HTML Widgets: Advanced Topics

HTMLWidgets.dataframeToD3()

R data frames are represented in “long” form (an array of named vectors) whereas d3 typically requires “wide” form (an array of objects each of which includes all names and values). Since the R representation is smaller in size and much faster to transmit over the network, we create the long-form representation of R data, and then transform the data in JavaScript using the dataframeToD3() helper function.

Here is an example of the long-form representation of an R data frame:

{
  "Sepal.Length": [5.1, 4.9, 4.7],
  "Sepal.Width": [3.5, 3, 3.2],
  "Petal.Length": [1.4, 1.4, 1.3],
  "Petal.Width": [0.2, 0.2, 0.2],
  "Species": ["setosa", "setosa", "setosa"]
} 

After we apply HTMLWidgets.dataframeToD3(), it will become:

[
  {
    "Sepal.Length": 5.1,
    "Sepal.Width": 3.5,
    "Petal.Length": 1.4,
    "Petal.Width": 0.2,
    "Species": "setosa"
  },
  {
    "Sepal.Length": 4.9,
    "Sepal.Width": 3,
    "Petal.Length": 1.4,
    "Petal.Width": 0.2,
    "Species": "setosa"
  },
  {
    "Sepal.Length": 4.7,
    "Sepal.Width": 3.2,
    "Petal.Length": 1.3,
    "Petal.Width": 0.2,
    "Species": "setosa"
  }
] 

As a real example, the simpleNetwork widget accepts a data frame containing network links on the R side, then transforms it to a d3 representation within the JavaScript renderValue function:

renderValue: function(x) {

  // convert links data frame to d3 friendly format
  var links = HTMLWidgets.dataframeToD3(x.links);
  
  // ... use the links, etc ...

}

HTMLWidgets.transposeArray2D()

Sometimes a 2-dimensional array requires a similar transposition. For this the transposeArray2D() function is provided. Here is an example array:

[
  [5.1, 4.9, 4.7, 4.6, 5, 5.4, 4.6, 5],
  [3.5, 3, 3.2, 3.1, 3.6, 3.9, 3.4, 3.4],
  [1.4, 1.4, 1.3, 1.5, 1.4, 1.7, 1.4, 1.5],
  [0.2, 0.2, 0.2, 0.2, 0.2, 0.4, 0.3, 0.2],
  ["setosa", "setosa", "setosa", "setosa", "setosa", "setosa", "setosa", "setosa"]
] 

HTMLWidgets.transposeArray2D() can transpose it to:

[
  [5.1, 3.5, 1.4, 0.2, "setosa"],
  [4.9, 3, 1.4, 0.2, "setosa"],
  [4.7, 3.2, 1.3, 0.2, "setosa"],
  [4.6, 3.1, 1.5, 0.2, "setosa"],
  [5, 3.6, 1.4, 0.2, "setosa"],
  [5.4, 3.9, 1.7, 0.4, "setosa"],
  [4.6, 3.4, 1.4, 0.3, "setosa"],
  [5, 3.4, 1.5, 0.2, "setosa"]
] 

As a real example, the dygraphs widget uses this function to transpose the “file” (data) argument it gets from the R side before passing it on to the dygraphs library:

renderValue: function(x) {
   
    // ... code excluded ...
    
    // transpose array
    x.attrs.file = HTMLWidgets.transposeArray2D(x.attrs.file);
    
    // ... more code excluded ...
}

Custom JSON serializer

You may find it necessary to customize the JSON serialization of widget data when the default serializer in htmlwidgets does not work in the way you have expected. For widget package authors, there are two levels of customization for the JSON serialization: you can either customize the default values of arguments for jsonlite::toJSON(), or just customize the whole function.

1. jsonlite::toJSON() has a lot of arguments, and we have already changed some of its default values. Below is the JSON serializer we use in htmlwidgets at the moment:

   function (x, ..., dataframe = "columns", null = "null", na = "null", 
       auto_unbox = TRUE, digits = getOption("shiny.json.digits", 
           16), use_signif = TRUE, force = TRUE, POSIXt = "ISO8601", 
       UTC = TRUE, rownames = FALSE, keep_vec_names = TRUE, strict_atomic = TRUE) 
   {
       if (strict_atomic) 
           x <- I(x)
       jsonlite::toJSON(x, dataframe = dataframe, null = null, na = na, 
           auto_unbox = auto_unbox, digits = digits, use_signif = use_signif, 
           force = force, POSIXt = POSIXt, UTC = UTC, rownames = rownames, 
           keep_vec_names = keep_vec_names, json_verbatim = TRUE, 
           ...)
   }
   <bytecode: 0x7fe511839ae0>

   For example, we convert data frames to JSON by columns instead of rows (the latter is jsonlite::toJSON’s default). If you want to change the default values of any arguments, you can attach an attribute TOJSON_ARGS to the widget data to be passed to createWidget(), e.g.

   fooWidget <- function(data, name, ...) {
     # ... process the data ...
     params <- list(foo = data, bar = TRUE)
     # customize toJSON() argument values
     attr(params, 'TOJSON_ARGS') <- list(digits = 7, na = 'string')
     htmlwidgets::createWidget(name, x = params, ...)
   }

   We changed the default value of digits from 16 to 7, and na from null to string in the above example. It is up to you, the package author, whether you want to expose such customization to users. For example, you can leave an extra argument in your widget function so that users can customize the behavior of the JSON serializer:

   fooWidget <- function(data, name, ..., JSONArgs = list(digits = 7)) {
     # ... process the data ...
     params <- list(foo = data, bar = TRUE)
     # customize toJSON() argument values
     attr(params, 'TOJSON_ARGS') <- JSONArgs
     htmlwidgets::createWidget(name, x = params, ...)
   }

   You can also use a global option htmlwidgets.TOJSON_ARGS to customize the JSON serializer arguments for all widgets in the current R session, e.g.

   options(htmlwidgets.TOJSON_ARGS = list(digits = 7, pretty = TRUE))

2. If you do not want to use jsonlite, you can completely override the serializer function by attaching an attribute TOJSON_FUNC to the widget data, e.g.

   fooWidget <- function(data, name, ...) {
     # ... process the data ...
     params <- list(foo = data, bar = TRUE)
     # customize the JSON serializer
     attr(params, 'TOJSON_FUNC') <- MY_OWN_JSON_FUNCTION
     htmlwidgets::createWidget(name, x = params, ...)
   }

   Here MY_OWN_JSON_FUNCTION can be an arbitrary R function that converts R objects to JSON. If you have also specified the TOJSON_ARGS attribute, it will be passed to your custom JSON function as well.

Note these features about custom JSON serializers require the shiny version to be greater than 0.11.1 if you render the widgets in Shiny apps.