req {shiny}

Check for required values

Description

Ensure that values are available ("truthy") before proceeding with a calculation or action. If any of the given values is not truthy, the operation is stopped by raising a "silent" exception (not logged by Shiny, nor displayed in the Shiny app's UI).

Usage

req(..., cancelOutput = FALSE)

Arguments

...

Values to check for truthiness.

cancelOutput

If TRUE and an output is being evaluated, stop processing as usual but instead of clearing the output, leave it in whatever state it happens to be in. If "progress", do the same as TRUE, but also keep the output in recalculating state; this is intended for cases when an in-progress calculation will not be completed in this reactive flush cycle, but is still expected to provide a result in the future.

Details

The req function was designed to be used in one of two ways. The first is to call it like a statement (ignoring its return value) before attempting operations using the required values:

rv <- reactiveValues(state = FALSE)
r <- reactive({
  req(input$a, input$b, rv$state)
  # Code that uses input$a, input$b, and/or rv$state...
})

In this example, if r() is called and any of input$a, input$b, and rv$state are NULL, FALSE, "", etc., then the req call will trigger an error that propagates all the way up to whatever render block or observer is executing.

The second is to use it to wrap an expression that must be truthy:

output$plot <- renderPlot({
  if (req(input$plotType) == "histogram") {
    hist(dataset())
  } else if (input$plotType == "scatter") {
    qplot(dataset(), aes(x = x, y = y))
  }
})

In this example, req(input$plotType) first checks that input$plotType is truthy, and if so, returns it. This is a convenient way to check for a value "inline" with its first use.

Value

The first value that was passed in.

Using req(FALSE)

You can use req(FALSE) (i.e. no condition) if you've already performed all the checks you needed to by that point and just want to stop the reactive chain now. There is no advantage to this, except perhaps ease of readability if you have a complicated condition to check for (or perhaps if you'd like to divide your condition into nested if statements).

Using cancelOutput = TRUE

When req(..., cancelOutput = TRUE) is used, the "silent" exception is also raised, but it is treated slightly differently if one or more outputs are currently being evaluated. In those cases, the reactive chain does not proceed or update, but the output(s) are left is whatever state they happen to be in (whatever was their last valid state).

Note that this is always going to be the case if this is used inside an output context (e.g. output$txt <- ...). It may or may not be the case if it is used inside a non-output context (e.g. reactive(), observe() or observeEvent()) — depending on whether or not there is an output$... that is triggered as a result of those calls. See the examples below for concrete scenarios.

Examples

## Only run examples in interactive R sessions
if (interactive()) {
  ui <- fluidPage(
    textInput('data', 'Enter a dataset from the "datasets" package', 'cars'),
    p('(E.g. "cars", "mtcars", "pressure", "faithful")'), hr(),
    tableOutput('tbl')
  )

  server <- function(input, output) {
    output$tbl <- renderTable({

      ## to require that the user types something, use: `req(input$data)`
      ## but better: require that input$data is valid and leave the last
      ## valid table up
      req(exists(input$data, "package:datasets", inherits = FALSE),
          cancelOutput = TRUE)

      head(get(input$data, "package:datasets", inherits = FALSE))
    })
  }

  shinyApp(ui, server)
}