Bespin Plugin Guide

Introduction

Bespin is built around a small core with most additional functionality placed in plugins. The driving goals behind plugins are:

The ability to load functionality lazily is very important, because we expect that Bespin will have a large number of features, only some of which any given person is likely to use at one time. The simplest example of this is syntax highlighters: maybe you write JavaScript most of the time, but someone else is developing Ruby code. If you only use JavaScript, that's the only syntax highlighter that will get loaded into your browser. And there are many more examples possible.

The Basics

A plugin consists of metadata and everything else. When you log into Bespin, the metadata for all of your plugins is loaded. Everything else (JavaScript, CSS, images, etc.) is loaded as needed. At its simplest, a plugin is a single .js file that contains a special section for the metadata. Here's a trivial example:

"define metadata";
({});
"end";

exports.someFunction = function() { };

Clearly, this example doesn't do anything useful. The first step in making a plugin do something is to define the appropriate metadata.

Bespin plugins feature extensions that plug into extension points. As an example of this, the simple syntax highlighter defines an extension point called "syntax.simple.highlighter". The JavaScript highlighter is one extension that plugs into that extension point. The metadata for the JavaScript highlighter looks like this:

{
    "provides": [
        {
            "ep": "syntax.simple.highlighter",
            "name": "JavaScript",
            "extensions": ["js", "json", "javascript", 
                "ecmascript", "jsm", "java"],
            "pointer": "#JavaScript"
        }
    ],
    "depends": ["SimpleSyntax"]
}

The metadata is an object specified in JSON format. provides is a list of extensions that are provided by this plugin. Each one of the extensions is given as an object with one attribute that is always there: "ep". ep is the extension point that the extension is for. In the example above, you can see that the plugin is providing an extension for the "syntax.simple.highlighter" extension point. The other metadata for the extension is specific to the extension point.

pointer is a common piece of metadata. Since this is pure JSON, and we want to lazily load the code anyhow, a pointer is a string that tells Bespin where to find the object that the extended code is going to need to perform the necessary work (in this case, the JavaScript syntax highlighting). A pointer is given in this format: plugin:path/to/module/in/plugin#memberInModule. The example above is about as simple as it gets. It's pointing to an object called JavaScript in the "package module" of the plugin (see more about the package module below). The name and extensions metadata are specific to the syntax.simple.highlighter extension point.

The depends list in the metadata is a list of the names of plugins upon which this plugin depends. Bespin will ensure that those plugins are loaded before this one. In the example above, the JavaScript highlighter plugin depends on the SimpleSyntax engine, which is itself a plugin.

Plugin File Structure

A plugin can be a single .js file with it's metadata bordered by directives that are contained in strings. Here's an example:

"define metadata";
({});
"end";

You must start the metadata with "define metadata"; and end it with "end";. The metadata itself is contained in an object. JavaScript does not allow you to put a bare object in a program, so you can either enclose the object in parentheses (as above) or set it to a variable. Bespin can handle either of those formats.

Often, a single file is not going to be enough. When you move beyond a single file, a plugin is defined as a directory with a plugin.json file in it. In a single file plugin, the "package module" is the plugin .js file. In a plugin directory, the "package module" is a file called "index.js".

Plugin Reloading

A key feature of the Bespin user experience is the ability to edit plugins from within Bespin and have the changes take effect immediately. Many types of Bespin plugins, such as syntax highlighters, don't have any UI which makes them easy to reload. If you write a plugin that has UI or any other resources that need to be cleaned up, you can point Bespin at a function to run before the plugin is to be reloaded. Here's an example:

"define metadata";
({
    "reloadPointer": "#cleanup"
});
"end";

exports.cleanup = function() {
    // remove the UI from the DOM, parent view, etc.
};

How To Learn More

We will be expanding on the plugin development docs over time. In the meantime, it's worth noting that all of Bespin's major functionality is implemented as plugins. In a checkout of the bespinclient repository, take a look at the plugins/supported directory for the bulk of Bespin's code.