Get notification on the front end when the websocket disconnects

[0x26res]

Feature Request

Description of Problem:

I use perspective for real time dashboard / monitoring, with a python server inspired by the python-tornado-streaming example.

If for whatever reason the browser disconnect from the server, I want to get a notification so I can let the user know that it's disconnected and the information in the dashboard is no longer up to date.

Prior to version 3, I used to do it this way:

                const websocket = perspective.websocket(websocket_uri);
                websocket._ws.onclose = function () {
                    viewer.style.display = "none"
                }

But since version 3, the _ws is no longer available.

Potential Solutions:

Here are some ideas:

• expose the websocket in the perspective.websocket object, so I can register an onclose callback
• add an option to clear the grid (or grey out) on disconnect (and on top try to automatically reconnect)

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[0x26res]

As a workaround, I've created a second websocket that connects to the same server and is only used to detect disconnect.

const viewer = document.getElementById("viewer");
const websocket = await perspective.websocket(websocket_uri);
const ws = new WebSocket(websocket_uri);
ws.onclose = function () {
    viewer.style.display = "none"
}

But it'd be better if we could either expose the underlying websocket in the client returned by perspective.websocket, or if we were able to register a callback for disconnection when calling perspective.websocket