shiny/man/reactivePoll.Rd

% Generated by roxygen2 (4.0.0): do not edit by hand
\name{reactivePoll}
\alias{reactivePoll}
\title{Reactive polling}
\usage{
reactivePoll(intervalMillis, session, checkFunc, valueFunc)
}
\arguments{
  \item{intervalMillis}{Approximate number of milliseconds
  to wait between calls to \code{checkFunc}. This can be
  either a numeric value, or a function that returns a
  numeric value.}

  \item{session}{The user session to associate this file
  reader with, or \code{NULL} if none. If non-null, the
  reader will automatically stop when the session ends.}

  \item{checkFunc}{A relatively cheap function whose values
  over time will be tested for equality; inequality
  indicates that the underlying value has changed and needs
  to be invalidated and re-read using \code{valueFunc}. See
  Details.}

  \item{valueFunc}{A function that calculates the
  underlying value. See Details.}
}
\value{
A reactive expression that returns the result of \code{valueFunc},
  and invalidates when \code{checkFunc} changes.
}
\description{
Used to create a reactive data source, which works by periodically polling a
non-reactive data source.
}
\details{
\code{reactivePoll} works by pairing a relatively cheap "check" function with
a more expensive value retrieval function. The check function will be
executed periodically and should always return a consistent value until the
data changes. When the check function returns a different value, then the
value retrieval function will be used to re-populate the data.

Note that the check function doesn't return \code{TRUE} or \code{FALSE} to
indicate whether the underlying data has changed. Rather, the check function
indicates change by returning a different value from the previous time it was
called.

For example, \code{reactivePoll} is used to implement
\code{reactiveFileReader} by pairing a check function that simply returns the
last modified timestamp of a file, and a value retrieval function that
actually reads the contents of the file.

As another example, one might read a relational database table reactively by
using a check function that does \code{SELECT MAX(timestamp) FROM table} and
a value retrieval function that does \code{SELECT * FROM table}.

The \code{intervalMillis}, \code{checkFunc}, and \code{valueFunc} functions
will be executed in a reactive context; therefore, they may read reactive
values and reactive expressions.
}
\examples{
\dontrun{
# Assume the existence of readTimestamp and readValue functions
shinyServer(function(input, output, session) {
  data <- reactivePoll(1000, session, readTimestamp, readValue)
  output$dataTable <- renderTable({
    data()
  })
})
}
}
\seealso{
\code{\link{reactiveFileReader}}
}