Why does shiny display validation message twice? - r

Question

How can we ensure the user sees the validation error message only once?
Even in Shiny validation page, they had displayed error message twice:
https://shiny.rstudio.com/articles/validation.html
Also, if I were using different language, the below link might have helped.
Knockout - validation showing same error message twice
Even though **Stackoverflow had similar question here but referring different issue **
Show validate error message only once
It meant something different.
I am referring to "Please select a data set" message displaying twice
library(shiny)
ui <- fluidPage(
titlePanel("Validation App"),
sidebarLayout(
sidebarPanel(
selectInput("data", label = "Data set",
choices = c("", "mtcars", "faithful", "iris"))
),
# Show a plot of the generated distribution
mainPanel(
tableOutput("table"),
plotOutput("plot")
)
)
)
server <- function(input, output) {
data <- reactive({
validate(
need(input$data != "", "Please select a data set")
)
get(input$data, 'package:datasets')
})
output$plot <- renderPlot({
hist(data()[, 1], col = 'forestgreen', border = 'white')
})
output$table <- renderTable({
head(data())
})
}
shinyApp(ui,server)
If there is an error, ideally only 1 time, user should be notified if not it might be annoying.

Answer 1

Because you are storing the message "Please select a data set" in the reactive object data() and then calling that object to be displayed twice, once in output$plot and once in output$table.
One way to refactor the app and still have a similar experience is the use a place holder in the input widget and then req() to check if the input value is truthy. If a value is not truthy ("falsey"?) then evaluation is stopped and you won't raise errors from downstream outputs that use data().
library(shiny)
ui <- fluidPage(
titlePanel("Validation App"),
sidebarLayout(
sidebarPanel(
selectInput("data", label = "Data set",
choices = c("Please select a dataset" = "", "mtcars", "faithful", "iris"))
),
# Show a plot of the generated distribution
mainPanel(
tableOutput("table"),
plotOutput("plot")
)
)
)
server <- function(input, output) {
data <- reactive({
req(input$data)
get(input$data, 'package:datasets')
})
output$plot <- renderPlot({
hist(data()[, 1], col = 'forestgreen', border = 'white')
})
output$table <- renderTable({
head(data())
})
}
shinyApp(ui,server)
Another option would be to relocate the validate() logic from the data() block to one of the outputs. That way the message will only be shown once, but you may have to implement another check on the data, which is why I prefer using req for things like this.