React + VSCode Webview = IPC

A small library to make two-way communication between a VS Code extension host and a React webview simple and type‑safe.

Two complementary paradigms are supported (you can use one or both):

UDF reducer IPC: Dispatch actions from the webview; the host computes a patch; the webview applies it via a reducer. (unidirectional dataflow)
RPC promises IPC: Call host functions from the webview and await typed results; the host can also push typed events to all connected webviews.

This README explains how to implement both and shows how they can coexist.

Install

Install the package in your VS Code extension project.

npm i react-vscode-webview-ipc

You’ll consume two entry points:
- react-vscode-webview-ipc/host for your extension host code
- react-vscode-webview-ipc/client for your React webview code

Concepts Overview

WebviewKey: a branded string identifying your view instance. Use a stable id (often your view type).
Messages:
- Reducer IPC: { type: 'act' } messages from webview → host; { type: 'patch' } messages from host → webview.
- RPC IPC: { type: 'request' } from webview → host; { type: 'response'|'error' } from host → webview; { type: 'event' } from host → webview broadcast.
Logging: webview logs are forwarded to the host’s Output channel.

UDF Reducer IPC (Action → Patch → Reduce)

Use this when your webview wants unidirectional state updates managed via a reducer.

Types and Building Blocks

On the webview:
- useVscodeState<S, A>(vscode, providerId, postReducer, initialState) returns [state, actor].
  - state: S – your current state
  - actor: A – a proxy with methods matching your action interface
- postReducer: StateReducer<S, A> maps each action key to (prevState, patch) => newState.
On the host:
- Extend BaseWebviewViewProvider<A> and implement:
  - webviewActionDelegate: ActionDelegate<A> – map action keys to host handlers that return a patch (sync or async)
  - generateWebviewHtml(webview, extensionUri) – return the webview HTML
  - handleMessage(message, webview) – handle any messages you want besides reducer IPC (e.g., your RPC requests)
- Optionally pass a WebviewApiProvider instance to the base class constructor to enable host→webview event broadcasting (RPC paradigm).

Minimal Example

Host (extension):

// src/extension/MyViewProvider.ts
import * as vscode from 'vscode';
import { BaseWebviewViewProvider, type ActionDelegate } from 'react-vscode-webview-ipc/host';
import type { WebviewKey } from 'react-vscode-webview-ipc/client';

// 1) Define the action interface A: methods return the “patch” type
interface MyActions {
  increment: (by: number) => number; // patch is a number
  setMessage: (msg: string) => { message: string }; // patch is an object
}

export class MyViewProvider extends BaseWebviewViewProvider<MyActions> {
  protected readonly webviewActionDelegate: ActionDelegate<MyActions> = {
    increment: (by) => by, // just echo back the increment amount as the patch
    setMessage: (msg) => ({ message: msg }),
  };

  constructor(
    private readonly id: WebviewKey,
    private readonly ctx: vscode.ExtensionContext
  ) {
    // You can also pass a WebviewApiProvider instance as 3rd arg to enable events
    super(id, ctx.extensionUri);
  }

  protected generateWebviewHtml(webview: vscode.Webview): string {
    const scriptUri = webview.asWebviewUri(
      vscode.Uri.joinPath(this.ctx.extensionUri, 'dist', 'webview.js')
    );
    const styleUri = webview.asWebviewUri(
      vscode.Uri.joinPath(this.ctx.extensionUri, 'dist', 'webview.css')
    );

    return `<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="Content-Security-Policy"
          content="default-src 'none'; img-src ${webview.cspSource} https:; style-src ${webview.cspSource} 'unsafe-inline'; script-src ${webview.cspSource};" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <link rel="stylesheet" href="${styleUri}">
    <title>My View</title>
  </head>
  <body>
    <div id="root"></div>
    <script src="${scriptUri}"></script>
  </body>
</html>`;
  }

  protected async handleMessage(_message: unknown, _webview: vscode.Webview): Promise<void> {
    // No-op for reducer-only example. Use this for RPC too (see later).
  }
}

