Unit Testing Shiny Apps - r

Question

So I have been writing a fairly detailled shiny app, and in the future will need updating as the functionality behind what is run is constantly changing.
What I need to be able to do is have unit tests (either using testthat or another library more useful for shiny apps) that enables me to run these tests in a more automated fashion.
I have written a simple shiny app. For the sake of testing in this would like a way to know that if I choose the number 20 in the numeric input then I get 400 as the output$out text. But want to be able to do this without actually running the app myself.
library(shiny)
ui <- fluidPage(title = 'Test App',
numericInput('num', 'Number', 50, 1, 100, 0.5),
'Numeric output',
textOutput('out')
)
server <- function(input, output, session) {
aux <- reactive(input$num ^ 2)
output$out <- renderText(aux())
}
shinyApp(ui = ui, server = server)

Answer 1

As many already mentioned, you can use the package shinytest combined with testthat.
Here a simple example:
library(shinytest)
library(testthat)
context("Test shiny app")
#open shiny app
app <- ShinyDriver$new('path_to_shiny_app')
test_that("app gets expected output", {
#set numeric input
app$setInputs(num = 20)
#get output
output <- app$getValue(name = "out")
#test
expect_equal(output, "400")
})
#stop shiny app
app$stop()

Answer 2

I see two potential approaches here – testing the underlying functionality, and performing tests of the web application itself. Note that the latter actually would require running the server, but is a more accurate representation of if your web app works or not.
By testing the underlying functionality, what I mean is refactoring the calculations you currently perform in the server to their own, independent functions. Instead of squaring the number directly in the server, you ought to separate the functionality from the server so it can be tested. For example, like so:
square_of_number <- function(n) return(n^2)
Now, you can separately test the square_of_number function for its expected output.
library('testthat')
square_of_number <- function(n) return(n^2)
expect_equal(square_of_number(4), 16)
Further, if you want to test the application itself, you could also create tests using a headless browser on the actual UI you generate with Shiny. One method as suggested in the comments is using Shinytest, but one approach that I'd suggest trying is:
Running the server with a specific port,
Interfacing this server with a tool like rvest or RSelenium to manipulate the page and then scrape the output,
then verifying said output with testthat.