NAME
Plugins - Generic plugins framework
SYNOPSIS
use Plugins;
$plugins = Plugins->new context => $context;
$plugins->readconfig($config_file, self => $self);
$plugins->initialize();
$plugins->invoke($method, @args);
$plugins->invoke_until($method, sub { scalar(@_) }, @args);
my $iterator = $plugins->iterator();
while (@results = &$iterator(@args)) {
}
for my $plugin ($plugins->plugins()) {
$plugin->invoke($method);
}
DESCRIPTION
Plugins provides a simple way for a programs to be assembled at runtime
via configuration files.
Generally, there are only a few ways that a module in a library can be
customized for an application: (1) You can change the code of the
module; (2) if it presents an object-oriented interface you can subclass
it; or (3) the module in question can have a configuration file that
allows you to do what you need. Plugins makes it easier to allow
behavior to be modified by a configuration file by allow other code to
be mixed in.
Plugins allows plugins have plugins and for all the plugins to share a
single configuration file. This isn't required but sometimes it's nice
to have everything in one place.
Each plugin is implemented by a perl module.
There can be more than one instance (object) for each plugin (class).
The Plugins module itself isn't complete and must be subclassed. An
example subclass that completes Plugins is Plugins::Style1. Plugins
written for use with the Plugins module generally don't need to know how
Plugins has been subclassed because the $context that is passed to a
plugin allows it to use Plugins directly even if its ancestor did not.
NAMING CONVENTION
Since plugins can have plugins, some plugins will have Plugins objects
themselves. Some won't. We will term a user of Plugins to be
*requestor*. A *requestor* that is not itself a plugin will be termed
the *root*. A plugin will be termed a *child* and the *requestor* that
invoked it will be termed the *parent*. A plugin that has plugins is a
*child requestor*.
CONSTRUCTION AND INITIALIAZATION
While there are similarities, using Plugins for the first time (by the
*root*) is different than using it as a plugin that has plugins (*child
requestor*).
*root* initialization.
The *root requestor* will need to create a plugins object. Since Plugins
needs to be subclassed it will create the object using the subclass. For
example, Plugins::Style1:
$plugins = Plugins::Style1->new(%args);
$plugins->readconfig($configfile, %args);
$plugins->initialize();
*child requestor* initialization
There are three steps for a *requestor* to take to start using plugins.
Each step is a call to Plugins:
$plugins = Plugins->new(context => $context, %args);
$plugins->readconfig($configfile, %args);
$plugins->initialize();
The most important argument for "Plugins::new()" is "context ="
$context>. The $context comes from the first argument to *child*->new()
when Plugins creates plugin object instances.
For *child requestors*, no %args are needed for new() except for
"context =" $context>.
Likewise, no %args are needed for "readconfig()".
%args for new()
There may be additional parameters depending on how Plugins is
subclassed.
The %args that "new()" userstands are:
configfile => "/config/file"
Provide a configuration file name. If this is done here then "undef"
can be passwd for the configfile to "readconfig()".
api => $api
Provide a Plugins::API object that will be passed to any plugins's
"new()" invocation.
%args for readconfig()
Readconfig has a positional argument: $configfile:
$plugins->readconfig($configfile, %args);
%args for "readconfig()" depend upon how Plugins is subclassed.
USING PLUGINS
Plugins provides the following methods for *requestors*:
invoke($method, @args)
This calls each plugin in turn. Plugins are called in the order in
which they were configured. No return value is defined. The calls
are not eval-wrapped so a "die" in a plugin is fatal.
invoke_until($method, $testfunc, @args)
This calls each plugin in turn. Plugins are called in the order in
which they were configured. After each call, the return value from
the plugin method invocation is evaluated with "&$testfunc()". If
"&$testfunc()" returns a true value, then the return value from the
plugin method invocation is returned and no further calls are made.
plugins()
This returns the list of plugin objects. This will be in the order
in which the plugins were defined.
iterator($method)
This returns an anonymous function that when invoked will call the
first plugin ("$plugin-"method(@args)). Each successive call will
call another plugin until it returns undef.
startconfig()
This method can be used instead of "readconfig()" to start the
configuration process. It does not read any configuration files. It
is required prior to "parseconfig()" or "initialize()".
parseconfig($configfile, %args)
This parses a configuration file. Unlike "readconfig()" it does not
call "startconfig()" first.
api($api)
This will set an $api variable that will be passed to "new()" when
creating new plugin instances. This is expected to be a Plugins::API
object.
RE-CONFIGURATION
The "readconfig()" method may be called more than once. Each time it is
called, "initialize()" may be called again. When "initialize()" is
called a second time, it calls "$plugin->shutdown()" for each of the old
plugins before calling "$plugin->new()" to create the new ones.
Each call to "readconfig()" starts fresh. If you want to add to an
existing set of plugins rather than replace them, use "parseconfig()"
instead of "readconfig()".
WRITING PLUGINS
Plugins should be subclasses of "Plugins::Plugin". Plugin objects
provide the following methods:
invoke($method, @args)
Basically this just does "$plugin->method(@args)". If $method does
not exist, just return undef.
new()
The default "new()" does the following:
sub new
{
my ($pkg, $pconfig, %args) = @_;
return bless {
context => $pconfig->{context},
api => $pconfig->{api},
config => \%args
}, $pkg;
}
You'll often want to override "new()".
Plugin-defined methods
Plugins will need to define methods to be called. What methods need to
be defined depends upon what they are a plugin for. Plugins for programs
that are a Daemon::Generic will generally need "preconfig()" and
"postconfig()" methods. Look at the documentation or code of the
*requestor*.
