Connections and Streaming

library(RJSONIO)

fromJSON() can read from files and R connections. readJSONStream() provides a lower-level connection-oriented parser interface.

File paths

path <- system.file("sampleData", "keys.json", package = "RJSONIO")
parsed <- fromJSON(path)

names(parsed)
#> [1] "menu"

Text connections

con <- textConnection("[1, 2, 3,\n4]")
parsed <- fromJSON(con)
close(con)

parsed
#> [1] 1 2 3 4

File connections

path <- system.file("sampleData", "usaPolygons.as", package = "RJSONIO")
con <- file(path)
parsed <- fromJSON(con)
close(con)

length(parsed)
#> [1] 3082

Parser callbacks

Callbacks can be used to observe parser events without building the default R object.

events <- character()

result <- fromJSON(path, function(type, value) {
  events <<- c(events, names(type))
  TRUE
})

result
#> NULL
head(events)
#> [1] "ARRAY_BEGIN" "ARRAY_BEGIN" "FLOAT"       "FLOAT"       "FLOAT"      
#> [6] "FLOAT"

Streaming parser

readJSONStream() accepts a connection and reads JSON content from it.

tmp <- tempfile(fileext = ".json")
writeLines("[1, 2, 3]", tmp, useBytes = TRUE)

con <- file(tmp, open = "rb")
streamed <- readJSONStream(con)
close(con)
unlink(tmp)

streamed
#> [1] 1 2 3

Practical notes

For ordinary package code, start with fromJSON() on a string, file path, or connection. Use parser callbacks or readJSONStream() when the application needs lower-level control over parsing.