15 Reactive building blocks

15.1.1 Exercises

1. l1 is a reactive values class with values a and b, whereas l2 is a list with a, a reactiveVal and b, a reactiveVal.

l1 <- reactiveValues(a = 1, b = 2)
l2 <- list(a = reactiveVal(1), b = reactiveVal(2))

# get reactive values in l1
l1$a
l1$b
# set reactive values in l1
l1$a <- 10
l1$b <- 20

# get reactive values in l2
l2$a()
l2$b()
# set reactive values in l2
l2$a(10) 
l2$b(20)

2. Not sure.

15.2.3 Exercises

1. Not sure.

library(shiny)

ui <- fluidPage(
  checkboxInput("error", "error?"),
  textOutput("result")
)

server <- function(input, output, session) {
  a <- reactive({
    if (req(input$error, cancelOutput = TRUE)) {
      "Error!"
    } else {
      1
    }
  })
  b <- reactive(a() + 1)
  c <- reactive(b() + 1)
  output$result <- renderText(c())
}

shinyApp(ui, server)

• If I use req() and remove the stop(), I get an error message: Error: non-numeric argument to binary operator.
• Not sure about what happens when I use cancelOutput argument, but the documentation states:

When req(..., cancelOutput = TRUE) is used, the “silent” exception is also raised, but it is treated slightly differently if one or more outputs are currently being evaluated. In those cases, the reactive chain does not proceed or update, but the output(s) are left is whatever state they happen to be in (whatever was their last valid state).

15.4.3 Exercises

library(shiny)

ui <- fluidPage(
  numericInput("x", "x", value = 50, min = 0, max = 100),
  actionButton("capture", "capture"),
  textOutput("out")
)

server <- function(input, output, session) {
  df <- eventReactive(input$capture, { 
    input$x
  })
  output$out <- renderText({ df() })
}

shinyApp(ui, server)

15.5.4 Exercises

1. Not sure.

2. Not sure.