The following methods are called directly by Plugins:
new($pconfig, @args)
Invoked to create an object instance of a plugin. The @args come
from the configuration file. The $pconfig is used to pass in
plugin-related configuration data. It's a reference to a hash and
"$pconfig->{context}" needs to be passed to any *child requestors*
so that they can fit themselves in properly.
shutdown()
Called when a object instance is no longer needed due to a reconfig
after the configuration file has been changed. On a reconfiguration,
all the old objects are destoryed and a new set are created. The
default "shutdown()" doesn't do anything.
AUTOLOAD with Plugins::API
The Plugins module has defines an "AUTOLOAD" function in
Plugins::Plugin base class to map unknown method invocations into
"invoke()". It does this through the Plugins::API module. Plugins
that invoke a method that isn't formally defined will have the
attempted method invocation redirected through
"$self->{myapi}->invoke()" or "$self->{api}->invoke()".
The default "new()" for plugins will capture a Plugins::API object
passed in from Plugins as "$self->{api}". Plugins that have plugins
might want to name their Plugins::API object "$self->{myapi}"
(assuming they have one).
Plugins with plugins
Plugins that themselves have plugins (*child requestors*) will need
to invoke Plugins themselves. If the overall program uses
Daemon::Generic, then doing this in a "preconfig()" method is
reccomended. In other contexts this may need to be done in "new()".
sub new
{
my ($pkg, $pconfig, %args) = @_;
return bless {
context => $pconfig->{context},
api => $pconfig->{api},
config => \%args
}, $pkg;
}
sub preconfig
{
my ($self, $configfile) = @_;
my $config = $self->{config}{configfile} || $configfile;
$self->{plugins} = new Plugins %{$self->{context};
$self->{plugins}->readconfig($config, self => $self);
$self->{plugins}->initialize();
$self->{plugins}->invoke('preconfig', $config);
}
It is expected that whichever subclass of Plugins is used by the
*parent* should also be used by the child. The following snippet
will supress this behavior. This is probably a bad idea unless the
*child* uses a different configuration file than then *parent*.
local($self->{context}{pkg_override}{$config});
$self->{plugins} = new Plugins context => $self->{context};
$self->{plugins}->readconfig($config, self => $self);
SUBCLASSING
If you don't want to use an existing configuration file format, you
don't have to.
Subclass Plugins and override a couple of methods to change the
behavior.
In addition to the methods defined above in the "USING PLUGINS" and
in "CONSTRUCTION AND INITIALIAZATION" that probably shoudn't be
changed, Plugins defines the following methods:
parseconfig($configfile, %args)
This is the method that actually parses a config file. This must
be overridden as the default just calls "die()".
The parameters come from either a direct call from the
*requestor* or are passed through unchanged from "readconfig()".
post_initialize($context, $plugin)
Called by "initialize_plugin()" after new'ing up an instance.
The default behavior is to do nothing. The $context is the same
context that is passwed to "new()" and "pkg_invoke()" and
"addplugin()".
genkey($context)
This is called to generate a unique identifier for each plugin
instance. Duplicate identifiers will result in a "die()". The
default "genkey()" combines the configuration file name with the
plugin package name and thus does not allow multiple instances
of the same package.
registerplugin(%context)
This add a plugin to a configuration. This is used after
"startconfig()" and before "initialize()". This will "require"
the plugin unless the symbol table for the plugin already
exists. This method should not be overridden.
addplugin(%context)
This is used to bypass the entire "startconfig()",
"registerplugin()", "invoke()" process. The plugin will be
registered and initialized all at once. This can be used after
"initialize()" to add additional plugins. This will "require"
the plugin unless the symbol table for the plugin already
exists. Duplicate registrations of the same plugin will replace
the old plugin ("shutdown()" will be called). This method should
not be overridden.
The %context is the same as for "%registerplugin()"
pkg_invoke($pkg, [$method, @args])
This method will "require" a plugin and (optionally) call a
class method. This method should not be overridden.
initialize_plugin($self, $context)
This is used by "addplugin()" and "initialize()" to create
plugin instances. It calls "new()" and then "post_initialize()".
This method should not be overridden.
%context for addplugin() and registerplugin()
Most of the %context has that is given to "addplugin()" and
"registerplugin()" should (in theory) be passed as $context to the
plugin's "new($context, %args)" and then as "context => $context" to
"Plugins::new()" if the is a *child requestor*.
The part that is not passed to "Plugins::new()" is the part that
defines the *child* plugin:
pkg => 'Some::Plugin::Module'
The perl package name for the plugin.
new_args => [@args]
The @args will be passed to the the plugin's "new()":
new $pkg (\%context, @args)
requestor => 'The::Parent::Module'
The perl package name of the *parent* module. This will be found
with "caller()" if it isn't supplied.
configfile => "/some/file"
Which configuration file referenced the plugin and caused it to
be loaded.
lineno => 38
The of where the plugin was registered in $configfile. Used for
error messages.
The rest will be passed to *child* "Plugins::new()" by way of
$context:
pkg_override => "Plugins::Subclass"
Plugins has been subclassed, this the name of the subclass.
Subclassing should usually be inherited by *child requestors*
and this is the mechenism that makes it happen.
Re-parsing the same configuration file
The tricky part of subclassing Plugins is that if a configruation
file is shared between *root requestors* and *child requestors* the
"parseconfig()" method will be invoked on the same file more than
once.
Store extra things in %context to work around this issue.
SEE ALSO
Plugins::Style1 Plugins::API
THANK THE AUTHOR
If you find this module useful and wish to show your appreciation to
the author, please give me a Request-For-Quote on your next
high-speed internet pipe order. I have good pricing for T1s, T3s,
OC3s etc.
LICENSE
Copyright (C) 2006, David Muir Sharnoff <
[email protected]>. This
module may be used and redistributed on the same terms as Perl
itself.
