Extend shinyWidgets: knobInput

November 21, 2017 ยท 9 minute read


shiny basic package contains several widgets, available here. This database is continuously extended through the shinyWidget package developed by the dreamRs team. Here I present how we add a new knobInput widget to shinyWidget, with Victor Perrier from the dreamRs team.

I always wanted to play with some knobs in shiny, similarly as what we do with sliders. However, in spite of some research, I found nothing already implemented. Later, I discovered this js library called jQuery knob. In parallel, I started to read some tutorials on “how to extend shiny” and decided to create a new RStudio project to include this new widget. Since I am not a expert in javascript and jQuery, I asked a question on stackoverflow and Victor Perrier nicely solved my problem. Then, we both decided to include this new widget in the shinyWidget package. Below is a short explanation on how to build such a custom input, which is not trivial but not so hard. I hope it will be useful for you, who are reading this blog post.

Here is the final result on shinyapps.io: shinyknob


The idea is simple:

  • First, we create the file containing the new widget: input-knob.R, because it is supposed to be a knob (very original). It is widely inspired on what you can find on the shiny github R folder. Below, I quickly describe its content:
# Knob Input
knobInput <- function(inputId, label, value, min = 0, max = 100, step = 1,
                      angleOffset = 0, angleArc = 360, cursor = FALSE,
                      thickness = NULL, lineCap = c("default", "round"), displayInput = TRUE,
                      displayPrevious = FALSE, rotation = c("clockwise", "anticlockwise"),
                      fgColor = NULL, inputColor = NULL, bgColor = NULL,
                      readOnly = FALSE, skin = NULL, width = NULL, height = NULL) {
  value <- shiny::restoreInput(id = inputId, default = value)
  lineCap <- match.arg(lineCap)
  rotation <- match.arg(rotation)
  knobParams <- dropNulls(list(
    id = inputId, type = "text", class = "knob-input", 
    value = value, `data-value` = value, `data-skin` = skin,
    `data-min` = min, `data-max` = max, `data-step` = step, 
    `data-angleoffset` = angleOffset, `data-anglearc` = angleArc,
    `data-displayprevious` = displayPrevious, `data-thickness` = thickness,
    `data-displayinput` = displayInput, `data-linecap` = lineCap,
    `data-fgcolor` = fgColor, `data-inputcolor` = inputColor,
    `data-bgcolor` = bgColor, `data-cursor` = cursor,
    `data-rotation` = rotation, `data-readonly` = readOnly,
    `data-width` = width, `data-height` = height
  knobParams <- lapply(knobParams, function(x) {
    if (identical(x, TRUE))
    else if (identical(x, FALSE))
    else x
  inputTag <- do.call(htmltools::tags$input, knobParams)
  knobInputTag <- htmltools::tags$div(
    class = "form-group shiny-input-container",
    style = if (!is.null(width)) paste0("width: ", htmltools::validateCssUnit(width), ";"),
    if (!is.null(label)) htmltools::tags$label(`for` = inputId, label),
    if (!is.null(label)) htmltools::tags$br(),
  dep <- htmltools::htmlDependency(
    name = "jquery-knob", version = "1.2.13",
    src = c(href = "jquery-knob"),
    script = c("jquery.knob.min.js",
  htmltools::attachDependencies(knobInputTag, dep)
  # Change the value of a knob input on the client
  updateKnobInput <- function(session, inputId, label = NULL, value = NULL, readOnly = NULL) {
    message <- dropNulls(list(label = label, value = value, readOnly = readOnly))
    session$sendInputMessage(inputId, message)
  dropNulls <- function(x) {
    x[!vapply(x, is.null, FUN.VALUE = logical(1))]

First of all, all the options available in jQuery knob are included in the function arguments. Thus, I highly recommend you to check the corresponding documentation. The value is recovered using shiny’s restoreInput function. linecap and rotation can have 2 different values. Therefore, the match.arg function allows to select the values. All parameters passed in the function are stored in a list called knobParams. Importantly, as shown in jQuery knob demo page, type is set as “text”. We choose to name the class “knob-input”, instead of “dial” (see the documentation), without any consequences. Null values are not considered (dropNulls function). Since we use a js library, TRUE and FALSE cannot be recognized, unless they are converted in true and false, respectively. This is done, using the lapply and identical functions. Our knobParam widget is wrapped in an input div (tags$input), as also shown in jQuery knob documentation:

<input type="text" class="dial" data-min="-50" data-max="50">

We store all dependencies in the dep object, via htmlDependency function from htmltools package. Notice that in the shinyWidget code, this part is slightly modified, since files are organized in another way. We also create an updateKnobInput function, which will allow the user to update the value and label of the knob (readOnly does not work at the moment). dropNulls is defined below but could be written in a utils.R file, for instance.

