EDIT: show customized slider value using Javascript

柳高卓
2023-12-01

Say if you really want to change the current value label for the slider to indicate a somewhat sophisticated value (date, transformed values etc.), you can write some small Javascript snippet to do that.

One example provided by the jslider control (which is what Shiny uses for your sliderInput) indicates that by calling the constructor of a slider with a calculate function, you can manually change the mapping from the current value (numeric) to a custom string (in this case, a date).

Without being too verbose, let's just say that the slider's Javascript object is called slider. Which you can always get by calling:

$(select).slider()

where select is a jQuery selector. Again, in this case it is #foo because the slider has an id foo set in ui.R.  When initiated, the slider.settings.calculate function appeared in the example will be bound to be the slider.nice function. So, we can simply override the nice function to achieve our goal.

Below is a modified ui.R with a piece of Javascript to do exactly the nice function overriding.

ui.R

shinyUI(pageWithSidebar(
  headerPanel("renderImage example"),
  sidebarPanel(
    sliderInput("foo", "Animation duration", min = -30,
                max = 30, value = 0, step = 1,
                animate = animationOptions(loop = TRUE, interval = 1000)),
    dateInput("ana", "Choose a date:", value = as.Date("2014-07-01"))
  ),

  mainPanel(
    singleton(tags$head(HTML(
      '
  <script type="text/javascript">
    $(document).ready(function() {
      var slider = $("#foo").slider();
      // override the default "nice" function.
      slider.nice = function(value) {
        var ref_date = new Date("2014-07-01");
        // each slider step is 1 day, translating to 24 * 3600 * 1000 milliseconds
        var slider_date = new Date(ref_date.getTime() + value * 24 * 3600 * 1000);
        return [slider_date.getUTCFullYear(), 
                slider_date.getUTCMonth() + 1, 
                slider_date.getUTCDate()].join("-");
      }
    })
  </script>
'))),
    # Use imageOutput to place the image on the page
    imageOutput("preImage")
  )
))

Another detail we might want to tweak is the label on both ends of the slider. To achieve this we could:

  1. replace the entire slider.domNode by calling slider.generateScales();
  2. directly modify the two <span>'s with class jslider-label.

For example if we take approach 2, we can modify the HTML() part of ui.R as:

    singleton(tags$head(HTML(
      '
  <script type="text/javascript">
    $(document).ready(function() {
      var slider = $("#foo").slider();
      // override the default "nice" function.
      var labels = slider.domNode.find(".jslider-label span");
      labels.eq(0).text("2014-06-01");
      labels.eq(1).text("2014-07-31");
      slider.nice = function(value) {
        var ref_date = new Date("2014-07-01");
        // each slider step is 1 day, translating to 24 * 3600 * 1000 milliseconds
        var slider_date = new Date(ref_date.getTime() + value * 24 * 3600 * 1000);
        return [slider_date.getUTCFullYear(), 
                slider_date.getUTCMonth() + 1, 
                slider_date.getUTCDate()].join("-");
      }
    })
  </script>
'))),

And the labels will display the dates as expected.

This is of course some quick hacking and may not work if some other customization is made to the slider.

I think somehow the Shiny team should consider adding a calculate option in sliderInput which directly maps to slider.settings.calculate, just to make things like this easier.

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