Uses the reactiveTrigger() construct in an R6 object class in order to make it useful in reactive settings, like a Shiny app (MWE included)

shiny-reactive-R6-object.R

library(shiny)

reactiveTrigger <- function() {
  counter <- reactiveVal( 0)
  list(
    depend = function() {
      counter()
      invisible()
    },
    trigger = function() {
      counter( isolate(counter()) + 1 )
    }
  )
}

counter <- R6::R6Class(
  public = list(
    initialize = function(reactive = FALSE) {
      private$reactive = reactive
      private$value = 0
      private$rxTrigger = reactiveTrigger()
    },
    setIncrement = function() {
      if (private$reactive) private$rxTrigger$trigger()
      private$value = private$value + 1
    },
    setDecrement = function() {
      if (private$reactive) private$rxTrigger$trigger()
      private$value = private$value -1
    },
    getValue = function() {
      if (private$reactive) private$rxTrigger$depend()
      return(private$value)
    }
  ),
  private = list(
    reactive = NULL,
    value = NULL,
    rxTrigger = NULL
  )
)

ui <- fluidPage(
  actionButton("minus", "-1"),
  actionButton("plus", "+1"),
  textOutput("value")
)

server <- function(input, output, session) {
  count <- counter$new(reactive = TRUE)
  
  observeEvent(input$minus, { count$setDecrement() })
  observeEvent(input$plus,  { count$setIncrement() })
  
  output$value <- renderText({ count$getValue() })
}

shinyApp(ui, server)