js/libs/ember-1.5.1.js
function(target, opts) {
    var willChange = (opts && opts.willChange) || 'arrayWillChange',
        didChange  = (opts && opts.didChange) || 'arrayDidChange';

    var hasObservers = get(this, 'hasArrayObservers');
    if (!hasObservers) Ember.propertyWillChange(this, 'hasArrayObservers');
    Ember.addListener(this, '@array:before', target, willChange);
    Ember.addListener(this, '@array:change', target, didChange);
    if (!hasObservers) Ember.propertyDidChange(this, 'hasArrayObservers');
    return this;
  }
function(target, opts) {
    var willChange = (opts && opts.willChange) || 'arrayWillChange',
        didChange  = (opts && opts.didChange) || 'arrayDidChange';

    var hasObservers = get(this, 'hasArrayObservers');
    if (!hasObservers) Ember.propertyWillChange(this, 'hasArrayObservers');
    Ember.addListener(this, '@array:before', target, willChange);
    Ember.addListener(this, '@array:change', target, didChange);
    if (!hasObservers) Ember.propertyDidChange(this, 'hasArrayObservers');
    return this;
  }
.......................................................... ARRAY OBSERVERS  
Adds an array observer to the receiving array. The array observer object
normally must implement two methods:

`arrayWillChange(observedObj, start, removeCount, addCount)` - This method will be
called just before the array is modified.
`arrayDidChange(observedObj, start, removeCount, addCount)` - This method will be
called just after the array is modified.

Both callbacks will be passed the observed object, starting index of the
change as well a a count of the items to be removed and added. You can use
these callbacks to optionally inspect the array during the change, clear
caches, or do any other bookkeeping necessary.

In addition to passing a target, you can also include an options hash
which you can use to override the method names that will be invoked on the
target.

@method addArrayObserver
@param {Object} target The observer object.
@param {Hash} opts Optional hash of configuration options including
`willChange` and `didChange` option.
@return {Ember.Array} receiver