Reactive testing for Shiny server functions and modules

A way to test the reactive interactions in Shiny applications. Reactive interactions are defined in the server function of applications and in modules.

testServer(app = NULL, expr, args = list(), session = MockShinySession$new())

Arguments

app	A server function (i.e. a function with input, output, and session), or a module function (i.e. a function with first argument id that calls moduleServer(). You can also provide an app, a path an app, or anything that as.shiny.appobj() can handle.
expr	Test code containing expectations. The objects from inside the server function environment will be made available in the environment of the test expression (this is done using a data mask with rlang::eval_tidy()). This includes the parameters of the server function (e.g. input, output, and session), along with any other values created inside of the server function.
args	Additional arguments to pass to the module function. If app is a module, and no id argument is provided, one will be generated and supplied automatically.
session	The MockShinySession object to use as the reactive domain. The same session object is used as the domain both during invocation of the server or module under test and during evaluation of expr.

Examples

# Testing a server function  ----------------------------------------------
server <- function(input, output, session) {
  x <- reactive(input$a * input$b)
}

testServer(server, {
  session$setInputs(a = 2, b = 3)
  stopifnot(x() == 6)
})
#> The value of x is 30


# Testing a module --------------------------------------------------------
myModuleServer <- function(id, multiplier = 2, prefix = "I am ") {
  moduleServer(id, function(input, output, session) {
    myreactive <- reactive({
      input$x * multiplier
    })
    output$txt <- renderText({
      paste0(prefix, myreactive())
    })
  })
}

testServer(myModuleServer, args = list(multiplier = 2), {
  session$setInputs(x = 1)
  # You're also free to use third-party
  # testing packages like testthat:
  #   expect_equal(myreactive(), 2)
  stopifnot(myreactive() == 2)
  stopifnot(output$txt == "I am 2")

  session$setInputs(x = 2)
  stopifnot(myreactive() == 4)
  stopifnot(output$txt == "I am 4")
  # Any additional arguments, below, are passed along to the module.
})