| setLoadActions {methods} | R Documentation |
These functions provide a mechanism for packages to specify computations to be done during the loading of the package namespace. Such actions are a flexible way to provide information only available at load time (such as locations in a dynamically linked library).
A call to
setLoadAction() or setLoadActions() specifies one or more functions
to be
called when the corresponding package is loaded, with the ... argument
names being used as identifying names for the actions.
getLoadActions reports the currently defined load actions,
given a package's namespace as its argument.
hasLoadAction returns TRUE if a load action
corresponding to the given name has previously been set for the
where namespace.
evalOnLoad() and evalqOnLoad() schedule a specific expression for
evaluation at load time.
setLoadAction(action, aname=, where=) setLoadActions(..., .where=) getLoadActions(where=) hasLoadAction(aname, where=) evalOnLoad(expr, where=, aname=) evalqOnLoad(expr, where=, aname=)
action, ... |
functions of one or more arguments, to be called when this package is loaded. The functions will be called with one argument (the package namespace) so all following arguments must have default values. If the elements of ... are named, these names will be used for the corresponding load metadata. |
where, .where |
the namespace of the package for which the list of load actions are defined. This argument is normally omitted if the call comes from the source code for the package itself, but will be needed if a package supplies load actions for another package. |
aname |
the name for the action. If an action is set without
supplying a name, the
default uses the position in the sequence of actions specified
( |
expr |
an expression to be evaluated in a load action in
environment |
The evalOnLoad() and evalqOnLoad() functions are for
convenience.
They construct a function to evaluate the expression and call
setLoadAction() to schedule a call to that function.
Each of the functions supplied as an argument to setLoadAction() or
setLoadActions() is saved as metadata in the
namespace, typically that of the package containing the call to
setLoadActions().
When this package's namespace is loaded, each of these functions will
be called.
Action functions are called in the order they are supplied to
setLoadActions().
The objects assigned have metadata names constructed from the names
supplied in the call; unnamed arguments are taken to be named by their
position in the list of actions (".1", etc.).
Multiple calls to setLoadAction() or setLoadActions() can be used in a package's
code; the actions will be scheduled after any previously specified,
except if the name given to setLoadAction() is that of an
existing action.
In typical applications, setLoadActions() is more convenient
when calling from the package's own code to set several actions.
Calls to setLoadAction() are more convenient if the action name
is to be constructed, which is more typical when one package
constructs load actions for another package.
Actions can be revised by assigning with the same name, actual or constructed, in a subsequent call. The replacement must still be a valid function, but can of course do nothing if the intention was to remove a previously specified action.
The functions must have at least one argument. They will be called
with one argument, the namespace of the package.
The functions will be called at the end of processing of S4 metadata, after
dynamically linking any libraries, the call to .onLoad(), if
any, and caching method and class definitions, but before the namespace is sealed.
Functions may therefore assign or modify objects in the namespace supplied as the argument in the call. The mechanism allows packages to save information not available until load time, such as values obtained from a dynamically linked library.
Load actions should be contrasted with user load hooks supplied by
setHook().
User hooks are generally provided from outside the package and are run
after the namespace has been sealed.
Load actions are part of the package code, and the list of actions is
normally established when the package is installed.
Load actions can be supplied directly in the source code for a package. It is also possible and useful to provide facilities in one package to create load actions in another package. The software needs to be careful to assign the action functions in the correct environment, namely the namespace of the target package.
setLoadAction() and setLoadActions() are called for their side effect and return no
useful value.
getLoadActions() returns a named list of the actions in the
supplied namespace.
hasLoadAction() returns TRUE if the specified action
name appears in the actions for this package.
## Not run:
## in the code for some package
## ... somewhere else
setLoadActions(function(attach)
cat(c("Loaded", "Unloaded")[attach], "at", Sys.time(), "\n"),
setCount = function(ns) assign("myCount", 1, envir = ns),
function(ns) assign("myPointer", getMyExternalPointer(), envir = ns))
... somewhere later
if(countShouldBe0)
setLoadAction(function(ns) assign("myCount", 0, envir = ns), "setCount")
## End(Not run)