Exercises

2. The following reactive graph simulates long running computation by using Sys.sleep().

1. Starting state is invalidated

2. The waiting times can be calculated via the reactive graph, but, for a double check, the following app confirms each waiting time:

# Waiting times per reactive value
## x1: 1 second
## x2: 2 seconds
## x3: 1 second

library(shiny)

ui <- fluidPage(
  radioButtons("increaseVal", 
               "Increase selected reactive value by 1",
               inline = TRUE,
               choices = paste0("x", 1:3)
  )
)


server <- function(input, output) {
  x1 <- reactiveVal(1)
  x2 <- reactiveVal(2)
  x3 <- reactiveVal(3)
  
  y1 <- reactive({
    Sys.sleep(1)
    x1()
  })
  y2 <- reactive({
    Sys.sleep(1)
    x2()
  })
  y3 <- reactive({
    Sys.sleep(1)
    x2() + x3() + y2() + y2()
  })
  
  
  observe({
    # Print current minute and seconds
    print(paste("Starting time:", stringr::str_sub(Sys.time(), 15, 20)))
    print(y1())
    print(paste("y1 finished:", stringr::str_sub(Sys.time(), 15, 20)))
    print(y2())
    print(paste("y2 finished:", stringr::str_sub(Sys.time(), 15, 20)))
    print(y3())
    print(paste("y3 finished:", stringr::str_sub(Sys.time(), 15, 20)))
  })
  
  # When the user increases some reactive value
  observeEvent(input$increaseVal, {
    message(input$increaseVal)
    # Example: x1(isolate(x1()) +1)
    eval(parse(text = 
                 paste0(
                   input$increaseVal, 
                   "(isolate(",
                   input$increaseVal, 
                   "()) + 1)"
                 )
    ))
  })
}

shinyApp(ui, server)