Endpoint Serializer with Hooks

This method allows serializers to return preexec, postexec, and aroundexec ( ) hooks in addition to a serializer. This is useful for graphics device serializers which need a preexec and postexec hook to capture the graphics output.

endpoint_serializer(
  serializer,
  preexec_hook = NULL,
  postexec_hook = NULL,
  aroundexec_hook = NULL
)

Arguments

serializer	Serializer method to be used. This method should already have its initialization arguments applied.
preexec_hook	Function to be run directly before a PlumberEndpoint calls its route method.
postexec_hook	Function to be run directly after a PlumberEndpoint calls its route method.
aroundexec_hook	Function to be run around a PlumberEndpoint call. Must handle a .next argument to continue execution.

Details

preexec and postexec hooks happened directly before and after a route is executed. These hooks are specific to a single PlumberEndpoint's route calculation.

Examples

# The definition of `serializer_device` returns
# * a `serializer_content_type` serializer
# * `aroundexec` hook
print(serializer_device)
#> function (type, dev_on, dev_off = grDevices::dev.off) 
#> {
#>     stopifnot(!missing(type))
#>     stopifnot(!missing(dev_on))
#>     stopifnot(is.function(dev_on))
#>     stopifnot(length(formals(dev_on)) > 0)
#>     if (!any(c("filename", "...") %in% names(formals(dev_on)))) {
#>         stop("`dev_on` must contain an arugment called `filename` or have `...`")
#>     }
#>     stopifnot(is.function(dev_off))
#>     endpoint_serializer(serializer = serializer_content_type(type), 
#>         aroundexec_hook = function(..., .next) {
#>             tmpfile <- tempfile()
#>             dev_on(filename = tmpfile)
#>             device_id <- dev.cur()
#>             dev_off_once <- once(function() dev_off(device_id))
#>             success <- function(value) {
#>                 dev_off_once()
#>                 if (!file.exists(tmpfile)) {
#>                   stop("The device output file is missing. Did you produce an image?", 
#>                     call. = FALSE)
#>                 }
#>                 con <- file(tmpfile, "rb")
#>                 on.exit({
#>                   close(con)
#>                 }, add = TRUE)
#>                 img <- readBin(con, "raw", file.info(tmpfile)$size)
#>                 img
#>             }
#>             cleanup <- function() {
#>                 dev_off_once()
#>                 on.exit({
#>                   unlink(tmpfile)
#>                 }, add = TRUE)
#>             }
#>             async <- FALSE
#>             on.exit({
#>                 if (!async) {
#>                   cleanup()
#>                 }
#>             }, add = TRUE)
#>             result <- promises::with_promise_domain(createGraphicsDevicePromiseDomain(device_id), 
#>                 {
#>                   .next(...)
#>                 })
#>             if (is.promising(result)) {
#>                 async <- TRUE
#>                 result %>% then(success) %>% finally(cleanup)
#>             }
#>             else {
#>                 success(result)
#>             }
#>         })
#> }
#> <bytecode: 0x5588e761bcd0>
#> <environment: namespace:plumber>