A Shiny app object, or a string naming an app directory.
file
A vector of names of output files. Should end with .png,
.pdf, or .jpeg. If several screenshots have to be taken and
only one filename is provided, then the function appends the index number
of the screenshot to the file name.
A named character vector or named list of environment
variables and values to set for the Shiny app's R process. These will be
unset after the process exits. This can be used to pass configuration
information to a Shiny app.