Lazy higher order reactives

dedupe.R

library(shiny)
library(dplyr)
library(ggplot2)

dedupe <- function(r) {
  x <- reactiveVal(isolate(r()))

  o <- observe({
    x(r())
  })
  
  observe({
    o$suspend()
    x()
  }, priority = 9999999)
  
  r <- reactive({
    o$resume()
    x()
  })
  # onInvalidated(r, o$suspend)
  r
}

ui <- fluidPage(
  checkboxInput("show", "Show plot", TRUE),
  sliderInput("hp_range", "Horsepower range", min(mtcars$hp), max(mtcars$hp), c(min(mtcars$hp), max(mtcars$hp))),
  conditionalPanel("input.show",
    plotOutput("plot")
  )
)

server <- function(input, output, session) {
  selected_cars <- reactive({
    print("Filtering")
    mtcars %>% filter(hp >= input$hp_range[1] & hp <= input$hp_range[2])
  }) %>% dedupe()
  
  output$plot <- renderPlot({
    print("Plotting")
    ggplot(selected_cars(), aes(x = mpg, y = disp)) + geom_point()
  })
}

shinyApp(ui, server)