Question

I’m facing a “change event not firing” issue on Backbone.js =/

Here my view of User model :

    window.UserView = Backbone.View.extend({

        ...

        initialize: function()
        {
            this.model.on('destroy', this.remove, this);

            this.model.on('change', function()
            {
               console.log('foo');
            });
        },

        render: function(selected)
        {
            var view = this.template(this.model.toJSON());

            $(this.el).html(view);

            return this;
        },

        transfer: function(e)
        {                
            var cas = listofcas;

            var transferTo = Users.getByCid('c1');
            var transferToCas = transferTo.get('cas');

            this.model.set('cas', cas);
            console.log('current model');
            console.log(this.model);

            //this.model.change();
            this.model.trigger("change:cas");
            console.log('trigger change');

            transferTo.set('cas', transferToCas);
            console.log('transferto model');
            console.log(transferTo);

            //transferTo.change();
            transferTo.trigger("change:cas");
            console.log('trigger change');

        }

    });

Here, the User model :

window.User = Backbone.Model.extend({

        urlRoot: $('#pilote-manager-app').attr('data-src'),

        initialize: function()
        {
            this.set('rand', 1);
            this.set('specialite', this.get('sfGuardUser').specialite);
            this.set('name', this.get('sfGuardUser').first_name + ' ' + this.get('sfGuardUser').last_name);
            this.set('userid', this.get('sfGuardUser').id);
            this.set('avatarsrc', this.get('sfGuardUser').avatarsrc);
            this.set('cas', new Array());

            if (undefined != this.get('sfGuardUser').SignalisationBouclePorteur) {

                var cas = new Array();

                _.each(this.get('sfGuardUser').SignalisationBouclePorteur, function(value)
                {
                    cas.push(value.Signalisation);
                });

                this.set('cas', cas);

            }
        }
    });

In User model, there is “cas” attribute, which is an array of objects.

I read in others topics that change events are not fire on model.set if attributes are not a value.

So, I try to trigger directly the change event with model.change() method.
But, I have no “foo” log in my console …

Answer 1

I’m pretty new to backbone and I was having this same problem.

After doing some research, I found a few posts that shed a little bit more light on why this was happening, and eventually things started to make sense:

Question 1

Question 2

The core reason has to do with the notion of reference equality versus set/member equality. It appears that to a large extent, reference equality is one of the primary techniques backbone uses to figure out when an attribute has changed.

I find that if I use techniques that generate a new reference like Array.slice() or _.clone(), the change event is recognized.

So for example, the following code does not trigger the event because I’m altering the same array reference:

this.collection.each(function (caseFileModel) {
    var labelArray = caseFileModel.get("labels");
    labelArray.push({ Key: 1, DisplayValue: messageData });
    caseFileModel.set({ "labels": labelArray });
});

While this code does trigger the event:

this.collection.each(function (caseFileModel) {
    var labelArray = _.clone(caseFileModel.get("labels")); // The clone() call ensures we get a new array reference - a requirement for the change event
    labelArray.push({ Key: 1, DisplayValue: messageData });
    caseFileModel.set({ "labels": labelArray });
});

NOTE: According to the Underscore API, _.clone() copies certain nested items by reference. The root/parent object is cloned though, so it will work fine for backbone. That is, if your array is very simple and does not have nested structures e.g. [1, 2, 3].

While my improved code above triggered the change event, the following did not because my array contained nested objects:

var labelArray = _.clone(this.model.get("labels"));
_.each(labelArray, function (label) {
    label.isSelected = (_.isEqual(label, selectedLabel));
});
this.model.set({ "labels": labelArray });

Now why does this matter? After debugging very carefully, I noticed that in my iterator I was referencing the same object reference backbone was storing. In other words, I had inadvertently reached into the innards of my model and flipped a bit. When I called setLabels(), backbone correctly recognized that nothing changed because it already knew I flipped that bit.

After looking around some more, people seem to generally say that deep copy operations in javascript are a real pain – nothing built-in to do it. So I did this, which worked fine for me – general applicability may vary:

var labelArray = JSON.parse(JSON.stringify(this.model.get("labels")));
_.each(labelArray, function (label) {
    label.isSelected = (_.isEqual(label, selectedLabel));
});
this.model.set({ "labels": labelArray });