Modifying a reactive value should trigger an observe chunk

Question

I'm modifying a reactive value inside an observeEvent. I have another observe chunk which calls this reactive value. I expect the observe chunk to be triggered at the end of the observeEvent chunk given this reactive value dependency but that seems to not be happening. Is that expected behaviour?

http://shiny.rstudio.com/reference/shiny/latest/reactiveValues.html says - "When you read a value from it, the calling reactive expression takes a reactive dependency on that value, and when you write to it, it notifies any reactive functions that depend on that value."

In the below example, everytime I click the button, I expect cat with 2 to be output, and depending on the value of counter, the cat with the 3 to be output too. That isn't the case, and the output typically prints the cat with 1 but not the cats with 2 and 3.

server.r

shinyServer ( 

   function(input, output, session) {

      lReactiveValues = reactiveValues(a = 1.1)

      voidaA = observeEvent(
         input$buttonCommit,
         {
            a = runif(1)
            cat(a,' 1\t')
            lReactiveValues$a = a
         }
      )

      voidB = observe({
         counter = runif(1)
         cat(counter,' 2\t'); 

         if (counter > 0.5) {

            cat('\n')

            cat(lReactiveValues$a,' 3\n')

         }
      }
      )

   }
)

ui.r

dashboardPage(

   dashboardHeader(

      title = "Analytics"

   ),

   ## Sidebar content
   dashboardSidebar(
   menuItem("Analysis", tabName = "tabAnalysis", icon = icon("calculator"))
   ),

   ## Body content
   dashboardBody(
      tabItems(
         tabItem(
            tabName = "tabAnalysis",
            actionButton("buttonCommit", "Commit!")
         )
      )
      #, style='width: 100%; height: 100%;'

   )
)

Answer 1

I'm not 100% sure I understand exactly what you want, but perhaps what you want is to just add lReactiveValues$a as the first line inside the voidB observer. See if that works like you expect.

If that was your problem, I think I understand the source of the problem. The way observers work is that they run first and record all the reactive dependencies they can find, and then whenever one of those dependencies change, the observer runs again. Since you have a randomizer there, it means that half the times the app runs, the line of code using lReactiveValues$a never gets run, so that observer doesn't know that lReactiveValues$a is a dependency. Which is why you'll notice inconsistent behaviour - sometimes voidB will run a few times but will eventually stop running when the random number is < 0.5.