Rxjs observing object updates and changes

Question

I am currently trying to observe any changes to a given object including all of it's elements.

The following code only fires when an object[x] is updates, but not if individually updating object[x]'s elements such as object[x][y]

<script>
  var elem = document.getElementById("test1");

var log = function(x) {
    elem.innerHTML += x + "<br/><br/><br/>";
};

var a = [{a:1,b:2},
         {a:2,b:5}
       ];


var source = Rx.Observable
.ofObjectChanges(a)
.map(function(x) {
    return JSON.stringify(x);
});


var subscription = source.subscribe(
    function (x) {log(x);},
    function (err) {log(err);},
    function () {log('Completed');}
);

a[0] = a[1];
</script>

This code runs and fires correctly.

however. if I instead to this

a[0]['a'] = 3;

Then nothing happens.

EDIT

A better way to phrase this, how can I observe changes from an array of objects?

Probably start with using `ofArrayChanges` instead. – paulpdaniels Sep 21 '15 at 18:43 — paulpdaniels, Sep 21 '15 at 18:43

electrichead · Accepted Answer · 2015-09-22T01:01:10.087

7

If you want only the nested object changes:

var source = rx.Observable.from(a).flatMap(function(item) {
  return rx.Observable.ofObjectChanges(item);
});

If you also want changes like a[0] = a[1]:

var source = rx.Observable.merge(
  rx.Observable.ofArrayChanges(a),
  rx.Observable.from(a).flatMap(function(item) {
    return rx.Observable.ofObjectChanges(item);
  })
);

The flatMap or selectMany (they are the same function) will allow you to iterate over a value and execute a function that returns an Observable. The values from all these Observables are "flattened" onto a new stream that is returned.

http://reactivex.io/documentation/operators/flatmap.html

edited Sep 22 '15 at 01:01

answered Sep 22 '15 at 00:29

electrichead

1,102
7
19

to make it work I needed to change rx into Rx and add the map function at the end of the merge. Cool! – Antonio Apr 16 '16 at 15:55
14

'rx.Observable.ofObjectChanges' depriciated. – serkan Aug 19 '16 at 00:40
5

Yep, seeing this method as depreciated, I would really like to see how to observe object changes in RxJS 5... – Marek M. Oct 03 '17 at 20:54

homam · Answer 2 · 2015-09-22T05:33:44.190

Perhaps something like this by merging two Observables (one for the array and the other observing the elements of the array):

var a = [
  {a:1,b:2},
  {a:2,b:5}
];


var source1 = Rx.Observable.ofArrayChanges(a).map(function(x) {
  return JSON.stringify(x);
});

var source2 = Rx.Observable
.fromArray(a.map(function(o, i) { return [o, i]; }))
.flatMap(function(oi) {
  return Rx.Observable.ofObjectChanges(oi[0])
  .map(function(x) {
    var y = {
      type: x.type,
      object: x.object,
      name: x.name,
      oldValue: x.oldValue,
      arrayIndex: oi[1] // pass the index of the member that changed
    };
    return JSON.stringify(y);
  });
})

source = source1.merge(source2)

var subscription = source.subscribe(
  function (x) {log(x);},
  function (err) {log(err);},
  function () {log('Completed');}
);


a[0] = a[1]
a[1]['b'] = 7

Thanks to @electrichead here we're not using concatMap because the sources that we made by ofObjectChanges and ofArrayChanges never complete.

score 0 · Answer 3 · answered Dec 31 '15 at 10:20

Here's a working example of Rx.Observable.ofNestedObjectChanges simple implementation, you can get the gist of it and implement you own.

http://jsbin.com/wekote/edit?js,console

        Rx.Observable.ofNestedObjectChanges = function(obj) {
            if (obj == null) { throw new TypeError('object must not be null or undefined.'); }
            if (typeof Object.observe !== 'function' && typeof Object.unobserve !== 'function') { throw new TypeError('Object.observe is not supported on your platform') }
            return new Rx.AnonymousObservable(function(observer) {
                function observerFn(changes) {
                    for(var i = 0, len = changes.length; i < len; i++) {
                        observer.onNext(changes[i]);
                    }
                }
                Object.observe(obj, observerFn);
                //Recursive observers hooks - same observerFn
                traverseObjectTree(obj, observerFn);

                function traverseObjectTree(element, observerFn){
                    for(var i=0;i<Object.keys(element).length;i++){
                        var myObj = element[Object.keys(element)[i]];
                        if(typeof myObj === "object"){
                            Object.observe(myObj, observerFn);
                            traverseObjectTree(myObj,observerFn);
                        }
                    }
                }

                return function () {
                    Object.unobserve(obj, observerFn);
                };
            });
        };



        //Test
        var json = {
            element : {
                name : "Yocto",
                job : {
                    title: "Designer"
                }
            },
            element1: {
                name : "Mokto"
            }
        };

        setTimeout(function(){
            json.element.job.title = "A Great Designer";
        },3000);


        var source = Rx.Observable.ofNestedObjectChanges(json);

        var subscription = source.subscribe(
            function (x) {
                console.log(x);
            },
            function (err) {
                console.log('Error: %s', err);
            },
            function () {
                console.log('Completed');
            });

        json.element.name = "Candy Joe";

Checked out in Safari & Chrome. Snippet provided by link appears to be broken. — akaRem, Jan 14 '17 at 18:47

Rxjs observing object updates and changes

3 Answers3

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