Shiiny: two selectInput with mutual exclusion

ShinyColumnsSelector.R

library(shiny)

runApp(
  list(ui=pageWithSidebar(
    headerPanel("Column Selector - first idea"),
    sidebarPanel(
      uiOutput("dyncolumns")
    ),
    mainPanel(
      tableOutput("subset"),
      verbatimTextOutput("count")
    )
  ),
  server=function(input, output, session) {
    # UI columns
    output$dyncolumns <- renderUI({
      Colnames <- names(mtcars)
      list(
        selectInput("x", "x", choices=Colnames, selected=Colnames[1]),
        selectInput("y", "y", choices=Colnames, selected=Colnames[2])
      )
    })
    
    ################################################
    #### handles selection of identical columns ####
    ################################################
    ##
    # call Ex the event: x is set to the same value as y, and Ey the symmetric event 
    ##
    # REACTIVE VALUES :
    R <- reactiveValues(oldx=0, oldy=0) 
    # oldx:  the previous value of x when Ex occurs
    # oldy:  the previous value of y when Ey occurs
    # REACTIVE VALUES :
    XY <- reactiveValues(x=NULL,  y=NULL, count=0)
    # x: the effective value of x
    # y: the effective value of y
    # count:  count each time x=y ('always 0' = 'app successful')
    observe({
      if(!is.null(XY$x) && !is.null(XY$y)) XY$count <- isolate(XY$count) + (XY$x==XY$y)
    })
    output$count <- renderText({ XY$count })
    ##
    #
    # 1) the case of Ex 
    #
    observe({
      if(!is.null(input$y)){
        R$oldx <- isolate(input$x)
      }
    })
    observe({
      if(!is.null(input$x)){
        if(input$x!=isolate(input$y)){
          R$oldx <- input$x
          XY$x <- input$x; XY$y <- isolate(input$y)
        }
        else{ # Ex occurs then we exchange x and y 
          oldx <- isolate(R$oldx)
          updateSelectInput(session, "y", choices=names(mtcars), selected=oldx)
          XY$x <- input$x; XY$y <- oldx 
        }
      }
    })
    ##
    #
    # 2) the case of Ey
    #
    observe({
      if(!is.null(input$x)){
        R$oldy <- isolate(input$y)
      }
    })
    observe({
      if(!is.null(input$y)){
        if(input$y!=isolate(input$x)){
          R$oldy <- input$y
          XY$x <- isolate(input$x); XY$y <- input$y
        }
        else{  # Ey occurs then we exchange x and y 
          oldy <- isolate(R$oldy)
          updateSelectInput(session, "x", choices=names(mtcars), selected=oldy)
          XY$x <- oldy; XY$y <- input$y
        }
      }
    })

    #################
    ## Subset data ##
    #################
    output$subset <- renderTable({
      if(!is.null(input$x)) head(subset(mtcars, select=c(XY$x,XY$y)))
    })
  }
  )
)