Register the provider in your extension activation:

// src/extension/activate.ts
import * as vscode from 'vscode';
import { MyViewProvider } from './MyViewProvider';

export function activate(context: vscode.ExtensionContext) {
  const viewType = 'myExtension.myView' as unknown as WebviewKey; // brand to WebviewKey
  const provider = new MyViewProvider(viewType, context);
  context.subscriptions.push(vscode.window.registerWebviewViewProvider(viewType, provider));
}

Webview (React):

// src/webview/App.tsx
import { useMemo } from 'react';
import {
  useVscodeState,
  type StateReducer,
  type WebviewKey,
} from 'react-vscode-webview-ipc/client';

declare function acquireVsCodeApi(): {
  postMessage(message: unknown): Thenable<boolean>;
  getState(): unknown;
  setState(state: unknown): void;
};

interface State {
  count: number;
  message: string;
}
interface MyActions {
  increment: (by: number) => number;
  setMessage: (msg: string) => { message: string };
}

const initial: State = { count: 0, message: '' };

const reducers: StateReducer<State, MyActions> = {
  increment: (prev, by) => ({ ...prev, count: prev.count + by }),
  setMessage: (prev, patch) => ({ ...prev, message: patch.message }),
};

export default function App() {
  const vscode = useMemo(() => acquireVsCodeApi(), []);
  const providerId = 'myExtension.myView' as unknown as WebviewKey;
  const [state, act] = useVscodeState(vscode, providerId, reducers, initial);

  return (
    <div>
      <p>Count: {state.count}</p>
      <p>Message: {state.message}</p>
      <button onClick={() => act.increment(1)}>+1</button>
      <button onClick={() => act.setMessage('Hello!')}>Set Message</button>
    </div>
  );
}

UDF Flow (Sequence)

sequenceDiagram
  participant W as Webview (React)
  participant VS as VS Code API
  participant H as Extension Host
  W->>VS: postMessage { type: 'act', providerId, key, params }
  VS->>H: onDidReceiveMessage(message)
  H->>H: webviewActionDelegate[key](...params) => patch
  H-->>VS: postMessage { type: 'patch', providerId, key, patch }
  VS-->>W: window message
  W->>W: postReducer[key](prev, patch) => newState

RPC Promises IPC (Typed Requests/Responses + Events)

Use this when your webview needs to call host functions and await results. The host can also broadcast typed events back to all connected webviews.

Types and Building Blocks

On the webview:
- Wrap your app in <WebviewProvider viewType contextKey>.
- Use createCtxKey<T>() to create a unique key tying the context to your API type T.
- Call const { api, addListener, removeListener, vscode } = useWebviewApi(ctxKey) inside components.
  - api.method(...) returns a promise (typed from your ClientCalls interface).
  - addListener('eventKey', cb) / removeListener(...) manage host-pushed events.
On the host:
- Create a WebviewApiProvider<HostEvents>() and pass it to your BaseWebviewViewProvider constructor (to register views for events).
- In your provider’s handleMessage, detect requests via isViewApiRequest(message), dispatch to your host API handlers, and respond with { type: 'response'|'error', id, value }.
- Use apiProvider.triggerEvent('eventKey', ...args) to broadcast events to connected webviews.

