App that implements exercise 2 of Mastering Shiny section 14.4.5

app.R

# this app implements exercise 2 of section 14.4.5 in Mastering Shiny
# as you update the inputs, you can check how long the computation takes
#  by seeing which of y1, y2, y3 changes
# example:
#  change x1 and the time for y1 will be different from when app is initialized
#  so it takes 1 second for graph to recompute when x1 changes

# exercise solution:
# when x1 changes: 1 second
# when x2 changes: 2 seconds
# when x3 changes: 1 second

library(shiny)

ui <- fluidPage(
    numericInput("x1", "x1", 1),
    numericInput("x2", "x2", 2),
    numericInput("x3", "x3", 3)

)

server <- function(input, output) {
    x1 <- reactive({input$x1})
    x2 <- reactive({input$x2})
    x3 <- reactive({input$x3})

    y1 <- reactive({
        start <- Sys.time()
        Sys.sleep(1)
        x1()
        Sys.time() - start
    })
    y2 <- reactive({
        start <- Sys.time()
        Sys.sleep(1)
        x2()
        Sys.time() - start
    })
    y3 <- reactive({
        start <- Sys.time()
        Sys.sleep(1)
        x2() + x3() + y2() + y2()
        Sys.time() - start
    })

    observe({
        print(y1())
        print(y2())
        print(y3())
        cat("---------- \n")
    })

}

shinyApp(ui = ui, server = server)