Computed Properties and Aggregate Data with @each

Often, you may have a computed property that relies on all of the items in an array to determine its value. For example, you may want to count all of the todo items in a controller to determine how many of them are completed.

Here's what that computed property might look like:

App.TodosController = Ember.Controller.extend({
  todos: [
    Ember.Object.create({ isDone: true }),
    Ember.Object.create({ isDone: false }),
    Ember.Object.create({ isDone: true })
  ],

  remaining: function() {
    var todos = this.get('todos');
    return todos.filterBy('isDone', false).get('length');
  }.property('todos.@each.isDone')
});

Note here that the dependent key (todos.@each.isDone) contains the special key @each. This instructs Ember.js to update bindings and fire observers for this computed property when one of the following four events occurs:

1. The isDone property of any of the objects in the todos array changes.
2. An item is added to the todos array.
3. An item is removed from the todos array.
4. The todos property of the controller is changed to a different array.

In the example above, the remaining count is 1:

App.todosController = App.TodosController.create();
App.todosController.get('remaining');
// 1

If we change the todo's isDone property, the remaining property is updated automatically:

var todos = App.todosController.get('todos');
var todo = todos.objectAt(1);
todo.set('isDone', true);

App.todosController.get('remaining');
// 0

todo = Ember.Object.create({ isDone: false });
todos.pushObject(todo);

App.todosController.get('remaining');
// 1

Note that @each only works one level deep. You cannot use nested forms like todos.@each.owner.name or todos.@each.owner.@each.name.