Advanced Customization

library(RJSONIO)

RJSONIO exposes customization points for parser events, string conversion, JavaScript variable output, and S4 serialization.

Parser handlers

basicJSONHandler() returns an object with update() and value() functions. The parser calls update() for each JSON token.

handler <- basicJSONHandler()
fromJSON("[1, 3, 10, 19]", handler$update)
#> NULL

handler$value()
#> [[1]]
#> [1] 1
#> 
#> [[2]]
#> [1] 3
#> 
#> [[3]]
#> [1] 10
#> 
#> [[4]]
#> [1] 19

A callback can also collect token names directly.

events <- character()

fromJSON('{"a": 1, "b": "x"}', function(type, value) {
  events <<- c(events, names(type))
  TRUE
})
#> NULL

unique(events)
#> [1] "OBJECT_BEGIN" "KEY"          "INTEGER"      "STRING"       "OBJECT_END"

String callbacks

Use stringFun to transform strings while parsing.

fromJSON('[1, "abc", "xyz"]',
         stringFun = function(value) sprintf("parsed_%s", value))
#> [[1]]
#> [1] 1
#> 
#> [[2]]
#> [1] "parsed_abc"
#> 
#> [[3]]
#> [1] "parsed_xyz"

fromJSON('["1", "2.3", "3.1415"]',
         stringFun = function(value) as.numeric(value))
#> [1] 1.0000 2.3000 3.1415

JavaScript variable output

asJSVars() formats named values as JavaScript-style assignments and returns the generated text invisibly.

js <- asJSVars(a = 1:3, b = "x")
cat(js)
#> a = [ 1, 2, 3 ] ;
#> 
#> b = [ "x" ] ;

Type annotations can be requested for common scalar values.

cat(asJSVars(.vars = list(count = 3L, label = "x"), types = TRUE))
#> count : int = [ 3 ] ;
#> 
#> label : String = [ "x" ] ;

S4 objects

S4 objects are serialized by their slots unless a more specific method is defined.

setClass("RJSONIOVignetteClass",
         representation(id = "integer", label = "character"))

object <- new("RJSONIOVignetteClass", id = 1L, label = "example")
cat(toJSON(object))
#> { "id": 1,"label": "example" }

Packages can define class-specific toJSON() methods when a different JSON shape is needed.

setClass("RJSONIOPoint", representation(x = "numeric", y = "numeric"))

setMethod("toJSON", "RJSONIOPoint",
          function(x, container = TRUE, collapse = "\n", ...,
                   .level = 1L, .withNames = TRUE, .na = "null",
                   .escapeEscapes = TRUE, pretty = FALSE, asIs = NA,
                   .inf = " Infinity") {
            toJSON(list(x = x@x, y = x@y),
                   container = container, collapse = collapse, ...,
                   .level = .level, .withNames = .withNames, .na = .na,
                   .escapeEscapes = .escapeEscapes, pretty = pretty,
                   asIs = asIs, .inf = .inf)
          })

point <- new("RJSONIOPoint", x = 1, y = 2)
fromJSON(toJSON(point))
#> [1] 1 2