Minimal Example

Shared types:

// Host receives these requests from the webview (must return promises)
import type { ClientCalls } from 'react-vscode-webview-ipc/client';
import type { HostCalls } from 'react-vscode-webview-ipc/client';

export interface MyClientApi extends ClientCalls {
  fetchGreeting: (name: string) => Promise<string>;
  saveCount: (count: number) => Promise<void>;
}

// Host can push these events to all webviews
export interface MyHostEvents extends HostCalls {
  onTick: (count: number) => void;
}

Host (extension):

import * as vscode from 'vscode';
import {
  BaseWebviewViewProvider,
  WebviewApiProvider,
  isViewApiRequest,
  type ViewApiResponse,
  type ViewApiError,
} from 'react-vscode-webview-ipc/host';
import type { MyClientApi, MyHostEvents } from './types';
import type { WebviewKey } from 'react-vscode-webview-ipc/client';

export class MyRpcViewProvider extends BaseWebviewViewProvider<{}> {
  protected readonly webviewActionDelegate = {}; // not using reducer actions here

  constructor(
    private readonly id: WebviewKey,
    private readonly ctx: vscode.ExtensionContext,
    private readonly api = new WebviewApiProvider<MyHostEvents>()
  ) {
    super(id, ctx.extensionUri, api);
  }

  protected generateWebviewHtml(webview: vscode.Webview): string {
    const scriptUri = webview.asWebviewUri(
      vscode.Uri.joinPath(this.ctx.extensionUri, 'dist', 'webview.js')
    );
    return `<div id="root"></div><script src="${scriptUri}"></script>`;
  }

  protected async handleMessage(message: unknown, webview: vscode.Webview): Promise<void> {
    if (isViewApiRequest<MyClientApi>(message)) {
      try {
        switch (message.key) {
          case 'fetchGreeting': {
            const [name] = message.params;
            const value = `Hello, ${name}!`;
            const response: ViewApiResponse<MyClientApi, 'fetchGreeting'> = {
              type: 'response',
              id: message.id,
              value,
            };
            await webview.postMessage(response);
            return;
          }
          case 'saveCount': {
            const [count] = message.params;
            // persist count...
            const response: ViewApiResponse<MyClientApi, 'saveCount'> = {
              type: 'response',
              id: message.id,
            };
            await webview.postMessage(response);
            return;
          }
        }
        const error: ViewApiError = {
          type: 'error',
          id: message.id,
          value: `Unknown method: ${String(message.key)}`,
        };
        await webview.postMessage(error);
      } catch (e) {
        const error: ViewApiError = {
          type: 'error',
          id: message.id,
          value: e instanceof Error ? e.message : String(e),
        };
        await webview.postMessage(error);
      }
    }
  }
}

// elsewhere in your extension, you can broadcast events
// provider.api.triggerEvent('onTick', currentCount)

Webview (React):

import React, { useEffect } from 'react';
import {
  WebviewProvider,
  useWebviewApi,
  createCtxKey,
  type CtxKey,
} from 'react-vscode-webview-ipc/client';
import type { MyClientApi, MyHostEvents } from './types';

const Ctx: CtxKey<MyClientApi> = createCtxKey<MyClientApi>('my-rpc');

function Inner() {
  const { api, addListener, removeListener } = useWebviewApi(Ctx);

  useEffect(() => {
    const onTick = (n: number) => console.log('tick', n);
    addListener('onTick', onTick as MyHostEvents['onTick']);
    return () => removeListener('onTick', onTick as MyHostEvents['onTick']);
  }, [addListener, removeListener]);

  useEffect(() => {
    (async () => {
      const greeting = await api.fetchGreeting('VS Code');
      console.log(greeting);
      await api.saveCount(42);
    })();
  }, [api]);

  return <div />;
}

export default function App() {
  return (
    <WebviewProvider<MyClientApi> viewType="myExtension.myView" contextKey={Ctx}>
      <Inner />
    </WebviewProvider>
  );
}

RPC Flow (Sequence)

sequenceDiagram
  participant W as Webview (React)
  participant VS as VS Code API
  participant H as Extension Host
  W->>VS: postMessage { type: 'request', id, key, params, context }
  VS->>H: onDidReceiveMessage(message)
  alt success
    H-->>VS: postMessage { type: 'response', id, value }
    VS-->>W: resolve pending promise
  else error
    H-->>VS: postMessage { type: 'error', id, value }
    VS-->>W: reject pending promise
  end
  H-->>VS: postMessage { type: 'event', key, value[] }
  VS-->>W: Invoke registered listeners for key

