Minimal reproducible example to test reactivity of selectInput with updateSelectInput

minimal_example.R

# app prototype -----------------------------------------------------------
library(shiny)

# define select chioces
input_choices <- data.frame(
  x = c(1, 1, 2, 2, 3, 3),
  y = c(11, 12, 21, 22, 31, 32)
)

# choices for x
x_choices <- unique(input_choices$x)

ui <- shinyUI(
  fluidPage(
    tagList(
      selectInput("x_selection", "X", x_choices),
      p(),
      selectInput("y_selection", "Y", c(""))
    )
  )
)

server <- shinyServer(
  function(input, output, session) {
    observeEvent(input$x_selection, {
      y_choices <-
        input_choices[
          input_choices$x == input$x_selection,
          "y"
        ]

      updateSelectInput(
        session,
        "y_selection",
        choices = y_choices
      )
    })
  }
)

shinyApp(ui, server)

# testing (run separately) ------------------------------------------------
# 
# library(testthat)
# library(shinytest)
# 
# test_that("Second selectInput is reactive", {
#   testServer(app = application, {
#       # init application
#       app <- ShinyDriver$new(path = "minimal_example.R")
#       
#       # check startup values
#       expect_equal(app$getValue("x_selection"), "1")
#       expect_equal(app$getValue("y_selection"), "11")
#       
#       # change x_selection input
#       app$setValue("x_selection", "2")
#       
#       # expect things to change
#       expect_equal(app$getValue("x_selection"), "2")
#       expect_equal(app$getValue("y_selection"), "21")
#       
#       # stop & remove app
#       app$finalize(); rm(app)
#   })
# })