8.1 Validation

8.1.1 Validating input

• giving feedback with the shinyFeedback package

UI: add useShinyFeedback()

ui <- fluidPage(
  shinyFeedback::useShinyFeedback(),
  numericInput("n", "n", value = 10),
  textOutput("half")
)

Server: use feedback(), feedbackWarning(), feedbackDanger(), and feedbackSuccess()

–> example app

8.1.2 Cancelling execution with req()

req() checks for required values before allowing a reactive producer to continue.

Without req() the complete reactive graph is computed (even without user input):

ui <- fluidPage(
  selectInput("language", "Language", choices = c("", "English", "Maori")),
  textInput("name", "Name"),
  textOutput("greeting")
)

server <- function(input, output, session) {
  greetings <- c(
    English = "Hello", 
    Maori = "Ki ora"
  )
  output$greeting <- renderText({
    paste0(greetings[[input$language]], " ", input$name, "!")
  })
}

Using req():

server <- function(input, output, session) {
  greetings <- c(
    English = "Hello", 
    Maori = "Ki ora"
  )
  output$greeting <- renderText({
    req(input$language, input$name)
    paste0(greetings[[input$language]], " ", input$name, "!")
  })
}

8.1.3 req() and validation

ui <- fluidPage(
  shinyFeedback::useShinyFeedback(),
  textInput("dataset", "Dataset name"), 
  tableOutput("data")
)

server <- function(input, output, session) {
  data <- reactive({
    req(input$dataset)
    
    exists <- exists(input$dataset, "package:datasets")
    shinyFeedback::feedbackDanger("dataset", !exists, "Unknown dataset")
    req(exists, cancelOutput = TRUE)

    get(input$dataset, "package:datasets")
  })
  
  output$data <- renderTable({
    head(data())
  })
}

Check out this app: https://hadley.shinyapps.io/ms-require-cancel/

8.1.4 Validate output

• validate(message) stops execution of the rest of the code and instead displays message in any downstream outputs

ui <- fluidPage(
  numericInput("x", "x", value = 0),
  selectInput("trans", "transformation", 
    choices = c("square", "log", "square-root")
  ),
  textOutput("out")
)

server <- function(input, output, session) {
  output$out <- renderText({
    if (input$x < 0 && input$trans %in% c("log", "square-root")) {
      validate(message = "x can not be negative for this transformation")
    }
    
    switch(input$trans,
      square = input$x ^ 2,
      "square-root" = sqrt(input$x),
      log = log(input$x)
    )
  })
}