R: Create a (single) reactive value

reactiveVal {shiny} | R Documentation

## Create a (single) reactive value

### Description

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.

### Usage

    reactiveVal(value = NULL, label = NULL)

### Arguments

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

### Details

`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.

### Value

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

### Examples

    ## 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()

    ## End(Not run)

    ## 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)

    }

[Package shiny version 1.1.0 Index]