How to unit test Shiny widgets?

cole · September 13, 2018, 3:43pm

Let's say I am building a Shiny widget with an input object on the server side. An example usage of the API:

ui <- fluidPage(
  myWidget("some_id")
)

server <- function(input, output, session) {
  observe(print(input$some_id))
}

If I am putting this widget into a package, how should I test it? I could build an example app like this and use shinytest, but that seems a bit heavyweight to use for CI / testing on CRAN / during development / etc. Again, the focus is not an app but a widget. Do I need a headless browser? Some particular type of mocking?

Do I scaffold a fake Shiny session around the functions for testing with testthat? I have been exploring some examples from the Shiny repo, but am having trouble wrapping my head around the best way to approach.

github.com

rstudio/shiny/blob/master/tests/testthat/test-bootstrap.r

context("bootstrap")

test_that("CSS unit validation", {
  # On error, return NA; on success, return result
  validateCssUnit_wrap <- function(x) {
    tryCatch(validateCssUnit(x), error = function(e) { NA_character_ })
  }

  # Test strings and expected results
  strings  <- c("100x", "10px", "10.4px", ".4px", "1px0", "px", "5",  "%", "5%", "auto", "1auto", "")
  expected <- c(NA,     "10px", "10.4px", ".4px", NA,       NA, "5px", NA, "5%", "auto", NA,      NA)
  results <- vapply(strings, validateCssUnit_wrap, character(1), USE.NAMES = FALSE)
  expect_equal(results, expected)

  # Numbers should return string with "px"
  expect_equal(validateCssUnit(100), "100px")
})


test_that("Repeated names for selectInput and radioButtons choices", {

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jcheng · September 18, 2018, 7:22pm

If the intention is to test both the R and JavaScript portions of the widget, then I don't think you have a choice but to use something that uses a headless browser (like shinytest). Personally yes I'd write a little app and hook it up to shinytest for CI; but don't do observe(print(input$some_id)), instead have a verbatimTextOutput/renderPrint so shinytest can verify the output.