These functions allow users to set actions to be taken before packages are attached/detached and namespaces are (un)loaded.
getHook(hookName)
setHook(hookName, value,
action = c("append", "prepend", "replace"))packageEvent(pkgname,
event = c("onLoad", "attach", "detach", "onUnload"))
character string: the hook name
character string: the package/namespace name
character string: an event for the package. Can be abbreviated.
A function or a list of functions, or for action = "replace"
, NULL
The action to be taken. Can be abbreviated.
For getHook
function, a list of functions (possibly empty).
For setHook
function, no return value.
For packageEvent
, the derived hook name (a character string).
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 was associated with events on
packages/namespaces: 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
or loaded by
other means, it can call initialization code. See
.onLoad
for a description of the package hook functions
called during initialization. Users can add their own initialization
code via the hooks provided by setHook()
, functions which will
be called as funname(pkgname, pkgpath)
inside a
try
call.
The sequence of events depends on which hooks are defined, and whether a package is attached or just loaded. In the case where all hooks are defined and a package is attached, the order of initialization events is as follows:
The package namespace is loaded.
The package's .onLoad
function is run.
If S4 methods dispatch is on, any actions set by
setLoadAction
are run.
The namespace is sealed.
The user's "onLoad"
hook is run.
The package is added to the search path.
The package's .onAttach
function is run.
The package environment is sealed.
The user's "attach"
hook is run.
A similar sequence (but in reverse) is run when a package is detached and its namespace unloaded:
The user's "detach"
hook is run.
The package's .Last.lib
function is run.
The package is removed from the search path.
The user's "onUnload"
hook is run.
The package's .onUnload
function is run.
The package namespace is unloaded.
Note that when an R session is finished, packages are not detached and namespaces are not unloaded, so the corresponding hooks will not be run.
Also note that some of the user hooks are run without the package being on the search path, so in those hooks objects in the package need to be referred to using the double (or triple) colon operator, as in the example.
If multiple hooks are added, they 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.
The hooks are stored in the environment .userHooksEnv
in the
base package, with ‘mangled’ names.
library
, detach
, loadNamespace
.
See ::
for a discussion of the double and triple colon operators.
Other hooks may be added later: functions plot.new
and
persp
already have them.
# NOT RUN {
setHook(packageEvent("grDevices", "onLoad"),
function(...) grDevices::ps.options(horizontal = FALSE))
# }
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