rstudio/shiny — pull request #457 by yihui (2014-04-25 21:54:23+00:00)
Title: A first attempt of the custom error type for unsatisfied input dependencies

Example:

```s
runApp(list(
ui = fluidPage(
checkboxGroupInput('foo', 'Check some letters', choices = head(LETTERS)),
selectizeInput('bar', 'Select a state', choices = state.name),
plotOutput('plot')
),
server = function(input, output) {
output$plot <- renderPlot({
renderError(
list(message = 'Check at least one letter!', condition = is.null(input$foo)),
list(message = 'Please choose a state.', condition = input$bar == '')
)
plot(1:10, main = paste(c(input$bar, input$foo), collapse = ', '))
})
}
))
```