shinycalculator

A calculator in a Shiny app

Install

Install the app from GitHub using:

# install.packages("remotes")
remotes::install_github("grddavies/shinycalculator")

Run

Run the app using:

shinycalculator::run_app()

Why?

I developed this app as a resource for teaching how to test R code using testthat and shiny apps using shinytest. Have a look here to see how the app is launched in a headless browser session using shinytest (and PhantomJS). A session is then simulated using the methods associated with the app object, and the expected behaviour tested using testthat.

Why else?

I'd like to point out some things (as well as the tests) which I think are cool about it:

• Modular code structure - the calculator is a shiny module itself, which consists of a screen and 20 instances of a calc_button module, which interact with the screen. If you want to update how all the buttons work, you only update the two functions in mod_calc_button.R.

• Quasiquotation - to give each button different functionality, one of the arguments passed to mod_calc_button_server() is a callback function (kind of - its a call rather than a function). The call is evaluated in the parent scope of the module server every time a button is pressed. When I first tried to do this, callback would only be evaluated once - it was being consumed by the first button press. I fixed that with this commit where callback is quoted when the module is first called, and then that call can be evaluated without being consumed.

• Safe evaluation of user input - rather than reinvent the wheel by constructing my own mathematical expression parser, the calculator uses the R interpreter to evaluate user input on the server. It would be a pretty major security issue if the user could submit arbitrary instructions to run on our server so I had a look at how to get around this. I found this rstudio community question which uses rlang to inspect the user input, step through the calls in the expression, and only evaluate the call if the function belongs to a list of whitelisted functions. That way the calculator can evaluate (1 + 1) * 10 but not system("sudo rm -rf /") if the whitelist is set up correctly!