js-plugins

An extension-point based open plugin framework

Open Plugin Framework for Node.js

This module is an extension-point based framework for loading plugins, inspired by Eclipse plugin system.

The scenario can explain why this module is developed:

Developer A made an excellent text editor which can be installed using

npm install xedit -g

Developer B is a language expert and he invents a new language. He loves xedit so much that he wants to make a plugin for it. A few days later, he published a module called xedit-lang-x.

User simply use

npm install xedit -g
npm install xedit-lang--g

than, xedit automatically recognized language x.

How it works

In the scenario above, xedit scans and loads plugins. In this way, xedit is called host in this framework. And xedit-lang-x is a plugin. A host will require js-plugins to manage plugins, while a plugin doesn't require js-plugins but need a special section in package.json to describe what the plugin provides.

Here's an example what xedit-lang-x written in the package.json:

{
    "name": "xedit-lang-x",
    ...
    "extensions": {
        "xedit:language": {
            "x": "./lib/xedit-lang-plugin"
        }
    }
}

In the example above, section extensions defines what extensions are provided by this module. The key of the hash is the extension-point name, and the value can be a path to the actual script file or a name representing an attribute from the object provided by index.js.

When xedit starts, it uses js-plugins to scan all installed modules and find out all available extensions from package.json files. When it is going to load a language plugin, it uses extension-point name xedit:language to load all matched extensions.

By default, js-plugins scans the following locations in sequence (latter ones may override former ones):

  • node_prefix/lib/node_modules/*/package.json
  • $HOME/.node_libraries/*/package.json
  • $HOME/.node_modules/*/package.json
  • require.main.paths/*/package.json
  • require.main.paths[0]/package.json

Usage

js-plugins provides a simple class Plugins:

var Plugins = require('js-plugins');
var pluginManager = new Plugins();

From pluginManager, we can

pluginManager.register(extensionPoint, name, factory);

to register a plugin mapped to extensionPoint/name. The factory is defined in two forms distinguished by the number of arguments:

function factory(datahostoptionscallback);    // the async version, the extension is created asynchronously 
                                                    // callback is defined as function callback(err, extension) 
function factory(datahostoptions);              // the sync version, the extension is returned directly 

Factories with argument number less than 3 are treated as sync version. The parameters are defined as:

  • data: opaque data which is recognized by host and plugin
  • host: instance of host
  • options: additional information, currently defined
    • extensionPoint: name of extension-point
    • name: name of extension
  • callback: for receiving the extension, in the form of function (err, extension)
pluginManager.connect(host, extensionPoint, options, callback)

The parameters are defined as:

  • host: the host instance
  • extensionPoint: name of extension-point
  • options: a hash defined as
    • data: passed to extension factory as the first argument
    • multi: when true, connect all available extensions registered to the same extension-point, when false, the first extension is connected
    • name: when is a string, explicitly specify the name of the extension, when is an array, names are iterated until an extension is connected
    • required: when true, error is thrown if no extensions found
  • callback: defined as function (err, extensions), it is possible extenions is a single instance or an array when multi is true.
pluginManager.scanSubdirs(dirs)

dirs is an array of directories, under each the first level sub-directories are scanned for package.json.

pluginManager.scan()

It scans all default directories in the sequence described above. The latter modules may override the former ones if the same extension-point + name is registered.

Plugins provides a global instance which can be accessed using Plugins.instance. All methods are promoted under Plugins directly. E.g. Plugins.scan is equivalent to Plugins.instance.scan. The recommended practise is to use the global instance as much as possible, and invoke Plugins.scan once at application start-up.

js-plugins is not needed. The only thing to do is presenting extensions section in package.json.

The extensions section is a hash with each key as an extension-point, and the value is also another hash with each key as extension name. The value can be in several form:

  • hash: this is the complete form, it defines 2 attributes:
    • module: if present, specify the path of script file which will be loaded using require, otherwise the top directory (containing package.json) is required;
    • object: when present, it means the required module doesn't provide a factory directory, but must use the value of object to get the factory
  • string without '/': equals to hash form without module but object
  • string with '/': equals to hash form with module but no object

See above to find out how to define a factory.

MIT/X11 License