freezeReactiveValue: Freeze a reactive value
In austin3087/https-github.com-rstudio-shiny: Web Application Framework for R

Description Usage Arguments See Also Examples

These functions freeze a reactiveVal(), or an element of a reactiveValues(). If the value is accessed while frozen, a "silent" exception is raised and the operation is stopped. This is the same thing that happens if req(FALSE) is called. The value is thawed (un-frozen; accessing it will no longer raise an exception) when the current reactive domain is flushed. In a Shiny application, this occurs after all of the observers are executed.

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freezeReactiveVal(x)

freezeReactiveValue(x, name)

`x`	For `freezeReactiveValue`, a `reactiveValues()` object (like `input`); for `freezeReactiveVal`, a `reactiveVal()` object.
`name`	The name of a value in the `reactiveValues()` object.

req()

## Only run this examples in interactive R sessions
if (interactive()) {

ui <- fluidPage(
  selectInput("data", "Data Set", c("mtcars", "pressure")),
  checkboxGroupInput("cols", "Columns (select 2)", character(0)),
  plotOutput("plot")
)

server <- function(input, output, session) {
  observe({
    data <- get(input$data)
    # Sets a flag on input$cols to essentially do req(FALSE) if input$cols
    # is accessed. Without this, an error will momentarily show whenever a
    # new data set is selected.
    freezeReactiveValue(input, "cols")
    updateCheckboxGroupInput(session, "cols", choices = names(data))
  })

  output$plot <- renderPlot({
    # When a new data set is selected, input$cols will have been invalidated
    # above, and this will essentially do the same as req(FALSE), causing
    # this observer to stop and raise a silent exception.
    cols <- input$cols
    data <- get(input$data)

    if (length(cols) == 2) {
      plot(data[[ cols[1] ]], data[[ cols[2] ]])
    }
  })
}

shinyApp(ui, server)
}