Overview

In the spirit of HTML widgets working just like plots in R, it’s important that HTML widgets intelligently size themselves to their container, whether it be the RStudio Viewer, a figure in knitr, or a UI panel within a Shiny application. The htmlwidgets framework provides a rich mechanism for specifying the sizing behavior of widgets.

This sizing mechanism is designed to address the following constraints that affect the natural size of a widget:

Widget sizing is handled in two steps:

  1. First, a sizing policy is specified for the widget. This is done via the sizingPolicy argument to the createWidget function. Most widgets can accept the default sizing policy (or override only one or two aspects of it) and get satisfactory sizing behavior (see details below).

  2. The sizing policy is used by the framework to compute the correct width and height for a widget given where it is being rendered. This size information is then passed to the initialize and resize methods of the widgets JavaScript binding. It’s up to the widget to then forward this size information to the underlying JavaScript library.

Specifying a sizing policy

The default HTML widget sizing policy treats the widget with the same sizing semantics as an R plot. When printed at the R console the widget is displayed within the RStudio Viewer and sized to fill the Viewer pane (modulo any padding). When rendered inside an R Markdown document the widget is sized based on the default size of figures in the document.

Note that for most widgets the default sizing behavior is fine and you won’t need to create a custom sizing policy. If you need a slightly different behavior than the default you can also selectively override the default behavior by calling the sizingPolicy function and passing the result to createWidget. For example:

htmlwidgets::createWidget(
  "sigma", 
  x, 
  width = width, 
  height = height,
  sizingPolicy = htmlwidgets::sizingPolicy(
    viewer.padding = 0,
    viewer.paneHeight = 500,
    browser.fill = TRUE
  )
)

Examples

The networkD3 package uses custom sizing policies for all of its widgets. The simpleNetwork widget eliminates padding (as d3 is already providing padding) and specifies that it wants to fill up as much space as possible when displayed in a standalone web browser:

sizingPolicy(padding = 0, browser.fill = TRUE)

The sankeyNetwork widget requires much more space than is afforded by the RStudio Viewer or a typical knitr figure so it disables those automatic sizing behaviors. It also provides a more reasonable default width and height for knitr documents:

sizingPolicy(viewer.suppress = TRUE,
             knitr.figure = FALSE,
             browser.fill = TRUE,
             browser.padding = 75,
             knitr.defaultWidth = 800,
             knitr.defaultHeight = 500)

Available options

Here are the various options that can be specified within a sizing policy:

Option Description
defaultWidth The default width used to display the widget. This parameter specifies the default width for viewing in all contexts (browser, viewer, and knitr) unless it is specifically overridden with e.g. browser.defaultWidth.
defaultHeight The default height used to display the widget. This parameter specifies the default height for viewing in all contexts (browser, viewer, and knitr) unless it is specifically overridden with e.g. browser.defaultHeight.
padding Padding around the widget (in pixels). This parameter specifies the padding for viewing in all contexts (browser and viewer) unless it is specifically overridden by e.g. browser.padding.
viewer.defaultWidth The default width used to display the widget within the RStudio Viewer.
viewer.defaultHeight The default height used to display the widget within the RStudio Viewer.
viewer.padding Padding around the widget when displayed in the RStudio Viewer (defaults to 15 pixels).
viewer.fill When displayed in the RStudio Viewer, automatically size the widget to the viewer dimensions (note that viewer.padding is still applied). Default to TRUE.
viewer.suppress Never display the widget within the RStudio Viewer (useful for widgets that require a large amount of space for rendering). Defaults to FALSE.
viewer.paneHeight Request that the RStudio Viewer be forced to a specific height when displaying this widget.
browser.defaultWidth The default width used to display the widget within a standalone web browser.
browser.defaultHeight The default height used to display the widget within a standalone web browser.
browser.padding Padding around the widget when displayed in a standalone browser (defaults to 40 pixels).
browser.fill When displayed in a standalone web browser, automatically size the widget to the browser dimensions (note that browser.padding is still applied). Defaults to FALSE.
knitr.defaultWidth The default width used to display the widget within documents generated by knitr (e.g. R Markdown).
knitr.defaultHeight The default height used to display the widget within documents generated by knitr (e.g. R Markdown).
knitr.figure Apply the default knitr fig.width and fig.height to the widget when it’s rendered within R Markdown documents. Defaults to TRUE.

JavaScript resize method

Specifying a sizing policy allows htmlwidgets to calculate the width and height of your widget based on where it’s being displayed. However, you still need to forward this sizing information on to the underlying JavaScript library you are creating a widget for.

Every JavaScript library handles dynamic sizing a bit differently. Some do it automatically, some have a resize() call to force a layout, and some require that size be set only along with data and other options. Whatever the case, the htmlwidgets framework will pass the computed sizes to both your factory function and resize function. Here’s an empty JavaScript binding that illustrates:

HTMLWidgets.widget({

  name: "demo",
  
  type: "output",
  
  factory: function(el, width, height) {
  
    return {
      renderValue: function(x) {
      
      },
      
      resize: function(width, height) {
        
      }
    };
  }
});

What you do with the passed width and height is up to you and depends on the re-sizing semantics of the underlying JavaScript library you are creating a widget for. A couple of illustrative examples are included in the next section.

Examples

dygraphs

In the dygraphs widget the implementation of re-sizing is relatively simple since the dygraphs library includes a resize() method to automatically size the graph to it’s enclosing HTML element:

resize: function(width, height) {
  if (dygraph)
    dygraph.resize();
}

forceNetwork

In the forceNetwork widget, the passed width and height are applied to the <svg> element that hosts the d3 network visualization, as well as forwarded on to the underlying d3 force simulation object:

factory: function(el, width, height) {

  // instance data
  var el = el;
  var force = d3.layout.force();

  d3.select(el).append("svg")
    .attr("width", width)
    .attr("height", height);
      
  return {
    renderValue: function(x) {
      // implementation excluded
    },
      
    resize: function(width, height) {
         
      d3.select(el).select("svg")
        .attr("width", width)
        .attr("height", height);

      force.size([width, height]).resume();
    }
  };
}

As you can see, re-sizing is handled in a wide variety of fashions in different JavaScript libraries. The resize method is intended to provide a flexible way to map the automatic sizing logic of htmlwidgets directly into the underlying library.