fetchTimestampHelpers: Methods to assist in writing and debugging custom...
In USGS-VIZLAB/vizlab: Utilities for building online data visualizations

These functions are used within vizlab fetchTimestamp() methods and may be useful within custom methods as well.

urlFetcher(id, location = NA, remoteURL)

writeTimestamp(new.timestamp, viz, timestamp.mtime = NA)

updateTimestamp(viz, verbose = FALSE)

formatTimestamp(timestamp)

locateTimestampFile(id)

readTimestamp(viz)

`id`	the viz id - should be the same as for the viz in the calling function
`location`	the fetched data file location - should be the same as for the viz in the calling function. May be NA if you're only passing the output to `fetchTimestamp()`; required for `fetch()`.
`remoteURL`	the new URL to use for fetching the data and/or timestamp (assuming `fetch()` or `fetchTimestamp()` is called on the results of this function)
`new.timestamp`	the new timestamp to write to file
`viz`	a viz item (e.g., from as.viz)
`timestamp.mtime`	the timestamp to assign to the timestamp file's metadata (using Sys.setFileTime - sets the mtime=modified time property); NA leaves the metadata alone
`verbose`	logical to pass on to `remake::make()`
`timestamp`	POSIXct timestamp to format into a character string

## Not run: 
# this custom method probably won't do what you want,
# just shows how to use the helper functions
fetchTimestamp.myfetcher <- function(viz) {
  timestamp.file <- locateTimestampFile(viz$id)
  old.timestamp <- readTimestamp(viz)
  new.timestamp <- getOnlineTimestamp() # made-up function, not implemented
  if(!file.exists(timestamp.file) || old.timestamp < new.timestamp) {
    writeTimestamp(new.timestamp, viz)
  }
  invisible()
}

## End(Not run)
## Not run: 
fetchTimestamp.myfetcher <- function(viz) {
  url <- constructURLFromViz(viz)
  urlLastModified(viz$id, remoteURL=url)
}

## End(Not run)