Shiny by RStudio

Create a (single) reactive value

reactiveVal(value = NULL, label = NULL)


value An optional initial value.
label An optional label, for debugging purposes (see showReactLog). If missing, a label will be automatically created.


A function. Call the function with no arguments to (reactively) read the value; call the function with a single argument to set the value.


The reactiveVal function is used to construct a "reactive value" object. This is an object used for reading and writing a value, like a variable, but with special capabilities for reactive programming. When you read the value out of a reactiveVal object, the calling reactive expression takes a dependency, and when you change the value, it notifies any reactives that previously depended on that value.


reactiveVal is very similar to reactiveValues, except that the former is for a single reactive value (like a variable), whereas the latter lets you conveniently use multiple reactive values by name (like a named list of variables). For a one-off reactive value, it's more natural to use reactiveVal. See the Examples section for an illustration.


## Not run: ------------------------------------ # # # Create the object by calling reactiveVal # r <- reactiveVal() # # # Set the value by calling with an argument # r(10) # # # Read the value by calling without arguments # r() ## --------------------------------------------- ## Only run examples in interactive R sessions if (interactive()) { ui <- fluidPage( actionButton("minus", "-1"), actionButton("plus", "+1"), br(), textOutput("value") ) # The comments below show the equivalent logic using reactiveValues() server <- function(input, output, session) { value <- reactiveVal(0) # rv <- reactiveValues(value = 0) observeEvent(input$minus, { newValue <- value() - 1 # newValue <- rv$value - 1 value(newValue) # rv$value <- newValue }) observeEvent(input$plus, { newValue <- value() + 1 # newValue <- rv$value + 1 value(newValue) # rv$value <- newValue }) output$value <- renderText({ value() # rv$value }) } shinyApp(ui, server) }