如何从Websocket连接中挑选特定的服务器消息？

Question

I have a WebSocket connection set up for a basic web chat server.

Now, whenever a message is received, it is sent to a function which outputs it on the screen.

socket.onmessage = function(msg){output(msg);}

However, there are certain technical commands which the user can send to the server through the connection which elicit a technical response, not meant to be output on the screen.

How can I grab the server response which immediately follows one of these technical messages?

Do I put a separate socket.onmessage right after the block of code which sends the technical message? I imagine that would take all future messages. I just want the next one .

Ideas?

Answer 1

WebSockets is asynchronous so trying to get the 'next' message received is not the right solution. There can be multiple message in flight in both directions at the same time. Also, most actions in Javascript are triggered by asynchronous events (timeout firing, or user clicking on something) which means you don't have synchronous control over when sends will happen. Also, the onmessage handler is a persistent setting: once it is set it receives all messages until it is unset. You need to have some sort of way of distinguishing control messages from data messages in the message it self. And if you need to correlate responses with sent messages then you will also need some kind of message sequence number (or other form of unique message ID).

For example, this sends a control message to the server and has a message handler which can distinguish between control and message and other messages:

var nextSeqNum = 0;
...
msg = {id: nextSeqNum, mtype: "control", data: "some data"};
waitForMsg = nextSeqNum;
nextSeqNum += 1;
ws.send(JSON.stringify(msg));
...
ws.onmessage = function (e) {
    msg = JSON.parse(e.data);
    if (msg.mtype === "control") {
        if (msg.id === waitForMsg) {
            // We got a response to our message
        } else {
            // We got an async control message from the server
        }
    } else {
        output(msg.data);
    }
};

Answer 2

You can packetise the data, I mean, by a special character/s, form a string like this :

"DataNotToBeShown"+"$$"+"DataToBeShown"; //if $$ is separating character

And then, you can split the string in javascript like this :

var recv=msg.data.split('$$');

So, the data not be shown is in recv[0] and data to be shown, in recv[1] . Then use however you want.