PropertyNotify event.window is null

I’m using this code snippet based on your post here

main.js

var x11 = require('x11');
x11.createClient(function (err, display) {
    var X = display.client;
    X.ChangeWindowAttributes(display.screen[0].root, {eventMask: x11.eventMask.PropertyChange});
    X.on('event', function (ev) {
        if (ev.name === 'PropertyNotify') {
            X.GetAtomName(ev.atom, function (err, name) {
                if (name === '_NET_ACTIVE_WINDOW') {
                    X.GetProperty(0, ev.window, ev.atom, X.atoms.WINDOW, 0, 4, function (err, prop) {
                        console.log('New active window:' + prop.data.readUInt32LE(0));
                    });
                }
            });
        }
    });
});

However, it seems that ev.window is null, as well as prop:

cj@lighthouse:~/workspace/moodring$ node main.js 
/home/cj/workspace/moodring/main.js:10
                        console.log('New active window:' + prop.data.readUInt32LE(0));
                                                               ^

TypeError: Cannot read property 'data' of undefined
    at /home/cj/workspace/moodring/main.js:10:64
    at ReadFixedRequest.callback (/home/cj/workspace/moodring/node_modules/x11/lib/xcore.js:490:39)
    at ReadFixedRequest.execute (/home/cj/workspace/moodring/node_modules/x11/lib/unpackstream.js:41:10)
    at UnpackStream.resume (/home/cj/workspace/moodring/node_modules/x11/lib/unpackstream.js:165:30)
    at UnpackStream.write (/home/cj/workspace/moodring/node_modules/x11/lib/unpackstream.js:102:10)
    at Socket.<anonymous> (/home/cj/workspace/moodring/node_modules/x11/lib/xcore.js:88:21)
    at emitOne (events.js:96:13)
    at Socket.emit (events.js:188:7)
    at readableAddChunk (_stream_readable.js:176:18)
    at Socket.Readable.push (_stream_readable.js:134:10)

What’s the proper way to listen for changes in the active (focused) window? I’m running Ubuntu 16.04 with Unity.

[specialblend]

@santigimeno that works perfectly, thank you so much! and thank you @sidorares for your help as well.

I’m quite new to node.js but I will be using both libraries (node-x11 and x11-prop) extensively and I’m happy to contribute to both as I learn more. Regards

[santigimeno]

Back to the OP issue. @SpecialBlend, it’s normal the wid you get is the root as the PropertyNotifythe event comes from the root window. To get the active window wid you need to get the _NET_ACTIVE_WINDOW property and then use that wid to get WM_CLASS, WM_NAME, etc. As an example, something like this would work: (I’m using https://github.com/santigimeno/node-x11-prop, that is just a fancy wrapper over GetProperty()):

var x11 = require('x11');
var x11prop = require('x11-prop');
var get_property = x11prop.get_property;


x11.createClient(function (err, display) {
    var X = display.client;
    X.ChangeWindowAttributes(display.screen[0].root, {eventMask: x11.eventMask.PropertyChange});
    X.on('event', function (ev) {
        console.log(ev);
        if (ev.name === 'PropertyNotify') {
            X.GetAtomName(ev.atom, function (err, name) {
                if (name === '_NET_ACTIVE_WINDOW') {
                   // get active window
                   get_property(X, ev.wid, '_NET_ACTIVE_WINDOW', 'WINDOW', function(err, wid) {
                       if (!err) {
                           console.log('_NET_ACTIVE_WINDOW: ' + wid);
                       }

                       if (wid) {
                           get_property(X, wid, 'WM_CLASS', 'STRING', function(err, data) {
                               if (!err) {
                                   console.log('WM_CLASS: ' + data);
                              }
                          });
                       }
                   });
                }
            });
        }
    });
});