WebSocket instead of HTTP-endpoint

Most modern web applications use WebSockets, when real-time data from servers need to be pushed out to the web applications, within a browser.

The implementation of the BACnet/WS in the DINGO-Stack, contains a HTTP-endpoint, that will proxy incoming HTTP-requests over on a open WebSocket-connection.

To use this option, the user has to follow these steps:

Create a subscription as described earlier, but modify the callback: "callback": "http://192.168.1.110/bacnetws/cov-http-to-ws-proxy" where the IP address in the example is the address of the DINGO device that is hosting the BACnet web-services.
Get the identifier (e.g. 1) provided for the created subscription, by parsing the HTTP-reply.
Use the identifier (e.g. 1) and create a WebSocket connection to ws://192.168.1.110/bacnetws/cov-http-to-ws-proxy/1?alt=json where the IP address in the example is the address of the DINGO device that is hosting the BACnet web-services.

Study the following JavaScript functions to see how the subscription and WebSocket connection is made .

function CreateSubscribtion(device,object) {
    // device could for example be: 1000
    // object could for example be: analog-input,18
    var xmlHttp = new XMLHttpRequest();
    xmlHttp.onreadystatechange = function () {
        if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
            // SUBSCRIPTION CREATED
            var headerString = xmlHttp.getResponseHeader("Location");
            if (headerString != null) {
                var urlParts = headerString.split("/");
                subscribtionID = urlParts[urlParts.length - 1];
                WebSocketSubscribe(urlParts[urlParts.length - 1]);
            }
        }
    }
    // CREATE BACnet/WS SUBSCRIPTION
    var host = "192.168.1.110";
    xmlHttp.open("POST", "http://" + host + "/bacnetws/.subs?alt=json", true); // true for asynchronous
    var subscription = {
        label: "My subscription",
        callback: "http://" + host + "/bacnetws/cov-http-to-ws-proxy",
        lifetime: "120",
        covs: {
            1: {
                path: "/.bacnet/.local/" + device + "/" + object,
                increment: "1"
            }
        }
    };
    xmlHttp.send(JSON.stringify(subscription));
}
 
function WebSocketSubscribe(id) {
    var host = "192.168.1.110";
    var wslocation = "ws://" + host + "/bacnetws/cov-http-to-ws-proxy/" + id + "?alt=json";
    if ("WebSocket" in window) {
        // CONNECTING TO WEBSOCKET
        ws = new WebSocket(wslocation);
        ws.onopen = function (event) {
            // CONNECTED
        };
        ws.onmessage = function (evt) {
            // RECIEVED DATA
            var msg = evt.data.toString();
            // Chrome and Firefox could not parse JSON to object,
            // not until we replace control characters out of the string.
            msg = msg.replace(/[\x00-\x1F\x7F-\x9F]/g, "");
            try {
                var obj = JSON.parse(msg);
                alert(obj["1"].$value);
            }
            catch (e) {
                alert(msg);
            }
        };
        ws.onclose = function () {
            // CONNECTION CLOSED
        };
        ws.onerror = function (error) {
            alert("ERROR: " + error.data);
        };
    }
    else {
        alert("This browser does not support WebSockets.");
    }
}

A web application (testclient.htm) exists on the file system, that uses WebSockets and can be studied furthered.