Data not available outside of observeEvent() statement?

[lc201203]

I have run into what seems like a very simple simple issue in Shiny but I cannot figure out what the issue is.

Basically, I want to set an observer to plot data that is generated by a separate observeEvent() statement. Consider the following app:

ui <- (fluidPage(
  actionButton("button","Press Me"),
  verbatimTextOutput("data")
))

server <- function (input,output) {
  observeEvent(input$button,{
    x <- 1
  })

  observe({
    output$data <- renderPrint(x)
  })
}

shinyApp (ui=ui,server=server)

Naturally, upon startup of the app there is an error message stating "object 'value' not found" since it has not been assigned a value on startup. However, when pressing the "Press Me' button I would expect that 'x' be assigned a value of 1, to which the observer would then print into the 'data' output. Instead, it continues to show the same error message.

If I modify the server to read as follows, it works:

server <- function (input,output) {
  observeEvent(input$button,{
    value <- 1
    output$data <- renderPrint(value)
  })
}

Why are variables assigned within an observeEvent statement (seemingly) only available within the statement itself?

[Joe Cheng]

Like functions, observers create their own variable scope. By default, variable writes are local to each function/observer.

If you want to make x available to both the observer and the renderPrint, you'd need to declare "x <- NULL" outside of the observer (but still inside the shiny server function) and then assign to it from inside the observer with the <<- operator, e.g. "x <<- 1".

But simple variables do not trigger reactivity--assigning to x will not, by default, cause renderPrint to re-run. You need to specifically tell Shiny to treat x as a reactive variable, by calling makeReactiveBinding.

I haven't tried this but it should work:

server <- function (input,output) {
x <- NULL
makeReactiveBinding("x")

observeEvent(input$button,{
x <<- 1
})

observe({
output$data <- renderPrint(x)
})
}

Further reading:
http://adv-r.had.co.nz/Functions.html#lexical-scoping
http://shiny.rstudio.com/articles/scoping.html

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[Xing Chen]

Hi, 

Based on your post, is there any trick to assign multiple variables as NULL?  What I need is :

server <- function (input,output) {

   x <- NULL

   y <- NULL

   ...

   z <- NULL

  observeEvent(input$button,{
    x <<- 1

    y <<- 1

     ...

    z <<- 1
  })

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