shiny multiple-interactive select

library(shiny)
df <- data.frame(
   brand=c("a","a","a","a","a","a","b","b","b","b","b","c","c","c","c","c"),
   model=c("a11","a11","a21","a21","a31","a31","b55","b55","b56","b56","b56","c33","c34","c34","c33","c34"),
   tax=c("y","x","y","x","x","y","x","x","x","y","y","x","y","x","y","y"),
   seller=c("A","B","B","C","C","D","A","D","B","C","D","B","A","C","C","D"),
   price=c(1000,1100,1500,1400,1200,1300,2000,2200,1900,1800,1700,800,900,850,950,880), stringsAsFactors = F)
app = shinyApp(
  ui = shinyUI(fluidPage(
   titlePanel("hirearchy data group"),
   sidebarLayout(
     sidebarPanel(
       selectInput("brand",label="choice the brand",choices=c("all", sort(unique(df$brand)))),
       selectInput("tax",label="choice the tax",choices=c("all", sort(unique(df$tax)))),
       selectInput("seller",label="choice the seller",choices=c("all", sort(unique(df$seller))))
     ),
     mainPanel(dataTableOutput("table"))
   ))),
  server = function(input, output, session) {
    vals = reactiveValues()
    vals$df = df
    vals$brand = "all"
    vals$tax = "all"
    vals$seller = "all"
    updateInput_f = function(panel_name){
      choice = c("all", sort(unique(vals$df[[panel_name]])))
      updateSelectInput(session, panel_name, choice = choice, selected = input[[panel_name]])
    }
    updateDF_f = function(){
      vals$df = df
      if (vals$brand != "all")
        vals$df = vals$df[vals$df$brand == vals$brand,]
      if (vals$tax != "all")
        vals$df = vals$df[vals$df$tax == vals$tax,]
      if (vals$seller != "all")
        vals$df = vals$df[vals$df$seller == vals$seller,]
    }
    brand = observe({
      input$brand
      isolate({
        vals$brand = ifelse(input$brand == "all", "all", input$brand)
        updateDF_f()
        updateInput_f("brand")
        updateInput_f("tax")
        updateInput_f("seller")
      })
    })
    tax = observe({
      input$tax
      isolate({
        vals$tax = ifelse(input$tax == "all", "all", input$tax)
        updateDF_f()
        updateInput_f("brand")
        updateInput_f("tax")
        updateInput_f("seller")
      })
    })
    seller = observe({
      input$seller
      isolate({
        vals$seller = ifelse(input$seller == "all", "all", input$seller)
        updateDF_f()
        updateInput_f("brand")
        updateInput_f("tax")
        updateInput_f("seller")
      })
    })
    output$table = renderDataTable({vals$df})
  }
)
runApp(app)