Using Both Paradigms Together

They are designed to coexist. The webview can dispatch reducer actions for state, and call RPC methods for imperative operations.
The library ensures messages don’t conflict:
- useVscodeState listens for { providerId, type: 'patch' } messages.
- WebviewProvider listens for { type: 'response'|'error'|'event' } messages and ignores messages with providerId present.
In your host provider, resolveWebviewView (from the base class) handles reducer act/patch automatically; implement handleMessage for RPC requests.

Logging

Webview: useLogger(tag, vscode) returns a logger that posts to the host output channel.
Host: getLogger(tag) returns an Output channel logger; BaseWebviewViewProvider automatically routes webview log messages to it.

Webview example:

import { useLogger } from 'react-vscode-webview-ipc/client';
const logger = useLogger('MyView', acquireVsCodeApi());
logger.info('hello');

Tips & Troubleshooting

Brand your view type to WebviewKey at the edges to keep types happy: const id = 'ext.view' as unknown as WebviewKey.
Always use stable providerIds; the reducer IPC ties messages to a specific provider.
Clean up listeners on unmount in the webview.
When posting RPC responses/errors from the host, always echo the same id you received.
If you use both paradigms, keep your reducer patches focused on state updates and use RPC for IO or long‑running tasks.

API Surface (Quick Reference)

Host exports (react-vscode-webview-ipc/host):

BaseWebviewViewProvider<A>
WebviewApiProvider<T extends HostCalls>
isViewApiRequest(message)
Logger, getLogger, disallowedLogKeys

Client exports (react-vscode-webview-ipc/client):

WebviewProvider<T extends ClientCalls>
useWebviewApi(ctxKey) and createCtxKey<T>()
useVscodeState<S, A>(vscode, providerId, postReducer, initial)
useLogger(tag, vscode)
Types: ClientCalls, HostCalls, CtxKey, WebviewKey, StateReducer

References

Legal

Apache 2.0 License
Visual Studio Code, VS Code, and the Visual Studio Code icon are trademarks of Microsoft Corporation.

Name		Name	Last commit message	Last commit date
Latest commit History 28 Commits
.claude		.claude
.github/workflows		.github/workflows
.vscode		.vscode
src		src
tests		tests
.gitignore		.gitignore
.prettierignore		.prettierignore
.prettierrc		.prettierrc
CODE_OF_CONDUCT.md		CODE_OF_CONDUCT.md
LICENSE.txt		LICENSE.txt
README.md		README.md
eslint.config.js		eslint.config.js
package-lock.json		package-lock.json
package.json		package.json
tsconfig.app.json		tsconfig.app.json
tsconfig.json		tsconfig.json
tsconfig.node.json		tsconfig.node.json
tsconfig.test.json		tsconfig.test.json
vite.config.ts		vite.config.ts
vitest.config.ts		vitest.config.ts

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React + VSCode Webview = IPC

Install

Concepts Overview

UDF Reducer IPC (Action → Patch → Reduce)

Types and Building Blocks

Minimal Example

UDF Flow (Sequence)

RPC Promises IPC (Typed Requests/Responses + Events)

Types and Building Blocks

Minimal Example

RPC Flow (Sequence)

Using Both Paradigms Together

Logging

Tips & Troubleshooting

API Surface (Quick Reference)

References

Legal

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React + VSCode Webview = IPC

Install

Concepts Overview

UDF Reducer IPC (Action → Patch → Reduce)

Types and Building Blocks

Minimal Example

UDF Flow (Sequence)

RPC Promises IPC (Typed Requests/Responses + Events)

Types and Building Blocks

Minimal Example

RPC Flow (Sequence)

Using Both Paradigms Together

Logging

Tips & Troubleshooting

API Surface (Quick Reference)

References

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