How can an observer find out the before and after values of the observed property in Ember.js?

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beforeObserver has been deprecated in Ember 1.10.

Instead, use a manual cache:

doSomething: Ember.observer( 'foo', function() {
  var foo = this.get('foo');
  var oldFoo = this.get('_oldFoo');

  if (foo === oldFoo) { return; }

  // Do stuff here

  this.set('_oldFoo', foo);
})

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