Deprecation Guide for beforeObserver

Before observers are deprecated due to the negative performance implications they have for Ember internals and applications.

Typically they were used to have access to the old value of a property when it's about to change, but you can get same functionality in an even more efficient way with just a few lines of code:

function fooObserver(obj){
  var newFoo = obj.get('foo');
  if (obj._oldFoo !== newFoo) {
    // do your stuff here
    obj._oldFoo = newFoo;
  }
}
addObserver(obj, 'foo', fooObserver);
fooObserver(obj); // Optionally call the observer immediately

The related functions Ember.addBeforeObserver, Ember.removeBeforeObserver, Ember.beforeObserversFor, and Function#beforeObserver are deprecated too.