x() + as.numeric(input$text2)})
  }
)

shinyServer(
  function(input, output) {
    output$text1 <- renderText({as.numeric(input$text1)+100  })
    output$text2 <- renderText({as.numeric(input$text1)+100 + 
        as.numeric(input$text2)})
  }
)

• Do runApp(display.mode='showcase')
• (While inconsequential) the second example has to add 100 twice every time text1 is updated for the second set of code
• Also note the somewhat odd syntax for reactive variables

• Sometimes you don't want shiny to immediately perform reactive calculations from widget inputs
• In other words, you want something like a submit button

shinyUI(pageWithSidebar(
  headerPanel("Hello Shiny!"),
  sidebarPanel(
      textInput(inputId="text1", label = "Input Text1"),
      textInput(inputId="text2", label = "Input Text2"),
      actionButton("goButton", "Go!")
  ),
  mainPanel(
      p('Output text1'),
      textOutput('text1'),
      p('Output text2'),
      textOutput('text2'),
      p('Output text3'),
      textOutput('text3')
  )
))

shinyServer(
  function(input, output) {
    output$text1 <- renderText({input$text1})
    output$text2 <- renderText({input$text2})
    output$text3 <- renderText({
        input$goButton
        isolate(paste(input$text1, input$text2))
    })
  }
)

• Notice it doesn't display output text3 until the go button is pressed
• input$goButton (or whatever you named it) gets increased by one for every time pushed
• So, when in reactive code (such as render or reactive) you can use conditional statements like below to only execute code on the first button press or to not execute code until the first or subsequent button press

if (input$goButton == 1){ Conditional statements }

Here's some replaced code from our previous server.R

output$text3 <- renderText({
    if (input$goButton == 0) "You have not pressed the button"
    else if (input$goButton == 1) "you pressed it once"
    else "OK quit pressing it"
})

• The sidebar layout with a main panel is the easiest.
• Using shinyUI(fluidpage( is much more flexible and allows tighter access to the bootstrap styles
• Examples here (http://shiny.rstudio.com/articles/layout-guide.html)
• fluidRow statements create rows and then the column function from within it can create columns
• Tabsets, navlists and navbars can be created for more complex apps

• For more complex layouts, direct use of html is preferred (http://shiny.rstudio.com/articles/html-ui.html)
• Also, if you know web development well, you might find using R to create web layouts kind of annoying
• Create a directory called www in the same directory with server.R
• Have an index.html page in that directory
• Your named input variables will be passed to server.R <input type="number" name="n" value="500" min="1" max="1000" />
• Your server.R output will have class definitions of the form shiny- <pre id="summary" class="shiny-text-output"></pre>

• Debugging shiny apps can be tricky
• We saw that runApp(displayMode = 'showcase') highlights execution while a shiny app runs
• Using cat in your code displays output to stdout (so R console)
• The browser() function can interupt execution and can be called conditionally (http://shiny.rstudio.com/articles/debugging.html)