Shiny button needed only once - button

Question

I want an event to be triggered for the first time only by clicking a button. After that I want it to be reactive to the slider input.
I tried the following:
ui <- fluidPage(
actionButton("go", "Go"),
sliderInput("n", label = "Sample size", min = 1, max = 100, value = 10),
plotOutput('samples')
)
server <- function(input, output, session){
activate = reactive({FALSE})
activate = eventReactive(input$go, {
isolate(TRUE)
})
samples = eventReactive(activate(), {
rnorm(input$n)
})
output$samples <- renderPlot({ hist(samples()) })
}
shinyApp(ui = ui, server = server)
I hoped it would make it reactive to input$n after input$go has been clicked once. But it isn't and still needs input$go to be clicked every time.

Answer 1

There are several ways to achieve that.
One way would be to store the value in a reactiveValues() or just use req(), see below.
The problem with using eventReactive(activate(), ... is that it only triggers the code inside if activate() is executed, which only happens if you click input$go.
Reproducible example with req():
ui <- fluidPage(
actionButton("go", "Go"),
sliderInput("n", label = "Sample size", min = 1, max = 100, value = 10),
plotOutput('samples')
)
server <- function(input, output, session){
output$samples <- renderPlot({
req(input$go > 0)
hist(rnorm(input$n))
})
}
shinyApp(ui = ui, server = server)
Reproducible example with reactiveValues():
ui <- fluidPage(
actionButton("go", "Go"),
sliderInput("n", label = "Sample size", min = 1, max = 100, value = 10),
plotOutput('samples')
)
server <- function(input, output, session){
global <- reactiveValues(showPlot = FALSE)
observeEvent(input$go, {
global$showPlot <- TRUE
})
samples = reactive({
rnorm(input$n)
})
output$samples <- renderPlot({
req(global$showPlot)
hist(samples())
})
}
shinyApp(ui = ui, server = server)