How to use built-in data instead of uploaded data - r

Question

I have an app like below. I want to either read-in data from file upload or use the built-in data. I thought I could put an action button and if somebody hit it the input data will mount on and it goes to the next levels. My problem is later in my real app, some widgets such as selectInput have to be updated and I want to be empty until user decided whether to use uploaded data or the built-in one.
library(shiny)
x <- mtcars
ui <- fluidPage(
fileInput(inputId = "uploadcsv", "", accept = '.csv'),
actionButton(inputId = "a", label = "action button"),
selectInput("select",label = h3("Select box"),choices = "",selected = 1)
)
server <- function(input, output, session) {
data <- reactive({
infile <- input$uploadcsv
if (is.null(infile))
return(NULL)
read.csv(infile$datapath, header = TRUE, sep = ",")
})
DataToUse <- NULL
observe(!is.null(input$uploadedcsv),
DataToUse <- data()
)
observeEvent(input$a,
DataToUse <- x
)
observe({
req(DataToUse)
if (max(DataToUse$cyl) %% 4 == 0){
numberofinterval <- max(DataToUse$cyl) %/% 4
} else {
numberofinterval <- (max(DataToUse$cyl) %/% 4)+1
}
NumPeriod <- seq(0, numberofinterval)
updateSelectInput(session, inputId = "select",
choices = NumPeriod,
selected = NumPeriod)
})
}
shinyApp(ui = ui, server = server)

Answer 1

Something like this should do:
library(shiny)
x <- mtcars
ui <- fluidPage(
fileInput(inputId = "uploadcsv", "", accept = '.csv'),
actionButton(inputId = "a", label = "action button"),
selectInput("select",label = h3("Select box"),choices = "",selected = 1)
)
server <- function(input, output, session) {
data <- reactive({
infile <- input$uploadcsv
if (is.null(infile))
return(NULL)
read.csv(infile$datapath, header = TRUE, sep = ",")
})
v <- reactiveValues()
v$DataToUse <- NULL
observeEvent(input$uploadcsv,{
if(!is.null(input$uploadcsv)){
v$DataToUse <- data()
}
})
observeEvent(input$a,v$DataToUse <- x)
observeEvent(v$DataToUse,{
req(v$DataToUse)
if (max(v$DataToUse$cyl) %% 4 == 0){
numberofinterval <- max(v$DataToUse$cyl) %/% 4
} else {
numberofinterval <- (max(v$DataToUse$cyl) %/% 4)+1
}
NumPeriod <- seq(0, numberofinterval)
updateSelectInput(session, inputId = "select",
choices = NumPeriod,
selected = NumPeriod)
})
}
shinyApp(ui = ui, server = server)