R: Hooks for Namespace Events

ns-hooks {base} | R Documentation

## 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

`libname` | a character string giving the library directory where the package defining the namespace was found.
`pkgname` | a character string giving the name of the package.
`libpath` | a character string giving the complete path to the package.

### 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 `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.

### 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.

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