Our Approach to Extensibility

VS Code has a very rich extensibility model and there are many ways to extend the tool. However, we do not provide direct access to the underlying UI DOM to extension writers. With VS Code, we’re continually trying to optimize use of the underlying web technologies to deliver an always available, highly responsive editor and we will continue to tune our use of the DOM as these technologies and our product evolve. To maintain performance and compatibility, we run extensions in their own host process and prevent direct access to the DOM. VS Code also includes a built-in set of UI components for common scenarios such as IntelliSense, so that these experiences are consistent across programming languages and extensions and extension developers do not need to build their own.

We realize that this approach may initially feel restrictive to extension developers. We’re always looking for ways to improve our extensibility model and expand the capabilities available to extensions. We look forward to hearing your feedback and ideas.

Core Concepts

When we set out to add extensibility to VS Code, we had a number of considerations in mind. The following sections provide some context as to a number of our core decisions. We also have a document that outlines many of the core patterns we have adopted within our APIs.

Stability - Extension Isolation

Extensions are wonderful but extensions can also affect startup performance or the overall stability of VS Code itself. To avoid these problems, VS Code loads and runs extensions in a separate process, the extension host process. A misbehaving extension cannot impact VS Code and in particular its startup time.

We have built this architecture with the end-user in mind, as this architecture allows us to ensure that the end-user is always in control of VS Code: the user can open, type or save files at any time, VS Code ensures a responsive UI irrespective of what extensions are doing.

The extension host is a Node.js process and it exposes the VS Code API to extension writers. VS Code provides debugging support for extensions running inside the extension host.

Performance - Extension Activation

VS Code loads extensions as late as possible and extensions that are not used during a session are not loaded and therefore do not consume memory. To help support lazy loading of extensions, VS Code defines so-called activation events. An activation event is fired by VS Code based on specific activities and an extension can define for which events it needs to be activated. For example, an extension for editing Markdown only needs to be activated when the user opens a Markdown file.

Extension Manifest

To activate an extension lazily, VS Code requires a description of your extension, the extension manifest which is a package.json file enriched with some additional VS Code specific fields. This includes the activation events that trigger the loading of the extension. VS Code provides a set of contribution points that an extension can add to. For example, when adding a command to VS Code, you provide the command definition through the commands contribution point. You define the contributions of your extension in the package.json. VS Code reads and interprets the manifest during start-up and prepares its UI accordingly.

As the extension host is a Node.js process, you can use the Node API in your extensions and even better you can reuse existing Node.js modules when implementing an extension. You define your module dependencies inside the package.json and you use npm to install a Node.js module.

See the package.json contribution points reference for more details.

Extensibility API

The approach to run the extensions isolated in a separate process allows VS Code to strictly control the API exposed to extenders. Initially VS Code provides a small API surface. In the future, we will expand the API based on requests from extenders. See the Extensibility API Overview for details on the current API.

VS Code is implemented using web technologies (HTML, CSS) and web technologies are very powerful when it comes to modifying and styling UI. You can easily add nodes to the DOM and implement a custom appearance using CSS. However, this power is not without its problems when it comes to evolving a complex application like VS Code. The structure can change and extensions that are tightly coupled to the UI would break. For this reason, VS Code took the defensive approach to not expose the DOM to extenders.

Protocol based extensions

A common extension pattern in VS Code is to execute extension code in a separate process that communicates with VS Code through a protocol. Examples of this in VS Code are the language servers and debug adapters. Typically this protocol uses stdin/stdout to communicate between the processes using a JSON payload. Using separate processes provides good isolation boundaries which helps VS Code preserve the stability of the core editor. In addition, this allows extenders to pick the programming language that is most appropriate for the particular extension implementation.

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