shiny/man/reactivePoll.Rd

\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}}
}