show warning to user in shiny in R - r

Question

How to show warning to user in shiny in R. The user's input is correct, but the output is not suitable to show. The aim is to remind the user only a subset data are shown due to too many. warning() is shown in console only. Thank you.
Here is a fake code to explain the question due to the original is long. There is a warning in the renderTable. it aims to check data if the data is big, only first several items will be shown.
ui.R
shinyUI(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.R
shinyServer(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({
warning("Warning message.")
head(data())
})
})

Answer 1

Update:
I put some more work into this and made the warning panel conditional.
However it only works if I include out the textOutput("warnstat") on every page. I assume because it is not setting the javascript variable output.warnstat unless I do this.
You could just build a warning panel into your UI, and set it accordingly. Here is a simple example, but it could be more elaborate than just a verabtim print statement.
ui.r
shinyUI(fluidPage(
titlePanel("Validation App"),
sidebarLayout(
sidebarPanel(
selectInput("data", label = "Data set",
choices = c("", "mtcars", "faithful", "iris"))
),
# Show a plot of the generated distribution
mainPanel(
conditionalPanel(condition = "output.warnstat == 'Error'",
verbatimTextOutput("warnmsg")),
tableOutput("table"),
plotOutput("plot")
)
)
))
server.r
shinyServer(function(input, output) {
errstat <- reactive({
ifelse (input$data=="mtcars",T,F)
})
data <- reactive({
validate(
need(input$data != "", "Please select a data set")
)
get(input$data, 'package:datasets')
})
output$plot <- renderPlot({
hist(data()[, 1], col = 'forestgreen', border = 'white')
})
output$table <- renderTable({
warning("Warning message.")
head(data())
})
output$warnmsg <- renderPrint({
if (errstat()){
print("Warning message - blah blah blah")
print(input$data)
head(data())
} else {
print("No error")
}
})
output$warnstat <- renderText({ifelse(errstat(),"Error","No error") })
outputOptions(output, "warnstat", suspendWhenHidden=FALSE)
})
With conditional warning panel:
Without conditional warning panel:

Answer 2

I use this wrapping function to capture errors, warnings and messages and display them as dismissible notifications to the user.
quietly <- function(.f) {
fun <- .f %>% purrr::quietly() %>% purrr::safely()
function(...) {
res <- fun(...)
if(!is.null(res$error)) { # safely output
showNotification(res$error$message, duration = 10, type="error")
return(res$result)
}
res <- res$result # quietly output
if(!is.null(res$warnings) && length(res$warnings) > 0) {
lapply(unique(res$warnings), showNotification, duration = 10, type="warning")
}
return(res$result)
}
}