Updating test inputs with shinytesters

The shinytesters package is a way to update input values whilst running tests through shiny::testServer.

When using testServer, any “update” function in shiny (or any shiny extension package) does nothing to the inputs within the session object, and therefore the tests litter in expressions to manually update inputs using session$setInputs. This can create a disconnect between what is expected to happen in the server-side and what is actually happening.

shinytesters enables this through the function create_test_update_fns. This function takes existing function definitions, and converts the body of the function to communicate with the test session object rather than the unavailable UI. By including what the ID and value arguments, it is enough to determine what the input is going to be after calling the update function.

In addition, it also takes all of the other arguments that are passed to the update function, and gives them an input value too, appending the argument name to the ID for an easy extraction in the test expression.

# Simple module where when a button is clicked, a select input is updated to 
# select "h" out of the letters and checkbox is selected
server_module_fn <- function(id) {
  moduleServer(id, function(input, output, session) {
    observeEvent(input$button, {
      updateSelectInput(inputId = "options", choices = letters, selected = "h")
      updateCheckboxInput(inputId = "check", value = TRUE)
    })
  })
}

test_that("When the button is clicked, the options update", {
  local_mocked_bindings(
    !!!create_test_update_fns(c("updateSelectInput", "updateCheckboxInput")),
    .package = "shiny" 
  )
  
  testServer(
    app = server_module_fn, 
    expr = {
      # Uninitialised modulehas NULL for inputs
      expect_null(input$options)
      
      # After clicking button, options choices and selected option update 
      session$setInputs(button = 1L)
      expect_identical(input$options, "h")
      expect_identical(input$options.choices, letters)
      expect_true(input$check)
    }
  )
})

Package Level Helpers

At the package level, the function use_shiny_testers is available as a blanket assignment of all update functions in the specified package.

test_that("When the button is clicked, the options update", {
  use_shiny_testers()
  
  testServer(
    app = server_module_fn, 
    expr = {
      # Uninitialised modulehas NULL for inputs
      expect_null(input$options)
      
      # After clicking button, options choices and selected option update 
      session$setInputs(button = 1L)
      expect_identical(input$options, "h")
      expect_identical(input$options.choices, letters)
      expect_true(input$check)
    }
  )
})