Additionally, your /www folder should contain both jquery.knob.min.js and knob-input-binding.js, the later is discussed below.

  • We create the related input-binding. This file should be put in the /www folder of the widget. To summarise, the input-binding will make the link between shiny and your new widget (see here).

var exportsKnob = window.Shiny = window.Shiny || {};
var $escapeKnob = exportsKnob.$escape = function(val) {
    return val.replace(/([!"#$%&'()*+,.\/:;<=>?@\[\\\]^`{|}~])/g, '\\$1');

function tron_skin() {

    // "tron" case
    if (this.$.data('skin') == 'tron') {
        this.cursorExt = 0.3;
        var a = this.arc(this.cv) // Arc
            pa // Previous arc
            , r = 1;
        this.g.lineWidth = this.lineWidth;
        if (this.o.displayPrevious) {
            pa = this.arc(this.v);
            this.g.strokeStyle = this.pColor;
            this.g.arc(this.xy, this.xy, this.radius - this.lineWidth, pa.s, pa.e, pa.d);
        this.g.strokeStyle = r ? this.o.fgColor : this.fgColor;
        this.g.arc(this.xy, this.xy, this.radius - this.lineWidth, a.s, a.e, a.d);
        this.g.lineWidth = 2;
        this.g.strokeStyle = this.o.fgColor;
        this.g.arc(this.xy, this.xy, this.radius - this.lineWidth + 1 + this.lineWidth * 2 / 3, 0, 2 * Math.PI, false);
        return false;

// Knob input binding

var knobInputBinding = new Shiny.InputBinding();

// An input binding must implement these methods
$.extend(knobInputBinding, {

    // This returns a jQuery object with the DOM element
    find: function(scope) {
        return $(scope).find('.knob-input');

    // this method will be called on initialisation
    initialize: function(el) {

        // extract the value from el
        // note here our knobInput does not yet exist
        var value = $(el).data("value");

        // initialize our knob based on the extracted state
        el.value = value;

        // Initialize knob
        $(el).knob({draw: tron_skin});

    // Given the DOM element for the input, return the value
    getValue: function(el) {
        return parseFloat(el.value);

    // Set up the event listeners so that interactions with the
    // input will result in data being sent to server.
    // callback is a function that queues data to be sent to
    // the server.
    subscribe: function(el, callback) {
        $(el).on('keyup.knobInputBinding', function(event) {
            // When called with true, it will use the rate policy,
            // which in this case is to debounce at 500ms.

        $(el).trigger('configure', {
            'change': function(v) {
            'release' : function (v) { 
        $(el).on('change.knobInputBinding', function(event) {
            // When called with false, it will NOT use the rate policy,
            // so changes will be sent immediately

    // Remove the event listeners
    unsubscribe: function(el) {

    // Receive messages from the server.
    // Messages sent by updateknobInput() are received by this function.
    receiveMessage: function(el, data) {
        if (data.hasOwnProperty('value')) {

        if (data.hasOwnProperty('readOnly')) {
                'configure', {
                    "readOnly": data.readOnly

        if (data.hasOwnProperty('label'))
            $(el).parent().parent().find('label[for="' + $escapeKnob(el.id) + '"]').text(data.label);


    // This returns a full description of the input's state.
    // Note that some inputs may be too complex for a full description of the
    // state to be feasible.
    getState: function(el) {
        return {
            label: $(el).parent().parent().find('label[for="' + $escapeKnob(el.id) + '"]').text(),
            value: el.value

    // The input rate limiting policy
    getRatePolicy: function() {
        return {
            // Can be 'debounce' or 'throttle'
            policy: 'debounce',
            delay: 500


Shiny.inputBindings.register(knobInputBinding, 'shiny.knobInput');

