SerializeData in CompositeView

I need to pass a value to the listView.template in order to be aware the template about the collection.length.
I think one option is to redefine the serializeData method in order to pass a parameter in this way.

var ListView = Marionette.CompositeView.extend({

    initialize: function () {
        this.collection.on('reset', this.serializeData, this);
        this.collection.on('remove', this.serializeData, this);
        this.collection.on('add', this.serializeData, this);
    },

    serializeData: function () {
        console.log(this.collection.length);
        return {
            has_tasks: this.collection.length > 0
        };
    },

    // other codes
});

When I start the app the collection is not yet fetched so:

1.a) the collection.legth = 0
2.b) the template get has_tasks = false as expected.

2.a) after the fetch the collection.length is > 0,
2.b) the serializeData is called and so it puts the has_tasks = true,
2.c) the template seems to be not rendered because it maintains the has_tasks = false

Any idea because 2.c?

Latest Marionette has solved this problem by calling an optional templateHelpers on the view to provide additional context to the view. Also your event binding is not Marionette-friendly as it it will not be auto-unbound correctly when the view is unloaded. So all you need to do in your view is:

var ListView = Marionette.CompositeView.extend({

    initialize: function () {
        this.bindTo(this.collection, "add", this.render, this);
        this.bindTo(this.collection, "remove", this.render, this);
        this.bindTo(this.collection, "reset", this.render, this);
    },

    templateHelpers: function () {
        console.log(this.collection.length);
        return {
            has_tasks: this.collection.length > 0
        };
    },

    // other codes
});

Note, however, that you probably don't want to rerender the entire view and all the sub-elements every time an item is added or removed. A better approach is to only update the count displayed. For instance:

var ListView = Marionette.CompositeView.extend({

    initialize: function () {
        this.bindTo(this.collection, "add", this.updateCount, this);
        this.bindTo(this.collection, "remove", this.updateCount, this);
        this.bindTo(this.collection, "reset", this.updateCount, this);
    },

    updateCount: function() {
        this.$('.count_span').html(this.collection.length);
    },

    // other codes
});