Handle nested observers in Shiny

nested-observers.R

library(shiny)

ui <- fluidPage(
  sliderInput("a", "Slider A", 0, 10, 1),
  sliderInput("b", "Slider B", 0, 10, 1),
  actionButton("create_observers", "Create observers")
)

server <- function(input, output, session) {
  slider_observers <- list()
  observeEvent(input$create_observers, {
    # Nested observers would be duplicated if not tracked and destroyed.
    # Comment out the line below and click "Create" repeatedly to see that happen.
    lapply(slider_observers, function(observer) observer$destroy())
    slider_observers$a <<- observe(message("Slider A has value ", input$a))
    slider_observers$b <<- observe(message("Slider B has value ", input$b))
  })
}

shinyApp(ui, server)