% File src/library/base/man/ns-hooks.Rd
% Part of the R package, http://www.R-project.org
% Copyright 1995-2007 R Core Development Team
% Distributed under GPL 2 or later

\name{ns-hooks}
\alias{.onLoad}
\alias{.onUnload}
\alias{.onAttach}
\title{Hooks for Name Space events}
\description{
  Packages with name spaces can supply functions to be called when
  loaded, attached or unloaded.
}

\usage{
.onLoad(libname, pkgname)
.onAttach(libname, pkgname)
.onUnload(libpath)
}
\arguments{
  \item{libname}{a character string giving the library directory where
    the package defining the namespace was found.}
  \item{pkgname}{a character string giving the name of the package.}
  \item{libpath}{a character string giving the complete path to the package.}
}
\details{
  These functions apply only to packages with name spaces.

  After loading, \code{\link{loadNamespace}} looks for a hook function named
  \code{.onLoad} and runs it before sealing the namespace and processing
  exports.

  If a name space is unloaded (via \code{\link{unloadNamespace}}),
  a hook function \code{.onUnload} is run before final unloading.
  
  Note that the code in \code{.onLoad} and \code{.onUnload} is run
  without the package being on the search path, but (unless circumvented)
  lexical scope will make objects in the namespace and its imports
  visible. (Do not use the double colon operator in \code{.onLoad} as
  exports have not been processed at the point it is run.)

  When the package is attached (via \code{\link{library}}), the hook
  function \code{.onAttach} is looked for and if found is called
  after the exported functions are attached and before the package
  environment is sealed.  This is less likely to be useful than
  \code{.onLoad}, which should be seen as the analogue of
  \code{\link{.First.lib}} (which is only used for packages without a
  name space).

  \code{.onLoad}, \code{.onUnload} and \code{.onAttach} are looked for
  as internal variables in the name space and should not be exported.
  
  If a function \code{\link{.Last.lib}} is visible in the package, it
  will be called when the package is detached: this does need to be exported.

  Anything needed for the functioning of the name space should be
  handled at load/unload times by the \code{.onLoad} and
  \code{.onUnload} hooks.  For example, DLLs can be loaded (unless done
  by a \code{useDynLib} directive in the \file{NAMESPACE} file) and
  initialized in \code{.onLoad} and unloaded in \code{.onUnload}.  Use
  \code{.onAttach} only for actions that are needed only when the
  package becomes visible to the user, for example a start-up message.
}
\seealso{
  \code{\link{setHook}} shows how users can set hooks on the same events.
}
\keyword{utilities}
