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

shiny-reactive-R6-object.R

library(shiny)

reactiveTrigger <- function() {
  counter <- reactiveVal(0L)
  list(
    listen = function() {
      counter()
      invisible()
    },
    trigger = function() {
      counter( isolate(counter()) + 1L ) %% 
        .Machine$integer.max  # prevent (very unlikely) integer overflow
    }
  )
}

Foo <- R6::R6Class(
  public = list(
    initialize = function(reactive = FALSE) {
      private$reactive = reactive
      private$value1 = 0
      private$value2 = 0
      
      # register reactive triggers
      private$triggers = list(
        "value1" = reactiveTrigger(),
        "value2" = reactiveTrigger()
        # ...
      )
    },
    set_value1 = function(x) {
      if (private$reactive) private$triggers$value1$trigger()
      private$value1 = x
    },
    get_value1 = function() {
      if (private$reactive) private$triggers$value1$listen()
      return(private$value1)
    },
    set_value2 = function(x) {
      if (private$reactive) private$triggers$value2$trigger()
      private$value2 = x
    },
    get_value2 = function() {
      if (private$reactive) private$triggers$value2$listen()
      return(private$value2)
    }
  ),
  private = list(
    reactive = NULL,
    value1 = NULL,
    value2 = NULL,
    triggers = NULL
  )
)

ui <- fluidPage(
  numericInput("new_value1", "new value 1", 0),
  numericInput("new_value2", "new value 2", 0),
  textOutput("value1"),
  textOutput("value2")
)

server <- function(input, output, session) {
  foo_obj <- Foo$new(reactive = TRUE)
  
  observe({ foo_obj$set_value1(input$new_value1) })
  observe({ foo_obj$set_value2(input$new_value2) })
  
  output$value1 <- renderText({ foo_obj$get_value1() })
  output$value2 <- renderText({ foo_obj$get_value2() })
}

shinyApp(ui, server)