Supplementing your R package with a Shiny app

http://deanattali.com/2015/04/21/r-package-shiny-app/

The R community is generally very fond of open-source-ness and the idea of releasing all code to the public. Writing packages has become such an easy experience now that Hadley’s 

devtools

 is
so powerful, and as a result there are new packages being released by useRs every single day.
A good package needs to have two things: useful functionality, and clear usage instructions. The former is a no-brainer, while the latter is what developers usually dread the most - the D-word
(Documentation. Yikes.). Proper documentation is essential so that others will know what your package can do and how to do it. And with the use of Shiny, we now have another great tool we can use to showcase a package’s capabilities.

Incorporating Shiny

In a nutshell, Shiny is a package that lets you run your R code
as an interactive webpage. What this means for package developers is that you can have an interactive webpage that lets users experiment with your package and see what it can do before having to read through the potentially lengthy function documentations/vignette.
As an example, I recently released a package for adding marginal plots to ggplot2. You be the judge: after that one-sentence description of some functionality, would you rather go straight to the README,
or see it in action first in a Shiny app online? I might be wrong, but I think it’s useful to interactively see what
the package can do.
Making a Shiny app doesn’t necessarily always make sense for every package, but there are certainly many times when it can be a great addition to a package’s “documentation”. I think that if a
new package has some functions that can be easily illustrated in a simple Shiny app, it’s worth it to take the extra 1-2 hours to develop it. This way, a user who finds your package and isn’t quite sure what to do with it
can try the Shiny app to see whether or not this is the functionality they were looking for. You can have several Shiny apps, each showing the usage of a particular function, or one app that is representative of a whole package. Whatever makes the most sense.
Of course, having a Shiny app is in no way a replacement to documentation, it’s just a useful add-on.
There are two ways to complement a package with a Shiny app that shows its main usage. These two methods are NOT mutually exclusive; I personally do both of them together:

1. Host the Shiny app online

You can host your Shiny app somewhere that is publicly available, such as shinyapps.io or
on your own Shiny Server. Then you can include
a link in the package’s README or vignette or function documentation that points to the Shiny app.
As an example, I host my own Shiny Server where I can host my Shiny
apps, and whenever I release a new package, I include a link in the README to a demo app.
The advantage of doing this is that people can play around with your package before even downloading it.

2. Include the app in the package and add a function to launch it

I recommend including the source code of the Shiny app in your package, and having a function such as 

runExample()

 that
will launch the app. Here are the steps to do this (I’ve learned a lot from looking at 

shiny::runExample

 source
code - thanks RStudio):
First, add 

shiny

 as
a dependency in your 

DESCRIPTION

 file.
Then place your Shiny app folder under 

inst/shiny-examples/

 and
add an R file called

runExample.R

. The package’s tree structure
should look like this

- mypacakge
|- inst
|- shiny-examples
|- myapp
|- ui.R
|- server.R
|- R
|- runExample.R
|- ...
|- DESCRIPTION
|- ...

Your 

runExample.R

 will
be simple - it will just look for the Shiny app and launch it

#' @export
runExample <- function() {
appDir <- system.file("shiny-examples", "myapp", package = "mypackage")
if (appDir == "") {
stop("Could not find example directory. Try re-installing `mypackage`.", call. = FALSE)
}

shiny::runApp(appDir, display.mode = "normal")
}

Of course, don’t forget to document this function! Now users can try out an app showcasing your package by running 

mypackage::runExample()

.
This method can easily support more than one Shiny app as well, simply place each app under 

inst/shiny-examples/

 and
change the runExample code to something like this

runExample <- function(example) {
# locate all the shiny app examples that exist
validExamples <- list.files(system.file("shiny-examples", package = "mypackage"))

validExamplesMsg <-
paste0(
"Valid examples are: '",
paste(validExamples, collapse = "', '"),
"'")

# if an invalid example is given, throw an error
if (missing(example) || !nzchar(example) ||
!example %in% validExamples) {
stop(
'Please run `runExample()` with a valid example app as an argument.\n',
validExamplesMsg,
call. = FALSE)
}

# find and launch the app
appDir <- system.file("shiny-examples", example, package = "mypackage")
shiny::runApp(appDir, display.mode = "normal")
}

Now running 

runExample("myapp")

 will
launch the “myapp” app, and running

runExample()

 will generate
a message telling the user what examples are allowed.