R: Functions to Get and Set Hooks for Load, Attach, Detach and Unload

UserHooks {base} | R Documentation

## Functions to Get and Set Hooks for Load, Attach, Detach and Unload

### Description

These functions allow users to set actions to be taken before packages are attached/detached and name spaces are (un)loaded.

### Usage

    getHook(hookName)
    setHook(hookName, value,
            action = c("append", "prepend", "replace"))

    packageEvent(pkgname,
                 event = c("onLoad", "attach", "detach", "onUnload"))

### Arguments

`hookName` | character string: the hook name
`pkgname` | character string: the package/name space name. If versioned install has been used, `pkgname` should be the unversioned name of the package (but any version information will be stripped).
`event` | character string: an event for the package
`value` | A function, or for `action="replace"`, `NULL`.
`action` | The action to be taken. The names can be appreviated.

### Details

`setHook` provides a general mechanism for users to register hooks, a list of functions to be called from system (or user) functions. The initial set of hooks is associated with events on packages/name spaces: these hooks are named via calls to `packageEvent`.

To remove a hook completely, call `setHook(hookName, NULL, "replace")`.

When an R package is attached by `library`, it can call initialization code via a function `.First.lib`, and when it is `detach`-ed it can tidy up via a function `.Last.lib`. Users can add their own initialization code via the hooks provided by these functions, functions which will be called as `funname(pkgname, pkgpath)` inside a `try` call. (The attach hook is called after `.First.lib` and the detach hook before `.Last.lib`.)

If a package has a name space, there are two further actions, when the name space is loaded (before being attached and after `.onLoad` is called ) and when it is unloaded (after being detached and before `.onUnload`). Note that code in these hooks is run without the package being on the search path, so objects in the package need to be referred to using the double colon operator as in the example. (Unlike `.onLoad`, the user hook is run after the name space has been sealed.)

Hooks are normally run in the order shown by `getHook`, but the `"detach"` and `"onUnload"` hooks are run in reverse order so the default for package events is to add hooks ‘inside’ existing ones.

Note that when an R session is finished, packages are not detached and name spaces are not unloaded, so the corresponding hooks will not be run.

The hooks are stored in the environment `.userHooksEnv` in the base package, with ‘mangled’ names.

### Value

For `getHook` function, a list of functions (possible empty). For `setHook` function, no return value. For `packageEvent`, the derived hook name (a character string).

### See Also

`library`, `detach`, `loadNamespace`.

Other hooks may be added later: `plot.new` and `persp` already have them.

### Examples

    setHook(packageEvent("grDevices", "onLoad"),
            function(...) grDevices::ps.options(horizontal=FALSE))

[Package base version 2.6.2 Index]