.onLoad(libname, pkgname)
.onAttach(libname, pkgname)
.onUnload(libpath)
.onDetach(libpath)
.Last.lib(libpath)
.onAttach
. These messages (and any essential
ones from .onLoad
) should use packageStartupMessage
so they can be silenced where they would be a distraction. There should be no calls to library
nor require
in these
hooks. The way for a package to load other packages is via the
Depends field in the DESCRIPTION file: this ensures
that the dependence is documented and packages are loaded in the
correct order. Loading a namespace should not change the search path,
so rather than attach a package, dependence of a namespace on another
package should be achieved by (selectively) importing from the other
package's namespace. Uses of library
with argument help
to display basic
information about the package should use format
on the
computed package information object and pass this to
packageStartupMessage
. There should be no calls to installed.packages
in startup
code: it is potentially very slow and may fail in versions of R
before 2.14.2 if package installation is going on in parallel. See
its help page for alternatives. Compiled code should be loaded (e.g., via
library.dynam
) in .onLoad
or a useDynLib
directive in the NAMESPACE file, and not in .onAttach
.
Similarly, compiled code should not be unloaded (e.g., via
library.dynam.unload
) in .Last.lib
nor
.onDetach
, only in .onUnload
.loadNamespace
looks for a hook function
named .onLoad
and calls it (with two unnamed arguments) before
sealing the namespace and processing exports. When the package is attached (via library
or
attachNamespace
), the hook function .onAttach
is
looked for and if found is called (with two unnamed arguments) before
the package environment is sealed.
If a function .onDetach
(as from R 3.0.0) is in the namespace
or .Last.lib
is exported from the package, it will be called
(with a single argument) when the package is detach
ed.
Beware that it might be called if .onAttach
has failed, so it should be
written defensively. (It is called within tryCatch
, so
errors will not stop the package being detached.)
If a namespace is unloaded (via unloadNamespace
), a hook
function .onUnload
is run (with a single argument) before final
unloading.
Note that the code in .onLoad
and .onUnload
should not
assume any package except the base package is on the search path.
Objects in the current package will be visible (unless this is
circumvented), but objects from other packages should be imported or
the double colon operator should be used.
.onLoad
, .onUnload
, .onAttach
and
.onDetach
are looked for as internal objects in the namespace
and should not be exported (whereas .Last.lib
should be).
Note that packages are not detached nor namespaces unloaded at the end
of an R session unless the user arranges to do so (e.g., via
.Last
).
Anything needed for the functioning of the namespace should be
handled at load/unload times by the .onLoad
and
.onUnload
hooks. For example, DLLs can be loaded (unless done
by a useDynLib
directive in the NAMESPACE file) and
initialized in .onLoad
and unloaded in .onUnload
. Use
.onAttach
only for actions that are needed only when the
package becomes visible to the user (for example a start-up message)
or need to be run after the package environment has been created.
setHook
shows how users can set hooks on the same events, and
lists the sequence of events involving all of the hooks. reg.finalizer
for hooks to be run at the end of a session.
loadNamespace
for more about namespaces.