There are three types of information you can get from a Vega chart, a signal, data (i.e. a dataset), and information associated with an event. A dataset or a signal must first be defined and named in the vegaspec.
vw_shiny_get_signal(outputId, name, body_value = "value")
vw_shiny_get_data(outputId, name, body_value = "value")
vw_shiny_get_event(outputId, event, body_value = "datum")character, shiny outputId for the vegawidget
character, name of the signal (defined in Vega specification)
being monitored
character or JS_EVAL, the body of a JavaScript
function that Vega will use to handle the signal or event; this function
must return a value
character, type of the event being monitored, e.g. "click",
for list of supported events, please see
Vega Event-Stream reference
shiny::reactive() function that returns the value returned by
body_value
These getter-functions are called from within
a Shiny server() function, where they act like
shiny::reactive(), returning a reactive expression.
To see these functions in action, you can run a shiny-demo:
vw_shiny_get_signal(): call vw_shiny_demo("signal-set-get")
vw_shiny_get_data(): call vw_shiny_demo("data-set-get")
vw_shiny_get_event(): call vw_shiny_demo("event-get")
In addition to the chart outputId, you will need to provide:
vw_shiny_get_signal(): the name of the signal, as defined in the Vega
specification
vw_shiny_get_data(): the name of the dataset, as defined in the Vega
specification
vw_shiny_get_event(): the event type, as defined in the
Vega Event-Stream reference
When the signal or data changes, or when the event fires, Vega needs to know which information you want returned to Shiny. To do this, you provide a JavaScript handler-function:
vw_shiny_get_signal(): the default handler,
vw_handler_signal("value"),
specifies that the value of the signal be returned.
vw_shiny_get_data(): the default handler,
vw_handler_data("value"),
specifies that the entire dataset be returned.
vw_shiny_get_event(): the default handler,
vw_handler_event("datum"),
specifies that the single row of data associated with graphical mark
be returned. For example, if you are monitoring a "click" event,
Vega would return the row of data that backs any mark
(like a point) that you click.
If you need to specify a different behavior for the handler, there are a
couple of options. This package provides
a library of handler-functions; call vw_handler_signal(),
vw_handler_data(), or vw_handler_event() without arguments to
list the available handlers.
If the library does not contain the handler you need, the body_value
argument will also accept a character string which will be used as
the body of the handler function.
For example, these calls are equivalent:
vw_shiny_get_signal(..., body_value = "value")
vw_shiny_get_signal(..., body_value = vw_handler_signal("value"))
vw_shiny_get_signal(..., body_value = "return value;")
If you use a custom-handler that you think may be useful for the handler-function library, please file an issue.