notify-collection.js

var Collection = require("ampersand-rest-collection");
var Model = require("ampersand-model");

var Product = Model.extend({
    props: {
        name: 'string',
        price: 'integer'
    }
});

var Products = Collection.extend({
    model: Product,
    initialize: function () {
        this.listenTo(this, 'change:price', this.notifyPriceChange);
    },
    notifyPriceChange: function () {
        this.trigger('priceChange', this.calculateTotal());
    },
    calculateTotal: function () {
        return this.pluck('price').reduce(function (a, b) {
            return a + b;
        });
    }
});

var products = new Products([{
    name: 'foo',
    price: 10
}, {
    name: 'bar',
    price: 15
}, {
    name: 'baz',
    price: 20
}]);

var Foo = Collection.extend({
    initialize: function () {
        this.listenTo(products, 'priceChange', this.logTotal);
    },
    logTotal: function (total) {
        console.log('Hey, prices changed! New total price is: ' + total);
    }
});

var foo = new Foo();

setTimeout(function () {
    products.first().set('price', 12);
}, 1000);

setTimeout(function () {
    products.first().set('price', 18);
}, 2000);

setTimeout(function () {
    products.first().set('price', 22);
}, 3000);