The first part of this binding contain a javascript code when the “tron” skin is used. It is just copied and paste from jQuery knob library. Then, we define the input binding as follows:

  • we declare a new variable (not surprisingly “knobInputBinding”).
  • we call the find method using the class “knob-input”, previously defined in our input-knob.R file. Crucial step!
  • the initialize function creates the knob using .knob() (see jQuery knob), where we set the value of the widget.
  • the method getValue returns the value of your widget(s).
  • subscribe method: one of the most important feature. Without this part, the value of your widget will not be updated in shiny when you change it. We can find several methods inside:
$(el).on('keyup.knobInputBinding', function(event) {
            // When called with true, it will use the rate policy,
            // which in this case is to debounce at 500ms.

keyup method is related to events on the user keyboard (especially multidirectional arrows).

$(el).trigger('configure', {
            'change': function(v) {
            'release' : function (v) { 

The configure method enables to dynamically configure the widget. The change method is used when the value of the input changes. Without release, scrolling with the mouse wheel will not update the value in shiny. We found relevant to set this kind of behavious.

Besides, the callback function is useful to set a rate policy for the widget (see below).

  • getRatePolicy will avoid to send to much requests to the server. For example, let’s say the user wants to move the knob from 1 to 100. Without rate policy, each change from 1 to 100 would be sent to the server, which is a bit ridiculous. Rate policy is called when callback has “true” as argument.
  • we register the input binding with the register function.

  • Finally, we create a short demonstration of this widget.

# knob --------------------------------------------------------------------

# Package

# Fun

# HTML dep
shiny::addResourcePath('jquery-knob', file.path(getwd(), "www"))

# ui ---- 
ui <- fluidPage(
  tags$h1("knob input examples"), 
  tags$h4("made by Victor Perrier (shinyWidgets) and David Granjon"),
  tags$h4("based on", a("jQuery knob", href = "https://github.com/aterrien/jQuery-Knob")),
      width = 4,
      knobInput(inputId = "knob1", label = "Default:", value = 10),
      verbatimTextOutput(outputId = "res1"),
      knobInput(inputId = "knob2", label = "Color:", value = 1, fgColor = "#D9534F", displayInput = FALSE),
      verbatimTextOutput(outputId = "res2"),
      knobInput(inputId = "knob3", label = "Read only:", value = 65, readOnly = TRUE),
      verbatimTextOutput(outputId = "res3")
      width = 4,
      knobInput(inputId = "knob6", label = "Cursor mode:", value = 50, thickness = 0.3, cursor = TRUE, width = 150, height = 150), # must specify width
      verbatimTextOutput(outputId = "res6"),
      knobInput(inputId = "knob7", label = "Display previous:", value = 50, min = -100, displayPrevious = TRUE, fgColor = "#428BCA", inputColor = "#428BCA"),
      verbatimTextOutput(outputId = "res7"),
        style = "background-color: #000;",
        knobInput(inputId = "knob8", label = tags$span("Tron skin:", style = "color: #FFF;"), value = 50, width = 75, fgColor = "#ffec03", inputColor = "#ffec03", skin = "tron")
      verbatimTextOutput(outputId = "res8")
      width = 4,
      knobInput(inputId = "knob11", label = "Angle offset:", value = 75, angleOffset = 90, lineCap = "round"),
      verbatimTextOutput(outputId = "res11"),
      knobInput(inputId = "knob12", label = "Angle offset and arc:", value = 50, angleOffset = -125, angleArc = 250),
      verbatimTextOutput(outputId = "res12"),
      knobInput(inputId = "knob13", label = "4-digit, step 0.1:", value = 0, min = -1000, max = 1000, step = 0.1, displayPrevious = TRUE),
      verbatimTextOutput(outputId = "res13")

# server ----
server <- function(input, output, session) {

  lapply(1:13, function(x) {
    output[[paste0("res", x)]] <- renderPrint(input[[paste0("knob", x)]])

# app ----
shinyApp(ui = ui, server = server)

Basically, there are 4 ways you can change the input:

  • using multidirectional arrows
  • entering text in the input field (when it is present, ie displayInput is set to TRUE)
  • dragging the knob with the mouse
  • scrolling with the mouse wheel.

For the moment, the readOnly argument does not work with the updateKnobInput function.