library(shiny)
library(gargoyle)

shiny::reactiveConsole(TRUE)
s <- shiny::MockShinySession$new()

init("pif", session = s)
#> [[1]]
#> reactiveVal: [1] "0"
test_for_on_function <- on(
  "pif",
  {
    cat("TEST fails")
  },
  session = s
)
test_for_on_function$.func()
#> Error in session$userData[[name]](): attempt to apply non-function
shiny::reactiveConsole(FALSE)

on <- function(
  name,
  expr,
  session = getDefaultReactiveDomain()
  ){
  ...
  observeEvent(
      substitute(gargoyle::watch(name)),
  ...
}