Hooks for Namespace Events

Description

Packages can supply functions to be called when loaded, attached, detached or unloaded.

Usage

.onLoad(libname, pkgname)
.onAttach(libname, pkgname)
.onUnload(libpath)
.onDetach(libpath)
.Last.lib(libpath)

Arguments

Details

After loading, 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 is in the namespace or .Last.lib is exported from the package, it will be called (with a single argument) when the package is detached. 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.

Good practice

Loading a namespace should where possible be silent, with startup messages given by .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.

See Also

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.