请问怎么返回socket.onerror的异常到connnect.tsx

[FickXu]

依赖库中监听了socket的onerror，在connect.tsx中我不知道怎么去获取返回的异常信息。我试着在依赖库返回一个promise，但在connect.tsx中它的状态为已完成，连接中断的时候（服务器主动断开）无法进入到catch中，请问这种情况该怎么捕获异常？

[yume-chan]

Can't understand what you want to do.

When the WebSocket connection is established, the Promise will be resolved. A settled Promise can't switch to other states anymore.

If server closes the WebSocket connection first, only close event will fire, and the listener for that event will close both the readable and writable streams. connect.tsx knows this because the pipe chain will stop and device.disconnected will be resolved.

[FickXu]

ws在onopen事件触发后直接resolved了，状态不会发生改变。之后服务器主动关闭了ws导致触发了onerror事件和onclose事件，最初我想在依赖包中的onerror中来处理异常信息返回，所以有了上面的疑问。
根据你的解释，我在ws连接后，判断了readable的locked字段的值，如果为false则表示可读流被关闭，从而得知ws已被关闭。请问这个字段可以作为判断依据吗？
这是我的部分代码：

[yume-chan]

No, the locked field indicates whether the stream currently has an active reader (has getReader or pipeTo called on it). When connect() returns, the stream is always available.

Your onclose event happens asynchronously, so you also need some asynchronous method to detect it (like a Promise or an event). This project generally doesn't use events (because it's easy to be missed, if the listener is attached later than the event is fired). Instead, streams will block if no reader is active, and Promise will latch on the state so any time then() is called, the same value is returned.

In this situation, connect() returns a duplex stream (a pair of readable stream and writable stream). Readable stream will only tell its active reader when it ends, so you can use a TransformStream to "intercept" this information, like this:

const streams = await backend.connect();
const readable = streams.readable.pipeThrough(new TransformStream({
    flush() {
        console.log('Ended');
    }
}));

Then pass this readable to Adb.authenticate() instead.

---

As said above, the only way to know when a readable stream end is to acquire this reader. In normal data flow, the ADB client will do that, and it has a disconnected field, which is a Promise to signal connection end. The promise will be resolved when the connection end gracefully and rejected if any error was thrown in the whole lifetime.

connect.tsx already has some code to handle that

ya-webadb/apps/demo/src/components/connect.tsx

Lines 188 to 193 in e779f80

	device.disconnected.then(async () => {
	await dispose();
	}, async (e) => {
	GlobalState.showErrorDialog(e);
	await dispose();
	});

So, your code can also be added here to handle disconnections.

---

However, with any of the above methods, you can only get when the steam ends, but not the error, simply due to WebSocket backend never forwards errors to outside.

This can be done by erroring out the readable stream when error happens, for example by adding this

socket.onerror = () => {
    controller.error(new Error('WebSocket error')); // or a custom error to wrap error code/message from WS
};

after here:

ya-webadb/libraries/adb-backend-ws/src/index.ts

Lines 36 to 39 in 6887d85

	start: (controller) => {
	socket.onmessage = ({ data }: { data: ArrayBuffer; }) => {
	controller.enqueue(new Uint8Array(data));
	};

After that, the disconnected promise will become rejected instead of resolved, and the error is what you thrown in backend.

[FickXu]

谢谢你耐心解答，我大致明白了连接的逻辑。
虽然在socket.onerror中我没有弄明白controller的流是如何产生并返回到页面。但根据你的解释我在ReadableWritablePair接口中新增了一个Promise类型的类WSError用来捕获socket.onerror中发生的错误，当它reject时，页面connect.tsx可以通过streams.wserror.catch获取这个异常信息。现在问题已经解决了，谢谢你。