How to make a dataset reactive in Shiny?

I would like to use a reactive dataset in my shiny app, such that any other objects that uses that dataset can be re-rendered according to the values in reactiveDf.

In this example I am outputting only one table, but in my app I have other charts and tables, and the idea is to trigger the rendering by subsetting reactiveDf only. Also, I would like to do that using dplyr.

library(shiny)
library(dplyr)
ui <- shinyUI(fluidPage(
   sidebarLayout(
      sidebarPanel(
        checkboxGroupInput('Category', '',
                           unique(mtcars$carb), selected = unique(mtcars$carb))),
      # Show table of the rendered dataset
      mainPanel(
         tableOutput("df")
      )
   )
))


server <- shinyServer(function(input, output) {

  reactiveDf <- reactive({tbl_df(mtcars) %>% 
                                  filter(carb %in% input$Category)})

   output$df <- renderTable({reactiveDf})
})

shinyApp(ui = ui, server = server)

Right now, when I run this app I get:

Listening on http://127.0.0.1:7032
Warning: Error in UseMethod: no applicable method for 'xtable' 
applied to an object of class "reactive"

And tableOutput() is not displayed.