shiny/_includes/tutorial/client-data.md

Getting Non-Input Data From the Client

On the server side, Shiny applications use the input object to receive user input from the client web browser. The values in input are set by UI objects on the client web page. There are also non-input values (in the sense that the user doesn't enter these values through UI components) that are stored in an object called session$clientData. These values include the URL, the pixel ratio (for high-resolution "Retina" displays), the hidden state of output objects, and the height and width of plot outputs.

Using session$clientData

To access session$clientData values, you need to pass a function to shinyServer() that takes session as an argument (session is a special object that is used for finer control over a user's app session). Once it's in there, you can access session$clientData just as you would input.

In the example below, the client browser will display out the components of the URL and also parse and print the query/search string (the part of the URL after a "?"):

server.R

{% highlight r %} shinyServer(function(input, output, session) {

Return the components of the URL in a string:

output$urlText <- renderText({ paste(sep = "", "protocol: ", session$clientData$url_protocol, "\n", "hostname: ", session$clientData$url_hostname, "\n", "pathname: ", session$clientData$url_pathname, "\n", "port: ", session$clientData$url_port, "\n", "search: ", session$clientData$url_search, "\n" ) })

Parse the GET query string

output$queryText <- renderText({ query <- parseQueryString(session$clientData$url_search)

# Return a string with key-value pairs
paste(names(query), query, sep = "=", collapse=", ")

}) }) {% endhighlight %}

ui.R

{% highlight r %} shinyUI(bootstrapPage( h3("URL components"), verbatimTextOutput("urlText"),

h3("Parsed query string"), verbatimTextOutput("queryText") )) {% endhighlight %}

This app will display the following:

Viewing all available values in clientData

The values in session$clientData will depend to some extent on the outputs. For example, a plot output object will report its height, width, and hidden status. The app below has a plot output, and displays all the values in session$clientData:

{% highlight r %} shinyServer(function(input, output, session) {

Store in a convenience variable

cdata <- session$clientData

Values from cdata returned as text

output$clientdataText <- renderText({ cnames <- names(cdata)

allvalues <- lapply(cnames, function(name) {
  paste(name, cdata[[name]], sep=" = ")
})
paste(allvalues, collapse = "\n")

})

A histogram

output$myplot <- renderPlot({ hist(rnorm(input$obs), main="Generated in renderPlot()") }) }) {% endhighlight %}

Notice that, just as with input, values in session$clientData can be accessed with session$clientData$myvar or session$clientData[['myvar']]. Or, equivalently, since we've saved it into a convenience variable cdata, we can use cdata$myvar or cdata[['myvar']].

ui.R

{% highlight r %} shinyUI(pageWithSidebar( headerPanel("Shiny Client Data"), sidebarPanel( sliderInput("obs", "Number of observations:", min = 0, max = 1000, value = 500) ), mainPanel( h3("clientData values"), verbatimTextOutput("clientdataText"), plotOutput("myplot") ) )) {% endhighlight %}

For the plot output output$myplot, there are three entries in clientData:

• output_myplot_height: The height of the plot on the web page, in pixels.
• output_myplot_width: The width of the plot on the web page, in pixels.
• output_myplot_hidden: If the object is hidden (not visible), this is TRUE. This is used because Shiny will by default suspend the output object when it is hidden. When suspended, the observer will not execute even when its inputs change.

Here is the view from the client, with all the clientData values: