Run rmarkdown::render() when files that match a pattern change within a directory

_watch-render-rmd.R

#!/usr/local/bin/Rscript --no-site-file --no-init-file --no-restore

# --vanilla ==> --no-site-file --no-init-file --no-restore --no-environ
# thus the above reads the .Renviron file(s)

## required packages ==
# install.packages(c("docopt",
#                    "testthat"))

## docopt.org ==
"Usage:
    _watch-render-rmd.R --patt=<patt>
    _watch-render-rmd.R -h | --help
Options:
    --patt=<patt>       File pattern
    -h, --help          Help
Examples:
    _watch-render-rmd.R --patt=\"lesson_[[:digit:]]+.Rmd\"
    _watch-render-rmd.R --patt=\".+\\.Rmd\"" -> doc

opts <- docopt::docopt(doc)

cwd <- getwd()

render <- function(added, deleted, modified) {
    file_changed <- ""

    write_and_bind_change <- function(f, txt) {
        arg <- deparse(substitute(f))
        writeLines(paste0("\nAt ", Sys.time(), " the file ", f, " was ", arg))
        file_changed <<- f
    }

    if (length(added) > 0) {
        write_and_bind_change(added)
    }
    if (length(deleted) > 0) {
        write_and_bind_change(deleted)
    }
    if (length(modified) > 0) {
        write_and_bind_change(modified)
    }

    writeLines(paste0("Re-rendering the document ", file_changed, " within ", cwd))

    output <- rmarkdown::render(input = file_changed,
                                output_format = 'html_document',
                                encoding = 'UTF-8',
                                quiet = TRUE)

    writeLines(paste0("At ", Sys.time(), " the document ", file_changed, " completed rendering: ", cwd, "/", output))
    writeLines(paste0("\n----------\n"))
    writeLines(paste0("Watching the directory ", cwd, " for \n    * additions\n    * deletions\n    * modifications"))

    # return TRUE to keep watching; FALSE to stop watching
    TRUE
}

writeLines(paste0("Watching the directory ", cwd, " for \n    * additions\n    * deletions\n    * modifications"))

testthat::watch(path = cwd,
                pattern = opts$patt,
                callback = render)