Ember.beforeObserver()

When observers fire, they are called with the arguments obj, keyName.

Note, @each.property observer is called per each add or replace of an element and it's not called with a specific enumeration item.

A _beforeObserver fires before a property changes.

A _beforeObserver is an alternative form of .observesBefore().

App.PersonView = Ember.View.extend({
  friends: [{ name: 'Tom' }, { name: 'Stefan' }, { name: 'Kris' }],

  valueDidChange: Ember.observer('content.value', function(obj, keyName) {
      // only run if updating a value already in the DOM
      if (this.get('state') === 'inDOM') {
        let color = obj.get(keyName) > this.changingFrom ? 'green' : 'red';
        // logic
      }
  }),

  friendsDidChange: Ember.observer('friends.@each.name', function(obj, keyName) {
    // some logic
    // obj.get(keyName) returns friends array
  })
});

Also available as Function.prototype.observesBefore if prototype extensions are enabled.