# WebSockets

> H3 内置跨平台 WebSocket 和服务器发送事件的实用工具。

您可以使用 [🔌 CrossWS](https://crossws.h3.dev/) 为 H3 服务器添加跨平台 WebSocket 支持。

<important>

h3 版本中内置的 WebSockets 支持仍在开发中。

</important>

## 用法

WebSocket 处理程序可以使用 `defineWebSocketHandler()` 工具定义，并像事件处理程序一样注册到任意路由。

您需要在 `serve` 函数中将 CrossWS 注册为服务器插件，并提供一个 `resolve` 函数以从路由中解析正确的钩子。

```js
import { H3, serve, defineWebSocketHandler } from "h3";

import { plugin as ws } from "crossws/server";

const app = new H3();

app.get("/_ws", defineWebSocketHandler({ message: console.log }));

serve(app, {
  plugins: [ws({ resolve: async (req) => (await app.fetch(req)).crossws })],
});
```

**完整示例：**

```js [websocket.mjs]
import { H3, serve, defineWebSocketHandler } from "h3";
import { plugin as ws } from "crossws/server";

export const app = new H3();

const demoURL =
  "https://raw.githubusercontent.com/h3js/crossws/refs/heads/main/playground/public/index.html";

app.get("/", () =>
  fetch(demoURL).then(
    (res) =>
      new Response(res.body, { headers: { "Content-Type": "text/html" } }),
  ),
);

app.get(
  "/_ws",
  defineWebSocketHandler({
    // upgrade(req) {},
    open(peer) {
      console.log("[open]", peer);

      // Send welcome to the new client
      peer.send("Welcome to the server!");

      // Join new client to the "chat" channel
      peer.subscribe("chat");

      // Notify every other connected client
      peer.publish("chat", `[system] ${peer} joined!`);
    },

    message(peer, message) {
      console.log("[message]", peer);

      if (message.text() === "ping") {
        // Reply to the client with a ping response
        peer.send("pong");
        return;
      }

      // The server re-broadcasts incoming messages to everyone
      peer.publish("chat", `[${peer}] ${message}`);

      // Echo the message back to the sender
      peer.send(message);
    },

    close(peer) {
      console.log("[close]", peer);
      peer.publish("chat", `[system] ${peer} has left the chat!`);
      peer.unsubscribe("chat");
    },
  }),
);

serve(app, {
  plugins: [ws({ resolve: async (req) => (await app.fetch(req)).crossws })],
});
```

## 服务器发送事件（SSE）

作为 WebSockets 的替代方案，您可以使用[服务器发送事件](https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events)。

H3 提供内置 API，可使用 `createEventStream(event)` 工具创建服务器发送事件。

### 示例

```js [server-sent-events.mjs]
import { H3, serve, createEventStream } from "h3";

export const app = new H3();

app.get("/", (event) => {
  const eventStream = createEventStream(event);

  // Send a message every second
  const interval = setInterval(async () => {
    await eventStream.push("Hello world");
  }, 1000);

  // cleanup the interval when the connection is terminated or the writer is closed
  eventStream.onClosed(() => {
    console.log("Connection closed");
    clearInterval(interval);
  });

  return eventStream.send();
});

serve(app);
```