Render variables from file input - r

Question

I am trying to develop an application, where I have designed my dashboard with two menu sub items.
Load
Prep
In Load, I am using File input, to allow the user to upload the file.
2 .In prep, I want to display the headers from the file in the form of checkgroupbox.
For that, I have tried the below code. In the below code, I have following problems.
1. The selection of variables, I need in Prep, is available in Load.
Select variables, I want to display them in checkbox, so the user can select the variables they want.
Here is my code, Could anyone help me how I could achieve it?
ui<-dashboardPage(
dashboardHeader(title = "Model"),
dashboardSidebar(
sidebarMenu(id="tabs",
menuItem("Data", tabName = "data", icon = icon("table"),startExpanded = TRUE,
menuSubItem("Load", tabName = "data1"),
menuSubItem("Prep", tabName = "prep")
),
menuItem("Visualisation",icon=icon("bar-chart-o"), tabName = "vis"),
menuItem("Result", icon=icon("cog"), tabName = "result")
)
),
dashboardBody(
tags$style(type="text/css",
".shiny-output-error { visibility: hidden; }",
".shiny-output-error:before { visibility: hidden; }"
),
tabItems(
tabItem(tabName = "data1",
fluidPage(
fluidRow(
fileInput("file1","Choose CSV File",
accept = c("text/csv",
"text/comma-seperated-values, text/plain",
".csv")
),
tags$hr(),
checkboxInput("header", "Header", TRUE),
radioButtons("sep","Separator",
choices=c(Comma=",",
semicolon=";",
Tab="\t"),
selected = ";")
),
mainPanel(
uiOutput("tb")
)
)
)
),
tabItem(tabName = "prep",
fluidPage(
fluidRow(
h5("Select Varibales")
),
mainPanel(
uiOutput("Pre")
)
))
)
)
server <- shinyServer(function(input,output){
data <- reactive({
file1 <- input$file1
if(is.null(file1)){return()}
read.csv(file = file1$datapath, sep=input$sep)
})
output$filedf <- renderTable({
if(is.null(data())){return()}
input$file1
})
output$sum <- renderTable({
if(is.null(data())){return()}
summary(data())
})
output$table <- renderTable({
if(is.null(data())){return()}
data()
})
output$tb <- renderUI({
if(is.null(data())){return()}
tabsetPanel(tabPanel("About file", tableOutput("filedf")),tabPanel("Data", tableOutput("table")),tabPanel("Summary", tableOutput("sum")))
})
#----- Data Preparation------
output$Pre <- renderUI({checkboxGroupInput(inputId = "select_vars",
label="Select Variables",
choices = names(data))
})
data_sel <- reactive({
req(input$select_vars)
data_sel<- data()%>% select(input$select_var)
})
})
shinyApp(ui,server)