shiny/man/observe.Rd

% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/reactives.R
\name{observe}
\alias{observe}
\title{Create a reactive observer}
\usage{
observe(x, env = parent.frame(), quoted = FALSE, label = NULL,
  suspended = FALSE, priority = 0, domain = getDefaultReactiveDomain(),
  autoDestroy = TRUE, ..stacktraceon = TRUE)
}
\arguments{
\item{x}{An expression (quoted or unquoted). Any return value will be
ignored.}

\item{env}{The parent environment for the reactive expression. By default,
this is the calling environment, the same as when defining an ordinary
non-reactive expression.}

\item{quoted}{Is the expression quoted? By default, this is \code{FALSE}.
This is useful when you want to use an expression that is stored in a
variable; to do so, it must be quoted with \code{quote()}.}

\item{label}{A label for the observer, useful for debugging.}

\item{suspended}{If \code{TRUE}, start the observer in a suspended state. If
\code{FALSE} (the default), start in a non-suspended state.}

\item{priority}{An integer or numeric that controls the priority with which
this observer should be executed. A higher value means higher priority: an
observer with a higher priority value will execute before all observers
with lower priority values. Positive, negative, and zero values are
allowed.}

\item{domain}{See \link{domains}.}

\item{autoDestroy}{If \code{TRUE} (the default), the observer will be
automatically destroyed when its domain (if any) ends.}

\item{..stacktraceon}{Advanced use only. For stack manipulation purposes; see
\code{\link{stacktrace}}.}
}
\value{
An observer reference class object. This object has the following
  methods:
  \describe{
    \item{\code{suspend()}}{
      Causes this observer to stop scheduling flushes (re-executions) in
      response to invalidations. If the observer was invalidated prior to
      this call but it has not re-executed yet then that re-execution will
      still occur, because the flush is already scheduled.
    }
    \item{\code{resume()}}{
      Causes this observer to start re-executing in response to
      invalidations. If the observer was invalidated while suspended, then it
      will schedule itself for re-execution.
    }
    \item{\code{destroy()}}{
      Stops the observer from executing ever again, even if it is currently
      scheduled for re-execution.
    }
    \item{\code{setPriority(priority = 0)}}{
      Change this observer's priority. Note that if the observer is currently
      invalidated, then the change in priority will not take effect until the
      next invalidation--unless the observer is also currently suspended, in
      which case the priority change will be effective upon resume.
    }
    \item{\code{setAutoDestroy(autoDestroy)}}{
      Sets whether this observer should be automatically destroyed when its
      domain (if any) ends. If autoDestroy is TRUE and the domain already
      ended, then destroy() is called immediately."
    }
    \item{\code{onInvalidate(callback)}}{
      Register a callback function to run when this observer is invalidated.
      No arguments will be provided to the callback function when it is
      invoked.
    }
  }
}
\description{
Creates an observer from the given expression.
}
\details{
An observer is like a reactive expression in that it can read reactive values
and call reactive expressions, and will automatically re-execute when those
dependencies change. But unlike reactive expressions, it doesn't yield a
result and can't be used as an input to other reactive expressions. Thus,
observers are only useful for their side effects (for example, performing
I/O).

Another contrast between reactive expressions and observers is their
execution strategy. Reactive expressions use lazy evaluation; that is, when
their dependencies change, they don't re-execute right away but rather wait
until they are called by someone else. Indeed, if they are not called then
they will never re-execute. In contrast, observers use eager evaluation; as
soon as their dependencies change, they schedule themselves to re-execute.

Starting with Shiny 0.10.0, observers are automatically destroyed by default
when the \link[=domains]{domain} that owns them ends (e.g. when a Shiny
session ends).
}
\examples{
values <- reactiveValues(A=1)

obsB <- observe({
  print(values$A + 1)
})

# Can use quoted expressions
obsC <- observe(quote({ print(values$A + 2) }), quoted = TRUE)

# To store expressions for later conversion to observe, use quote()
expr_q <- quote({ print(values$A + 3) })
obsD <- observe(expr_q, quoted = TRUE)

# In a normal Shiny app, the web client will trigger flush events. If you
# are at the console, you can force a flush with flushReact()
shiny:::flushReact()
}