Backbone listenTo callback arguments

Question

I'm new to Backbone and I'm confused by a snippet of the sample todo app code: http://backbonejs.org/docs/todos.html

On the AppView object there's a function called addOne that takes an argument:

addOne: function(todo) {
  var view = new TodoView({model: todo});
  this.$("#todo-list").append(view.render().el);
},

This function is called whenever the add event is triggered on the Todos model. There doesn't seem to be anything in here that tells the listener that it needs to pass in a todo argument to the addOne function:

this.listenTo(Todos, 'add', this.addOne);

When the event gets triggered and this.addOne is called, how does Backbone know to provide addOne with it's todo argument, since it doesn't seem to be specified in the listenTo invocation?

Answer 1

Backbone.Collection.add & Backbone.Collection.create will trigger the add event passing model, this, options as arguments like this.

.trigger('add', model, this, options);

this.addOne is then passed these arguments when called by listenTo.