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Use a custom function to convert the JavaScript data into an R object

When using Shiny.onInputChange(name, data) (as described here), you are passing in a JavaScript object (data) and expect it to get converted to an R object (input$name). This conversion happens by serializing and deserializing the data to and from JSON. Usually input$name will look exactly like you'd expect it to, but it is possible for the conversion process to not do exactly what you want. Alternatively, you may just want to alter the data slightly in R before presenting it to Shiny.

This is where the shiny::registerInputHandler() function comes in: it allows you to transform the data passed in from JavaScript before it gets used as input$. For example, suppose you use Shiny.onInputChange("myobj", value) to send a value from JavaScript to R, but you want input$myobj to be automatically converted into a list. There are two simple steps you'd need to follow:

  1. In JavaScript, change Shiny.onInputChange("myobj", value) to Shiny.onInputChange("myobj:mylist", value). Notice that we append :<type> to the name of the object. This specifices the type of object that is being passed to Shiny, so that Shiny will know what handler to call when deserialization its value.

  2. In R, define the following function:

shiny::registerInputHandler("mylist", function(data, ...) {
}, force = TRUE)

Now if you access input$myobj in shiny, the value will be wrapped in a list. Of course this particular example isn't terribly useful, but this principle can be applied in real apps (for example, you can see how I've used it in timevis).