Fix spurious duplicate values being sent by selectInput

[jcheng5]

Fixes #2162. I described the root cause in my comment.

The fix I made is to explicitly detect this condition in the selectize input binding, and suppress the event that (temporarily) sets the value to "". So the selectize input binding does cause an "NY" event to be raised when you backspace, but now it's correctly squelched by the client side dupe-detecting logic.

Testing notes

Here is a slightly improved reprex of the original issue. Without this fix, you will see "NY" printed at first; and if you click into the select box and backspace, you'll see "NY" printed again. With this fix, you'll only see "NY" printed the first time. In no case should you be able to get it to show an empty string for the value.

shinyApp(
  ui = fluidPage(
    selectInput("state", "Choose a state:",
      c("NY", "NJ", "CT")
    ),
    textOutput("result")
  ),
  server = function(input, output, session) {
    output$result <- renderText({
      print(input$state)
      paste("You chose", input$state)
    })
  }
)

This example adds a "Please choose one" prompt, which causes "" to now be a valid state. Both before and after this fix, backspacing should cause "" to be printed at the console, and "Please choose one" should appear in the select box.

shinyApp(
  ui = fluidPage(
    selectInput("state", "Choose a state:",
      c("Please choose one" = "", "NY", "NJ", "CT")
    ),
    textOutput("result")
  ),
  server = function(input, output, session) {
    output$result <- renderText({
      print(input$state)
      paste("You chose", input$state)
    })
  }
)

This example is like the first one except it uses selectizeInput instead of selectInput; the defaults are slightly different, in that selectizeInput allows you to have nothing selected. So with and without the fix, you should see backspace cause "" to be printed at the console, and clicking focus away from the select box control should leave it empty.

shinyApp(
  ui = fluidPage(
    selectizeInput("state", "Choose a state:",
      c("NY", "NJ", "CT")
    ),
    textOutput("result")
  ),
  server = function(input, output, session) {
    output$result <- renderText({
      print(input$state)
      paste("You chose", input$state)
    })
  }
)

[alandipert]

I think if you use an arrow function for the change.selectInputBinding callback then you don't need to explicitly capture this (just refer to this in arrow function).

[jcheng5]

Thanks, fixed. I thought we couldn't use ES2015 here but you're right, we can.