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

Question

Please see the solution to another question provided by ud3323: http://jsfiddle.net/ud3323/ykL69/. This solution highlights the changed value using the red color. I have an

Answer 1

Take a look at beforeObservers. A beforeObserver fires before a property changes. So if you get the property in a beforeObserver, you will have the the pre-change value. You set up this type of observer using Ember.addBeforeObserver or via the observesBefore function prototype extension.

That would give you what you need to achieve your goal. I've created the following JSBin based on your fiddle to demonstrate this here: http://emberjs.jsbin.com/qewudu/2/edit

UPDATED on 2014-10-07 to reflect behavior in Ember 1.7.x.

UPDATE on 2015-02: beforeObserver has been deprecated. See other answers to this question.