Backbone collection counter

Question

I've a collection witch can paginate over a collection in the backend, so onscroll I lot another batch of items. I know the count of items in on the server. I'm looking for a way to extend all my collections with a getCounter method. The idea is to set the server counter initially and when ever a user add or remove items from the collection the counter updates. Whats will be the best approach for this?

I think the problem is that the add event is also fired when I fetch new items from the server. Otherwise I could just bind to add and remove events.

Answer 1

You need to control which additions and removals should count, and which not. Two options come to mind.

1. Silence all operations which should not count with the silent:true option, and hook up your counter to the add, remove and reset events. This will put some limitations to the general use of the collection, because you can't utilize their events in all cases.

2. Override the add, remove and reset methods to accept an extra option, which tells you whether the counter should be updated or not. Something like:

   var PaginatedCollection = Backbone.Collection.extend({
   
       initialize: function() {
           this._counter = 0;
       },
   
       add: function(models, options) {
           if(options && options.count)
               this._counter += (_.isArray(models) ? models.length : 1);
           Backbone.Collection.prototype.add.apply(this, arguments);
       },
   
       remove: function(models, options) {
           if(options && options.count)
               this._counter -= (_.isArray(models) ? models.length : 1);
           Backbone.Collection.prototype.remove.apply(this, arguments);         
       },
   
       reset: function(models, options) {
           if(options && options.count)
               this._counter = (_.isArray(models) ? models.length : 0);
           Backbone.Collection.prototype.reset.apply(this, arguments)
       }
   });
   

   And pass the count:true option, when the added or removed item should be counted:

   var SomeCollection = PaginatedCollection.extend({ });
   
   var someCollection = new SomeCollection();
   someCollection.add(model, { count: true });
   someCollection.remove(model, { count: true });
   

(Code samples untested)