Lazy File Writer

Generates a closure that writes binary (raw) data to a file.

file_writer(path, append = FALSE)

Arguments

path

file name or path on disk

append

open file in append mode

Value

Function with signature writer(data = raw(), close = FALSE)

Details

The writer function automatically opens the file on the first write and closes when it goes out of scope, or explicitly by setting close = TRUE. This can be used for the data callback in multi_add() or curl_fetch_multi() such that we only keep open file handles for active downloads. This prevents running out of file descriptors when performing thousands of concurrent requests.

Examples

# Doesn't open yet
tmp <- tempfile()
writer <- file_writer(tmp)

# Now it opens
writer(charToRaw("Hello!\n"))
#> [1] 7
writer(charToRaw("How are you?\n"))
#> [1] 13

# Close it!
writer(charToRaw("All done!\n"), close = TRUE)
#> [1] 10

# Check it worked
readLines(tmp)
#> [1] "Hello!"       "How are you?" "All done!"